189e33ea7SMauro Carvalho Chehab========= 289e33ea7SMauro Carvalho ChehabLivepatch 389e33ea7SMauro Carvalho Chehab========= 489e33ea7SMauro Carvalho Chehab 589e33ea7SMauro Carvalho ChehabThis document outlines basic information about kernel livepatching. 689e33ea7SMauro Carvalho Chehab 789e33ea7SMauro Carvalho Chehab.. Table of Contents: 889e33ea7SMauro Carvalho Chehab 9*7af6fbddSMark Brown.. contents:: :local: 1089e33ea7SMauro Carvalho Chehab 1189e33ea7SMauro Carvalho Chehab 1289e33ea7SMauro Carvalho Chehab1. Motivation 1389e33ea7SMauro Carvalho Chehab============= 1489e33ea7SMauro Carvalho Chehab 1589e33ea7SMauro Carvalho ChehabThere are many situations where users are reluctant to reboot a system. It may 1689e33ea7SMauro Carvalho Chehabbe because their system is performing complex scientific computations or under 1789e33ea7SMauro Carvalho Chehabheavy load during peak usage. In addition to keeping systems up and running, 1889e33ea7SMauro Carvalho Chehabusers want to also have a stable and secure system. Livepatching gives users 1989e33ea7SMauro Carvalho Chehabboth by allowing for function calls to be redirected; thus, fixing critical 2089e33ea7SMauro Carvalho Chehabfunctions without a system reboot. 2189e33ea7SMauro Carvalho Chehab 2289e33ea7SMauro Carvalho Chehab 2389e33ea7SMauro Carvalho Chehab2. Kprobes, Ftrace, Livepatching 2489e33ea7SMauro Carvalho Chehab================================ 2589e33ea7SMauro Carvalho Chehab 2689e33ea7SMauro Carvalho ChehabThere are multiple mechanisms in the Linux kernel that are directly related 2789e33ea7SMauro Carvalho Chehabto redirection of code execution; namely: kernel probes, function tracing, 2889e33ea7SMauro Carvalho Chehaband livepatching: 2989e33ea7SMauro Carvalho Chehab 3089e33ea7SMauro Carvalho Chehab - The kernel probes are the most generic. The code can be redirected by 3189e33ea7SMauro Carvalho Chehab putting a breakpoint instruction instead of any instruction. 3289e33ea7SMauro Carvalho Chehab 3389e33ea7SMauro Carvalho Chehab - The function tracer calls the code from a predefined location that is 3489e33ea7SMauro Carvalho Chehab close to the function entry point. This location is generated by the 3589e33ea7SMauro Carvalho Chehab compiler using the '-pg' gcc option. 3689e33ea7SMauro Carvalho Chehab 3789e33ea7SMauro Carvalho Chehab - Livepatching typically needs to redirect the code at the very beginning 3889e33ea7SMauro Carvalho Chehab of the function entry before the function parameters or the stack 3989e33ea7SMauro Carvalho Chehab are in any way modified. 4089e33ea7SMauro Carvalho Chehab 4189e33ea7SMauro Carvalho ChehabAll three approaches need to modify the existing code at runtime. Therefore 4289e33ea7SMauro Carvalho Chehabthey need to be aware of each other and not step over each other's toes. 4389e33ea7SMauro Carvalho ChehabMost of these problems are solved by using the dynamic ftrace framework as 4489e33ea7SMauro Carvalho Chehaba base. A Kprobe is registered as a ftrace handler when the function entry 4589e33ea7SMauro Carvalho Chehabis probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from 4689e33ea7SMauro Carvalho Chehaba live patch is called with the help of a custom ftrace handler. But there are 4789e33ea7SMauro Carvalho Chehabsome limitations, see below. 4889e33ea7SMauro Carvalho Chehab 4989e33ea7SMauro Carvalho Chehab 5089e33ea7SMauro Carvalho Chehab3. Consistency model 5189e33ea7SMauro Carvalho Chehab==================== 5289e33ea7SMauro Carvalho Chehab 5389e33ea7SMauro Carvalho ChehabFunctions are there for a reason. They take some input parameters, get or 5489e33ea7SMauro Carvalho Chehabrelease locks, read, process, and even write some data in a defined way, 5589e33ea7SMauro Carvalho Chehabhave return values. In other words, each function has a defined semantic. 5689e33ea7SMauro Carvalho Chehab 5789e33ea7SMauro Carvalho ChehabMany fixes do not change the semantic of the modified functions. For 5889e33ea7SMauro Carvalho Chehabexample, they add a NULL pointer or a boundary check, fix a race by adding 5989e33ea7SMauro Carvalho Chehaba missing memory barrier, or add some locking around a critical section. 6089e33ea7SMauro Carvalho ChehabMost of these changes are self contained and the function presents itself 6189e33ea7SMauro Carvalho Chehabthe same way to the rest of the system. In this case, the functions might 6289e33ea7SMauro Carvalho Chehabbe updated independently one by one. 6389e33ea7SMauro Carvalho Chehab 6489e33ea7SMauro Carvalho ChehabBut there are more complex fixes. For example, a patch might change 6589e33ea7SMauro Carvalho Chehabordering of locking in multiple functions at the same time. Or a patch 6689e33ea7SMauro Carvalho Chehabmight exchange meaning of some temporary structures and update 6789e33ea7SMauro Carvalho Chehaball the relevant functions. In this case, the affected unit 6889e33ea7SMauro Carvalho Chehab(thread, whole kernel) need to start using all new versions of 6989e33ea7SMauro Carvalho Chehabthe functions at the same time. Also the switch must happen only 7089e33ea7SMauro Carvalho Chehabwhen it is safe to do so, e.g. when the affected locks are released 7189e33ea7SMauro Carvalho Chehabor no data are stored in the modified structures at the moment. 7289e33ea7SMauro Carvalho Chehab 7389e33ea7SMauro Carvalho ChehabThe theory about how to apply functions a safe way is rather complex. 7489e33ea7SMauro Carvalho ChehabThe aim is to define a so-called consistency model. It attempts to define 7589e33ea7SMauro Carvalho Chehabconditions when the new implementation could be used so that the system 7689e33ea7SMauro Carvalho Chehabstays consistent. 7789e33ea7SMauro Carvalho Chehab 7889e33ea7SMauro Carvalho ChehabLivepatch has a consistency model which is a hybrid of kGraft and 7989e33ea7SMauro Carvalho Chehabkpatch: it uses kGraft's per-task consistency and syscall barrier 8089e33ea7SMauro Carvalho Chehabswitching combined with kpatch's stack trace switching. There are also 8189e33ea7SMauro Carvalho Chehaba number of fallback options which make it quite flexible. 8289e33ea7SMauro Carvalho Chehab 8389e33ea7SMauro Carvalho ChehabPatches are applied on a per-task basis, when the task is deemed safe to 8489e33ea7SMauro Carvalho Chehabswitch over. When a patch is enabled, livepatch enters into a 8589e33ea7SMauro Carvalho Chehabtransition state where tasks are converging to the patched state. 8689e33ea7SMauro Carvalho ChehabUsually this transition state can complete in a few seconds. The same 8789e33ea7SMauro Carvalho Chehabsequence occurs when a patch is disabled, except the tasks converge from 8889e33ea7SMauro Carvalho Chehabthe patched state to the unpatched state. 8989e33ea7SMauro Carvalho Chehab 9089e33ea7SMauro Carvalho ChehabAn interrupt handler inherits the patched state of the task it 9189e33ea7SMauro Carvalho Chehabinterrupts. The same is true for forked tasks: the child inherits the 9289e33ea7SMauro Carvalho Chehabpatched state of the parent. 9389e33ea7SMauro Carvalho Chehab 9489e33ea7SMauro Carvalho ChehabLivepatch uses several complementary approaches to determine when it's 9589e33ea7SMauro Carvalho Chehabsafe to patch tasks: 9689e33ea7SMauro Carvalho Chehab 9789e33ea7SMauro Carvalho Chehab1. The first and most effective approach is stack checking of sleeping 9889e33ea7SMauro Carvalho Chehab tasks. If no affected functions are on the stack of a given task, 9989e33ea7SMauro Carvalho Chehab the task is patched. In most cases this will patch most or all of 10089e33ea7SMauro Carvalho Chehab the tasks on the first try. Otherwise it'll keep trying 10189e33ea7SMauro Carvalho Chehab periodically. This option is only available if the architecture has 10289e33ea7SMauro Carvalho Chehab reliable stacks (HAVE_RELIABLE_STACKTRACE). 10389e33ea7SMauro Carvalho Chehab 10489e33ea7SMauro Carvalho Chehab2. The second approach, if needed, is kernel exit switching. A 10589e33ea7SMauro Carvalho Chehab task is switched when it returns to user space from a system call, a 10689e33ea7SMauro Carvalho Chehab user space IRQ, or a signal. It's useful in the following cases: 10789e33ea7SMauro Carvalho Chehab 10889e33ea7SMauro Carvalho Chehab a) Patching I/O-bound user tasks which are sleeping on an affected 10989e33ea7SMauro Carvalho Chehab function. In this case you have to send SIGSTOP and SIGCONT to 11089e33ea7SMauro Carvalho Chehab force it to exit the kernel and be patched. 11189e33ea7SMauro Carvalho Chehab b) Patching CPU-bound user tasks. If the task is highly CPU-bound 11289e33ea7SMauro Carvalho Chehab then it will get patched the next time it gets interrupted by an 11389e33ea7SMauro Carvalho Chehab IRQ. 11489e33ea7SMauro Carvalho Chehab 11589e33ea7SMauro Carvalho Chehab3. For idle "swapper" tasks, since they don't ever exit the kernel, they 11689e33ea7SMauro Carvalho Chehab instead have a klp_update_patch_state() call in the idle loop which 11789e33ea7SMauro Carvalho Chehab allows them to be patched before the CPU enters the idle state. 11889e33ea7SMauro Carvalho Chehab 11989e33ea7SMauro Carvalho Chehab (Note there's not yet such an approach for kthreads.) 12089e33ea7SMauro Carvalho Chehab 12189e33ea7SMauro Carvalho ChehabArchitectures which don't have HAVE_RELIABLE_STACKTRACE solely rely on 12289e33ea7SMauro Carvalho Chehabthe second approach. It's highly likely that some tasks may still be 12389e33ea7SMauro Carvalho Chehabrunning with an old version of the function, until that function 12489e33ea7SMauro Carvalho Chehabreturns. In this case you would have to signal the tasks. This 12589e33ea7SMauro Carvalho Chehabespecially applies to kthreads. They may not be woken up and would need 12689e33ea7SMauro Carvalho Chehabto be forced. See below for more information. 12789e33ea7SMauro Carvalho Chehab 12889e33ea7SMauro Carvalho ChehabUnless we can come up with another way to patch kthreads, architectures 12989e33ea7SMauro Carvalho Chehabwithout HAVE_RELIABLE_STACKTRACE are not considered fully supported by 13089e33ea7SMauro Carvalho Chehabthe kernel livepatching. 13189e33ea7SMauro Carvalho Chehab 13289e33ea7SMauro Carvalho ChehabThe /sys/kernel/livepatch/<patch>/transition file shows whether a patch 13389e33ea7SMauro Carvalho Chehabis in transition. Only a single patch can be in transition at a given 13489e33ea7SMauro Carvalho Chehabtime. A patch can remain in transition indefinitely, if any of the tasks 13589e33ea7SMauro Carvalho Chehabare stuck in the initial patch state. 13689e33ea7SMauro Carvalho Chehab 13789e33ea7SMauro Carvalho ChehabA transition can be reversed and effectively canceled by writing the 13889e33ea7SMauro Carvalho Chehabopposite value to the /sys/kernel/livepatch/<patch>/enabled file while 13989e33ea7SMauro Carvalho Chehabthe transition is in progress. Then all the tasks will attempt to 14089e33ea7SMauro Carvalho Chehabconverge back to the original patch state. 14189e33ea7SMauro Carvalho Chehab 14289e33ea7SMauro Carvalho ChehabThere's also a /proc/<pid>/patch_state file which can be used to 14389e33ea7SMauro Carvalho Chehabdetermine which tasks are blocking completion of a patching operation. 14489e33ea7SMauro Carvalho ChehabIf a patch is in transition, this file shows 0 to indicate the task is 14589e33ea7SMauro Carvalho Chehabunpatched and 1 to indicate it's patched. Otherwise, if no patch is in 14689e33ea7SMauro Carvalho Chehabtransition, it shows -1. Any tasks which are blocking the transition 14789e33ea7SMauro Carvalho Chehabcan be signaled with SIGSTOP and SIGCONT to force them to change their 14889e33ea7SMauro Carvalho Chehabpatched state. This may be harmful to the system though. Sending a fake signal 14989e33ea7SMauro Carvalho Chehabto all remaining blocking tasks is a better alternative. No proper signal is 15089e33ea7SMauro Carvalho Chehabactually delivered (there is no data in signal pending structures). Tasks are 15189e33ea7SMauro Carvalho Chehabinterrupted or woken up, and forced to change their patched state. The fake 15289e33ea7SMauro Carvalho Chehabsignal is automatically sent every 15 seconds. 15389e33ea7SMauro Carvalho Chehab 15489e33ea7SMauro Carvalho ChehabAdministrator can also affect a transition through 15589e33ea7SMauro Carvalho Chehab/sys/kernel/livepatch/<patch>/force attribute. Writing 1 there clears 15689e33ea7SMauro Carvalho ChehabTIF_PATCH_PENDING flag of all tasks and thus forces the tasks to the patched 15789e33ea7SMauro Carvalho Chehabstate. Important note! The force attribute is intended for cases when the 15889e33ea7SMauro Carvalho Chehabtransition gets stuck for a long time because of a blocking task. Administrator 15989e33ea7SMauro Carvalho Chehabis expected to collect all necessary data (namely stack traces of such blocking 16089e33ea7SMauro Carvalho Chehabtasks) and request a clearance from a patch distributor to force the transition. 16189e33ea7SMauro Carvalho ChehabUnauthorized usage may cause harm to the system. It depends on the nature of the 16289e33ea7SMauro Carvalho Chehabpatch, which functions are (un)patched, and which functions the blocking tasks 16389e33ea7SMauro Carvalho Chehabare sleeping in (/proc/<pid>/stack may help here). Removal (rmmod) of patch 16489e33ea7SMauro Carvalho Chehabmodules is permanently disabled when the force feature is used. It cannot be 16589e33ea7SMauro Carvalho Chehabguaranteed there is no task sleeping in such module. It implies unbounded 16689e33ea7SMauro Carvalho Chehabreference count if a patch module is disabled and enabled in a loop. 16789e33ea7SMauro Carvalho Chehab 16889e33ea7SMauro Carvalho ChehabMoreover, the usage of force may also affect future applications of live 16989e33ea7SMauro Carvalho Chehabpatches and cause even more harm to the system. Administrator should first 17089e33ea7SMauro Carvalho Chehabconsider to simply cancel a transition (see above). If force is used, reboot 17189e33ea7SMauro Carvalho Chehabshould be planned and no more live patches applied. 17289e33ea7SMauro Carvalho Chehab 17389e33ea7SMauro Carvalho Chehab3.1 Adding consistency model support to new architectures 17489e33ea7SMauro Carvalho Chehab--------------------------------------------------------- 17589e33ea7SMauro Carvalho Chehab 17689e33ea7SMauro Carvalho ChehabFor adding consistency model support to new architectures, there are a 17789e33ea7SMauro Carvalho Chehabfew options: 17889e33ea7SMauro Carvalho Chehab 17989e33ea7SMauro Carvalho Chehab1) Add CONFIG_HAVE_RELIABLE_STACKTRACE. This means porting objtool, and 18089e33ea7SMauro Carvalho Chehab for non-DWARF unwinders, also making sure there's a way for the stack 18189e33ea7SMauro Carvalho Chehab tracing code to detect interrupts on the stack. 18289e33ea7SMauro Carvalho Chehab 18389e33ea7SMauro Carvalho Chehab2) Alternatively, ensure that every kthread has a call to 18489e33ea7SMauro Carvalho Chehab klp_update_patch_state() in a safe location. Kthreads are typically 18589e33ea7SMauro Carvalho Chehab in an infinite loop which does some action repeatedly. The safe 18689e33ea7SMauro Carvalho Chehab location to switch the kthread's patch state would be at a designated 18789e33ea7SMauro Carvalho Chehab point in the loop where there are no locks taken and all data 18889e33ea7SMauro Carvalho Chehab structures are in a well-defined state. 18989e33ea7SMauro Carvalho Chehab 19089e33ea7SMauro Carvalho Chehab The location is clear when using workqueues or the kthread worker 19189e33ea7SMauro Carvalho Chehab API. These kthreads process independent actions in a generic loop. 19289e33ea7SMauro Carvalho Chehab 19389e33ea7SMauro Carvalho Chehab It's much more complicated with kthreads which have a custom loop. 19489e33ea7SMauro Carvalho Chehab There the safe location must be carefully selected on a case-by-case 19589e33ea7SMauro Carvalho Chehab basis. 19689e33ea7SMauro Carvalho Chehab 19789e33ea7SMauro Carvalho Chehab In that case, arches without HAVE_RELIABLE_STACKTRACE would still be 19889e33ea7SMauro Carvalho Chehab able to use the non-stack-checking parts of the consistency model: 19989e33ea7SMauro Carvalho Chehab 20089e33ea7SMauro Carvalho Chehab a) patching user tasks when they cross the kernel/user space 20189e33ea7SMauro Carvalho Chehab boundary; and 20289e33ea7SMauro Carvalho Chehab 20389e33ea7SMauro Carvalho Chehab b) patching kthreads and idle tasks at their designated patch points. 20489e33ea7SMauro Carvalho Chehab 20589e33ea7SMauro Carvalho Chehab This option isn't as good as option 1 because it requires signaling 20689e33ea7SMauro Carvalho Chehab user tasks and waking kthreads to patch them. But it could still be 20789e33ea7SMauro Carvalho Chehab a good backup option for those architectures which don't have 20889e33ea7SMauro Carvalho Chehab reliable stack traces yet. 20989e33ea7SMauro Carvalho Chehab 21089e33ea7SMauro Carvalho Chehab 21189e33ea7SMauro Carvalho Chehab4. Livepatch module 21289e33ea7SMauro Carvalho Chehab=================== 21389e33ea7SMauro Carvalho Chehab 21489e33ea7SMauro Carvalho ChehabLivepatches are distributed using kernel modules, see 21589e33ea7SMauro Carvalho Chehabsamples/livepatch/livepatch-sample.c. 21689e33ea7SMauro Carvalho Chehab 21789e33ea7SMauro Carvalho ChehabThe module includes a new implementation of functions that we want 21889e33ea7SMauro Carvalho Chehabto replace. In addition, it defines some structures describing the 21989e33ea7SMauro Carvalho Chehabrelation between the original and the new implementation. Then there 22089e33ea7SMauro Carvalho Chehabis code that makes the kernel start using the new code when the livepatch 22189e33ea7SMauro Carvalho Chehabmodule is loaded. Also there is code that cleans up before the 22289e33ea7SMauro Carvalho Chehablivepatch module is removed. All this is explained in more details in 22389e33ea7SMauro Carvalho Chehabthe next sections. 22489e33ea7SMauro Carvalho Chehab 22589e33ea7SMauro Carvalho Chehab 22689e33ea7SMauro Carvalho Chehab4.1. New functions 22789e33ea7SMauro Carvalho Chehab------------------ 22889e33ea7SMauro Carvalho Chehab 22989e33ea7SMauro Carvalho ChehabNew versions of functions are typically just copied from the original 23089e33ea7SMauro Carvalho Chehabsources. A good practice is to add a prefix to the names so that they 23189e33ea7SMauro Carvalho Chehabcan be distinguished from the original ones, e.g. in a backtrace. Also 23289e33ea7SMauro Carvalho Chehabthey can be declared as static because they are not called directly 23389e33ea7SMauro Carvalho Chehaband do not need the global visibility. 23489e33ea7SMauro Carvalho Chehab 23589e33ea7SMauro Carvalho ChehabThe patch contains only functions that are really modified. But they 23689e33ea7SMauro Carvalho Chehabmight want to access functions or data from the original source file 23789e33ea7SMauro Carvalho Chehabthat may only be locally accessible. This can be solved by a special 23889e33ea7SMauro Carvalho Chehabrelocation section in the generated livepatch module, see 23989e33ea7SMauro Carvalho ChehabDocumentation/livepatch/module-elf-format.rst for more details. 24089e33ea7SMauro Carvalho Chehab 24189e33ea7SMauro Carvalho Chehab 24289e33ea7SMauro Carvalho Chehab4.2. Metadata 24389e33ea7SMauro Carvalho Chehab------------- 24489e33ea7SMauro Carvalho Chehab 24589e33ea7SMauro Carvalho ChehabThe patch is described by several structures that split the information 24689e33ea7SMauro Carvalho Chehabinto three levels: 24789e33ea7SMauro Carvalho Chehab 24889e33ea7SMauro Carvalho Chehab - struct klp_func is defined for each patched function. It describes 24989e33ea7SMauro Carvalho Chehab the relation between the original and the new implementation of a 25089e33ea7SMauro Carvalho Chehab particular function. 25189e33ea7SMauro Carvalho Chehab 25289e33ea7SMauro Carvalho Chehab The structure includes the name, as a string, of the original function. 25389e33ea7SMauro Carvalho Chehab The function address is found via kallsyms at runtime. 25489e33ea7SMauro Carvalho Chehab 25589e33ea7SMauro Carvalho Chehab Then it includes the address of the new function. It is defined 25689e33ea7SMauro Carvalho Chehab directly by assigning the function pointer. Note that the new 25789e33ea7SMauro Carvalho Chehab function is typically defined in the same source file. 25889e33ea7SMauro Carvalho Chehab 25989e33ea7SMauro Carvalho Chehab As an optional parameter, the symbol position in the kallsyms database can 26089e33ea7SMauro Carvalho Chehab be used to disambiguate functions of the same name. This is not the 26189e33ea7SMauro Carvalho Chehab absolute position in the database, but rather the order it has been found 26289e33ea7SMauro Carvalho Chehab only for a particular object ( vmlinux or a kernel module ). Note that 26389e33ea7SMauro Carvalho Chehab kallsyms allows for searching symbols according to the object name. 26489e33ea7SMauro Carvalho Chehab 26589e33ea7SMauro Carvalho Chehab - struct klp_object defines an array of patched functions (struct 26689e33ea7SMauro Carvalho Chehab klp_func) in the same object. Where the object is either vmlinux 26789e33ea7SMauro Carvalho Chehab (NULL) or a module name. 26889e33ea7SMauro Carvalho Chehab 26989e33ea7SMauro Carvalho Chehab The structure helps to group and handle functions for each object 27089e33ea7SMauro Carvalho Chehab together. Note that patched modules might be loaded later than 27189e33ea7SMauro Carvalho Chehab the patch itself and the relevant functions might be patched 27289e33ea7SMauro Carvalho Chehab only when they are available. 27389e33ea7SMauro Carvalho Chehab 27489e33ea7SMauro Carvalho Chehab 27589e33ea7SMauro Carvalho Chehab - struct klp_patch defines an array of patched objects (struct 27689e33ea7SMauro Carvalho Chehab klp_object). 27789e33ea7SMauro Carvalho Chehab 27889e33ea7SMauro Carvalho Chehab This structure handles all patched functions consistently and eventually, 27989e33ea7SMauro Carvalho Chehab synchronously. The whole patch is applied only when all patched 28089e33ea7SMauro Carvalho Chehab symbols are found. The only exception are symbols from objects 28189e33ea7SMauro Carvalho Chehab (kernel modules) that have not been loaded yet. 28289e33ea7SMauro Carvalho Chehab 28389e33ea7SMauro Carvalho Chehab For more details on how the patch is applied on a per-task basis, 28489e33ea7SMauro Carvalho Chehab see the "Consistency model" section. 28589e33ea7SMauro Carvalho Chehab 28689e33ea7SMauro Carvalho Chehab 28789e33ea7SMauro Carvalho Chehab5. Livepatch life-cycle 28889e33ea7SMauro Carvalho Chehab======================= 28989e33ea7SMauro Carvalho Chehab 29089e33ea7SMauro Carvalho ChehabLivepatching can be described by five basic operations: 29189e33ea7SMauro Carvalho Chehabloading, enabling, replacing, disabling, removing. 29289e33ea7SMauro Carvalho Chehab 29389e33ea7SMauro Carvalho ChehabWhere the replacing and the disabling operations are mutually 29489e33ea7SMauro Carvalho Chehabexclusive. They have the same result for the given patch but 29589e33ea7SMauro Carvalho Chehabnot for the system. 29689e33ea7SMauro Carvalho Chehab 29789e33ea7SMauro Carvalho Chehab 29889e33ea7SMauro Carvalho Chehab5.1. Loading 29989e33ea7SMauro Carvalho Chehab------------ 30089e33ea7SMauro Carvalho Chehab 30189e33ea7SMauro Carvalho ChehabThe only reasonable way is to enable the patch when the livepatch kernel 30289e33ea7SMauro Carvalho Chehabmodule is being loaded. For this, klp_enable_patch() has to be called 30389e33ea7SMauro Carvalho Chehabin the module_init() callback. There are two main reasons: 30489e33ea7SMauro Carvalho Chehab 30589e33ea7SMauro Carvalho ChehabFirst, only the module has an easy access to the related struct klp_patch. 30689e33ea7SMauro Carvalho Chehab 30789e33ea7SMauro Carvalho ChehabSecond, the error code might be used to refuse loading the module when 30889e33ea7SMauro Carvalho Chehabthe patch cannot get enabled. 30989e33ea7SMauro Carvalho Chehab 31089e33ea7SMauro Carvalho Chehab 31189e33ea7SMauro Carvalho Chehab5.2. Enabling 31289e33ea7SMauro Carvalho Chehab------------- 31389e33ea7SMauro Carvalho Chehab 31489e33ea7SMauro Carvalho ChehabThe livepatch gets enabled by calling klp_enable_patch() from 31589e33ea7SMauro Carvalho Chehabthe module_init() callback. The system will start using the new 31689e33ea7SMauro Carvalho Chehabimplementation of the patched functions at this stage. 31789e33ea7SMauro Carvalho Chehab 31889e33ea7SMauro Carvalho ChehabFirst, the addresses of the patched functions are found according to their 31989e33ea7SMauro Carvalho Chehabnames. The special relocations, mentioned in the section "New functions", 32089e33ea7SMauro Carvalho Chehabare applied. The relevant entries are created under 32189e33ea7SMauro Carvalho Chehab/sys/kernel/livepatch/<name>. The patch is rejected when any above 32289e33ea7SMauro Carvalho Chehaboperation fails. 32389e33ea7SMauro Carvalho Chehab 32489e33ea7SMauro Carvalho ChehabSecond, livepatch enters into a transition state where tasks are converging 32589e33ea7SMauro Carvalho Chehabto the patched state. If an original function is patched for the first 32689e33ea7SMauro Carvalho Chehabtime, a function specific struct klp_ops is created and an universal 32789e33ea7SMauro Carvalho Chehabftrace handler is registered\ [#]_. This stage is indicated by a value of '1' 32889e33ea7SMauro Carvalho Chehabin /sys/kernel/livepatch/<name>/transition. For more information about 32989e33ea7SMauro Carvalho Chehabthis process, see the "Consistency model" section. 33089e33ea7SMauro Carvalho Chehab 33189e33ea7SMauro Carvalho ChehabFinally, once all tasks have been patched, the 'transition' value changes 33289e33ea7SMauro Carvalho Chehabto '0'. 33389e33ea7SMauro Carvalho Chehab 33489e33ea7SMauro Carvalho Chehab.. [#] 33589e33ea7SMauro Carvalho Chehab 33689e33ea7SMauro Carvalho Chehab Note that functions might be patched multiple times. The ftrace handler 33789e33ea7SMauro Carvalho Chehab is registered only once for a given function. Further patches just add 33889e33ea7SMauro Carvalho Chehab an entry to the list (see field `func_stack`) of the struct klp_ops. 33989e33ea7SMauro Carvalho Chehab The right implementation is selected by the ftrace handler, see 34089e33ea7SMauro Carvalho Chehab the "Consistency model" section. 34189e33ea7SMauro Carvalho Chehab 34289e33ea7SMauro Carvalho Chehab That said, it is highly recommended to use cumulative livepatches 34389e33ea7SMauro Carvalho Chehab because they help keeping the consistency of all changes. In this case, 34489e33ea7SMauro Carvalho Chehab functions might be patched two times only during the transition period. 34589e33ea7SMauro Carvalho Chehab 34689e33ea7SMauro Carvalho Chehab 34789e33ea7SMauro Carvalho Chehab5.3. Replacing 34889e33ea7SMauro Carvalho Chehab-------------- 34989e33ea7SMauro Carvalho Chehab 35089e33ea7SMauro Carvalho ChehabAll enabled patches might get replaced by a cumulative patch that 35189e33ea7SMauro Carvalho Chehabhas the .replace flag set. 35289e33ea7SMauro Carvalho Chehab 35389e33ea7SMauro Carvalho ChehabOnce the new patch is enabled and the 'transition' finishes then 35489e33ea7SMauro Carvalho Chehaball the functions (struct klp_func) associated with the replaced 35589e33ea7SMauro Carvalho Chehabpatches are removed from the corresponding struct klp_ops. Also 35689e33ea7SMauro Carvalho Chehabthe ftrace handler is unregistered and the struct klp_ops is 35789e33ea7SMauro Carvalho Chehabfreed when the related function is not modified by the new patch 35889e33ea7SMauro Carvalho Chehaband func_stack list becomes empty. 35989e33ea7SMauro Carvalho Chehab 36089e33ea7SMauro Carvalho ChehabSee Documentation/livepatch/cumulative-patches.rst for more details. 36189e33ea7SMauro Carvalho Chehab 36289e33ea7SMauro Carvalho Chehab 36389e33ea7SMauro Carvalho Chehab5.4. Disabling 36489e33ea7SMauro Carvalho Chehab-------------- 36589e33ea7SMauro Carvalho Chehab 36689e33ea7SMauro Carvalho ChehabEnabled patches might get disabled by writing '0' to 36789e33ea7SMauro Carvalho Chehab/sys/kernel/livepatch/<name>/enabled. 36889e33ea7SMauro Carvalho Chehab 36989e33ea7SMauro Carvalho ChehabFirst, livepatch enters into a transition state where tasks are converging 37089e33ea7SMauro Carvalho Chehabto the unpatched state. The system starts using either the code from 37189e33ea7SMauro Carvalho Chehabthe previously enabled patch or even the original one. This stage is 37289e33ea7SMauro Carvalho Chehabindicated by a value of '1' in /sys/kernel/livepatch/<name>/transition. 37389e33ea7SMauro Carvalho ChehabFor more information about this process, see the "Consistency model" 37489e33ea7SMauro Carvalho Chehabsection. 37589e33ea7SMauro Carvalho Chehab 37689e33ea7SMauro Carvalho ChehabSecond, once all tasks have been unpatched, the 'transition' value changes 37789e33ea7SMauro Carvalho Chehabto '0'. All the functions (struct klp_func) associated with the to-be-disabled 37889e33ea7SMauro Carvalho Chehabpatch are removed from the corresponding struct klp_ops. The ftrace handler 37989e33ea7SMauro Carvalho Chehabis unregistered and the struct klp_ops is freed when the func_stack list 38089e33ea7SMauro Carvalho Chehabbecomes empty. 38189e33ea7SMauro Carvalho Chehab 38289e33ea7SMauro Carvalho ChehabThird, the sysfs interface is destroyed. 38389e33ea7SMauro Carvalho Chehab 38489e33ea7SMauro Carvalho Chehab 38589e33ea7SMauro Carvalho Chehab5.5. Removing 38689e33ea7SMauro Carvalho Chehab------------- 38789e33ea7SMauro Carvalho Chehab 38889e33ea7SMauro Carvalho ChehabModule removal is only safe when there are no users of functions provided 38989e33ea7SMauro Carvalho Chehabby the module. This is the reason why the force feature permanently 39089e33ea7SMauro Carvalho Chehabdisables the removal. Only when the system is successfully transitioned 39189e33ea7SMauro Carvalho Chehabto a new patch state (patched/unpatched) without being forced it is 39289e33ea7SMauro Carvalho Chehabguaranteed that no task sleeps or runs in the old code. 39389e33ea7SMauro Carvalho Chehab 39489e33ea7SMauro Carvalho Chehab 39589e33ea7SMauro Carvalho Chehab6. Sysfs 39689e33ea7SMauro Carvalho Chehab======== 39789e33ea7SMauro Carvalho Chehab 39889e33ea7SMauro Carvalho ChehabInformation about the registered patches can be found under 39989e33ea7SMauro Carvalho Chehab/sys/kernel/livepatch. The patches could be enabled and disabled 40089e33ea7SMauro Carvalho Chehabby writing there. 40189e33ea7SMauro Carvalho Chehab 40289e33ea7SMauro Carvalho Chehab/sys/kernel/livepatch/<patch>/force attributes allow administrator to affect a 40389e33ea7SMauro Carvalho Chehabpatching operation. 40489e33ea7SMauro Carvalho Chehab 40589e33ea7SMauro Carvalho ChehabSee Documentation/ABI/testing/sysfs-kernel-livepatch for more details. 40689e33ea7SMauro Carvalho Chehab 40789e33ea7SMauro Carvalho Chehab 40889e33ea7SMauro Carvalho Chehab7. Limitations 40989e33ea7SMauro Carvalho Chehab============== 41089e33ea7SMauro Carvalho Chehab 41189e33ea7SMauro Carvalho ChehabThe current Livepatch implementation has several limitations: 41289e33ea7SMauro Carvalho Chehab 41389e33ea7SMauro Carvalho Chehab - Only functions that can be traced could be patched. 41489e33ea7SMauro Carvalho Chehab 41589e33ea7SMauro Carvalho Chehab Livepatch is based on the dynamic ftrace. In particular, functions 41689e33ea7SMauro Carvalho Chehab implementing ftrace or the livepatch ftrace handler could not be 41789e33ea7SMauro Carvalho Chehab patched. Otherwise, the code would end up in an infinite loop. A 41889e33ea7SMauro Carvalho Chehab potential mistake is prevented by marking the problematic functions 41989e33ea7SMauro Carvalho Chehab by "notrace". 42089e33ea7SMauro Carvalho Chehab 42189e33ea7SMauro Carvalho Chehab 42289e33ea7SMauro Carvalho Chehab 42389e33ea7SMauro Carvalho Chehab - Livepatch works reliably only when the dynamic ftrace is located at 42489e33ea7SMauro Carvalho Chehab the very beginning of the function. 42589e33ea7SMauro Carvalho Chehab 42689e33ea7SMauro Carvalho Chehab The function need to be redirected before the stack or the function 42789e33ea7SMauro Carvalho Chehab parameters are modified in any way. For example, livepatch requires 42889e33ea7SMauro Carvalho Chehab using -fentry gcc compiler option on x86_64. 42989e33ea7SMauro Carvalho Chehab 43089e33ea7SMauro Carvalho Chehab One exception is the PPC port. It uses relative addressing and TOC. 43189e33ea7SMauro Carvalho Chehab Each function has to handle TOC and save LR before it could call 43289e33ea7SMauro Carvalho Chehab the ftrace handler. This operation has to be reverted on return. 43389e33ea7SMauro Carvalho Chehab Fortunately, the generic ftrace code has the same problem and all 43489e33ea7SMauro Carvalho Chehab this is handled on the ftrace level. 43589e33ea7SMauro Carvalho Chehab 43689e33ea7SMauro Carvalho Chehab 43789e33ea7SMauro Carvalho Chehab - Kretprobes using the ftrace framework conflict with the patched 43889e33ea7SMauro Carvalho Chehab functions. 43989e33ea7SMauro Carvalho Chehab 44089e33ea7SMauro Carvalho Chehab Both kretprobes and livepatches use a ftrace handler that modifies 44189e33ea7SMauro Carvalho Chehab the return address. The first user wins. Either the probe or the patch 44289e33ea7SMauro Carvalho Chehab is rejected when the handler is already in use by the other. 44389e33ea7SMauro Carvalho Chehab 44489e33ea7SMauro Carvalho Chehab 44589e33ea7SMauro Carvalho Chehab - Kprobes in the original function are ignored when the code is 44689e33ea7SMauro Carvalho Chehab redirected to the new implementation. 44789e33ea7SMauro Carvalho Chehab 44889e33ea7SMauro Carvalho Chehab There is a work in progress to add warnings about this situation. 449