1
2=============
3eBPF verifier
4=============
5
6The safety of the eBPF program is determined in two steps.
7
8First step does DAG check to disallow loops and other CFG validation.
9In particular it will detect programs that have unreachable instructions.
10(though classic BPF checker allows them)
11
12Second step starts from the first insn and descends all possible paths.
13It simulates execution of every insn and observes the state change of
14registers and stack.
15
16At the start of the program the register R1 contains a pointer to context
17and has type PTR_TO_CTX.
18If verifier sees an insn that does R2=R1, then R2 has now type
19PTR_TO_CTX as well and can be used on the right hand side of expression.
20If R1=PTR_TO_CTX and insn is R2=R1+R1, then R2=SCALAR_VALUE,
21since addition of two valid pointers makes invalid pointer.
22(In 'secure' mode verifier will reject any type of pointer arithmetic to make
23sure that kernel addresses don't leak to unprivileged users)
24
25If register was never written to, it's not readable::
26
27  bpf_mov R0 = R2
28  bpf_exit
29
30will be rejected, since R2 is unreadable at the start of the program.
31
32After kernel function call, R1-R5 are reset to unreadable and
33R0 has a return type of the function.
34
35Since R6-R9 are callee saved, their state is preserved across the call.
36
37::
38
39  bpf_mov R6 = 1
40  bpf_call foo
41  bpf_mov R0 = R6
42  bpf_exit
43
44is a correct program. If there was R1 instead of R6, it would have
45been rejected.
46
47load/store instructions are allowed only with registers of valid types, which
48are PTR_TO_CTX, PTR_TO_MAP, PTR_TO_STACK. They are bounds and alignment checked.
49For example::
50
51 bpf_mov R1 = 1
52 bpf_mov R2 = 2
53 bpf_xadd *(u32 *)(R1 + 3) += R2
54 bpf_exit
55
56will be rejected, since R1 doesn't have a valid pointer type at the time of
57execution of instruction bpf_xadd.
58
59At the start R1 type is PTR_TO_CTX (a pointer to generic ``struct bpf_context``)
60A callback is used to customize verifier to restrict eBPF program access to only
61certain fields within ctx structure with specified size and alignment.
62
63For example, the following insn::
64
65  bpf_ld R0 = *(u32 *)(R6 + 8)
66
67intends to load a word from address R6 + 8 and store it into R0
68If R6=PTR_TO_CTX, via is_valid_access() callback the verifier will know
69that offset 8 of size 4 bytes can be accessed for reading, otherwise
70the verifier will reject the program.
71If R6=PTR_TO_STACK, then access should be aligned and be within
72stack bounds, which are [-MAX_BPF_STACK, 0). In this example offset is 8,
73so it will fail verification, since it's out of bounds.
74
75The verifier will allow eBPF program to read data from stack only after
76it wrote into it.
77
78Classic BPF verifier does similar check with M[0-15] memory slots.
79For example::
80
81  bpf_ld R0 = *(u32 *)(R10 - 4)
82  bpf_exit
83
84is invalid program.
85Though R10 is correct read-only register and has type PTR_TO_STACK
86and R10 - 4 is within stack bounds, there were no stores into that location.
87
88Pointer register spill/fill is tracked as well, since four (R6-R9)
89callee saved registers may not be enough for some programs.
90
91Allowed function calls are customized with bpf_verifier_ops->get_func_proto()
92The eBPF verifier will check that registers match argument constraints.
93After the call register R0 will be set to return type of the function.
94
95Function calls is a main mechanism to extend functionality of eBPF programs.
96Socket filters may let programs to call one set of functions, whereas tracing
97filters may allow completely different set.
98
99If a function made accessible to eBPF program, it needs to be thought through
100from safety point of view. The verifier will guarantee that the function is
101called with valid arguments.
102
103seccomp vs socket filters have different security restrictions for classic BPF.
104Seccomp solves this by two stage verifier: classic BPF verifier is followed
105by seccomp verifier. In case of eBPF one configurable verifier is shared for
106all use cases.
107
108See details of eBPF verifier in kernel/bpf/verifier.c
109
110Register value tracking
111=======================
112
113In order to determine the safety of an eBPF program, the verifier must track
114the range of possible values in each register and also in each stack slot.
115This is done with ``struct bpf_reg_state``, defined in include/linux/
116bpf_verifier.h, which unifies tracking of scalar and pointer values.  Each
117register state has a type, which is either NOT_INIT (the register has not been
118written to), SCALAR_VALUE (some value which is not usable as a pointer), or a
119pointer type.  The types of pointers describe their base, as follows:
120
121
122    PTR_TO_CTX
123			Pointer to bpf_context.
124    CONST_PTR_TO_MAP
125			Pointer to struct bpf_map.  "Const" because arithmetic
126			on these pointers is forbidden.
127    PTR_TO_MAP_VALUE
128			Pointer to the value stored in a map element.
129    PTR_TO_MAP_VALUE_OR_NULL
130			Either a pointer to a map value, or NULL; map accesses
131			(see maps.rst) return this type, which becomes a
132			PTR_TO_MAP_VALUE when checked != NULL. Arithmetic on
133			these pointers is forbidden.
134    PTR_TO_STACK
135			Frame pointer.
136    PTR_TO_PACKET
137			skb->data.
138    PTR_TO_PACKET_END
139			skb->data + headlen; arithmetic forbidden.
140    PTR_TO_SOCKET
141			Pointer to struct bpf_sock_ops, implicitly refcounted.
142    PTR_TO_SOCKET_OR_NULL
143			Either a pointer to a socket, or NULL; socket lookup
144			returns this type, which becomes a PTR_TO_SOCKET when
145			checked != NULL. PTR_TO_SOCKET is reference-counted,
146			so programs must release the reference through the
147			socket release function before the end of the program.
148			Arithmetic on these pointers is forbidden.
149
150However, a pointer may be offset from this base (as a result of pointer
151arithmetic), and this is tracked in two parts: the 'fixed offset' and 'variable
152offset'.  The former is used when an exactly-known value (e.g. an immediate
153operand) is added to a pointer, while the latter is used for values which are
154not exactly known.  The variable offset is also used in SCALAR_VALUEs, to track
155the range of possible values in the register.
156
157The verifier's knowledge about the variable offset consists of:
158
159* minimum and maximum values as unsigned
160* minimum and maximum values as signed
161
162* knowledge of the values of individual bits, in the form of a 'tnum': a u64
163  'mask' and a u64 'value'.  1s in the mask represent bits whose value is unknown;
164  1s in the value represent bits known to be 1.  Bits known to be 0 have 0 in both
165  mask and value; no bit should ever be 1 in both.  For example, if a byte is read
166  into a register from memory, the register's top 56 bits are known zero, while
167  the low 8 are unknown - which is represented as the tnum (0x0; 0xff).  If we
168  then OR this with 0x40, we get (0x40; 0xbf), then if we add 1 we get (0x0;
169  0x1ff), because of potential carries.
170
171Besides arithmetic, the register state can also be updated by conditional
172branches.  For instance, if a SCALAR_VALUE is compared > 8, in the 'true' branch
173it will have a umin_value (unsigned minimum value) of 9, whereas in the 'false'
174branch it will have a umax_value of 8.  A signed compare (with BPF_JSGT or
175BPF_JSGE) would instead update the signed minimum/maximum values.  Information
176from the signed and unsigned bounds can be combined; for instance if a value is
177first tested < 8 and then tested s> 4, the verifier will conclude that the value
178is also > 4 and s< 8, since the bounds prevent crossing the sign boundary.
179
180PTR_TO_PACKETs with a variable offset part have an 'id', which is common to all
181pointers sharing that same variable offset.  This is important for packet range
182checks: after adding a variable to a packet pointer register A, if you then copy
183it to another register B and then add a constant 4 to A, both registers will
184share the same 'id' but the A will have a fixed offset of +4.  Then if A is
185bounds-checked and found to be less than a PTR_TO_PACKET_END, the register B is
186now known to have a safe range of at least 4 bytes.  See 'Direct packet access',
187below, for more on PTR_TO_PACKET ranges.
188
189The 'id' field is also used on PTR_TO_MAP_VALUE_OR_NULL, common to all copies of
190the pointer returned from a map lookup.  This means that when one copy is
191checked and found to be non-NULL, all copies can become PTR_TO_MAP_VALUEs.
192As well as range-checking, the tracked information is also used for enforcing
193alignment of pointer accesses.  For instance, on most systems the packet pointer
194is 2 bytes after a 4-byte alignment.  If a program adds 14 bytes to that to jump
195over the Ethernet header, then reads IHL and addes (IHL * 4), the resulting
196pointer will have a variable offset known to be 4n+2 for some n, so adding the 2
197bytes (NET_IP_ALIGN) gives a 4-byte alignment and so word-sized accesses through
198that pointer are safe.
199The 'id' field is also used on PTR_TO_SOCKET and PTR_TO_SOCKET_OR_NULL, common
200to all copies of the pointer returned from a socket lookup. This has similar
201behaviour to the handling for PTR_TO_MAP_VALUE_OR_NULL->PTR_TO_MAP_VALUE, but
202it also handles reference tracking for the pointer. PTR_TO_SOCKET implicitly
203represents a reference to the corresponding ``struct sock``. To ensure that the
204reference is not leaked, it is imperative to NULL-check the reference and in
205the non-NULL case, and pass the valid reference to the socket release function.
206
207Direct packet access
208====================
209
210In cls_bpf and act_bpf programs the verifier allows direct access to the packet
211data via skb->data and skb->data_end pointers.
212Ex::
213
214    1:  r4 = *(u32 *)(r1 +80)  /* load skb->data_end */
215    2:  r3 = *(u32 *)(r1 +76)  /* load skb->data */
216    3:  r5 = r3
217    4:  r5 += 14
218    5:  if r5 > r4 goto pc+16
219    R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp
220    6:  r0 = *(u16 *)(r3 +12) /* access 12 and 13 bytes of the packet */
221
222this 2byte load from the packet is safe to do, since the program author
223did check ``if (skb->data + 14 > skb->data_end) goto err`` at insn #5 which
224means that in the fall-through case the register R3 (which points to skb->data)
225has at least 14 directly accessible bytes. The verifier marks it
226as R3=pkt(id=0,off=0,r=14).
227id=0 means that no additional variables were added to the register.
228off=0 means that no additional constants were added.
229r=14 is the range of safe access which means that bytes [R3, R3 + 14) are ok.
230Note that R5 is marked as R5=pkt(id=0,off=14,r=14). It also points
231to the packet data, but constant 14 was added to the register, so
232it now points to ``skb->data + 14`` and accessible range is [R5, R5 + 14 - 14)
233which is zero bytes.
234
235More complex packet access may look like::
236
237
238    R0=inv1 R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp
239    6:  r0 = *(u8 *)(r3 +7) /* load 7th byte from the packet */
240    7:  r4 = *(u8 *)(r3 +12)
241    8:  r4 *= 14
242    9:  r3 = *(u32 *)(r1 +76) /* load skb->data */
243    10:  r3 += r4
244    11:  r2 = r1
245    12:  r2 <<= 48
246    13:  r2 >>= 48
247    14:  r3 += r2
248    15:  r2 = r3
249    16:  r2 += 8
250    17:  r1 = *(u32 *)(r1 +80) /* load skb->data_end */
251    18:  if r2 > r1 goto pc+2
252    R0=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R1=pkt_end R2=pkt(id=2,off=8,r=8) R3=pkt(id=2,off=0,r=8) R4=inv(id=0,umax_value=3570,var_off=(0x0; 0xfffe)) R5=pkt(id=0,off=14,r=14) R10=fp
253    19:  r1 = *(u8 *)(r3 +4)
254
255The state of the register R3 is R3=pkt(id=2,off=0,r=8)
256id=2 means that two ``r3 += rX`` instructions were seen, so r3 points to some
257offset within a packet and since the program author did
258``if (r3 + 8 > r1) goto err`` at insn #18, the safe range is [R3, R3 + 8).
259The verifier only allows 'add'/'sub' operations on packet registers. Any other
260operation will set the register state to 'SCALAR_VALUE' and it won't be
261available for direct packet access.
262
263Operation ``r3 += rX`` may overflow and become less than original skb->data,
264therefore the verifier has to prevent that.  So when it sees ``r3 += rX``
265instruction and rX is more than 16-bit value, any subsequent bounds-check of r3
266against skb->data_end will not give us 'range' information, so attempts to read
267through the pointer will give "invalid access to packet" error.
268
269Ex. after insn ``r4 = *(u8 *)(r3 +12)`` (insn #7 above) the state of r4 is
270R4=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) which means that upper 56 bits
271of the register are guaranteed to be zero, and nothing is known about the lower
2728 bits. After insn ``r4 *= 14`` the state becomes
273R4=inv(id=0,umax_value=3570,var_off=(0x0; 0xfffe)), since multiplying an 8-bit
274value by constant 14 will keep upper 52 bits as zero, also the least significant
275bit will be zero as 14 is even.  Similarly ``r2 >>= 48`` will make
276R2=inv(id=0,umax_value=65535,var_off=(0x0; 0xffff)), since the shift is not sign
277extending.  This logic is implemented in adjust_reg_min_max_vals() function,
278which calls adjust_ptr_min_max_vals() for adding pointer to scalar (or vice
279versa) and adjust_scalar_min_max_vals() for operations on two scalars.
280
281The end result is that bpf program author can access packet directly
282using normal C code as::
283
284  void *data = (void *)(long)skb->data;
285  void *data_end = (void *)(long)skb->data_end;
286  struct eth_hdr *eth = data;
287  struct iphdr *iph = data + sizeof(*eth);
288  struct udphdr *udp = data + sizeof(*eth) + sizeof(*iph);
289
290  if (data + sizeof(*eth) + sizeof(*iph) + sizeof(*udp) > data_end)
291	  return 0;
292  if (eth->h_proto != htons(ETH_P_IP))
293	  return 0;
294  if (iph->protocol != IPPROTO_UDP || iph->ihl != 5)
295	  return 0;
296  if (udp->dest == 53 || udp->source == 9)
297	  ...;
298
299which makes such programs easier to write comparing to LD_ABS insn
300and significantly faster.
301
302Pruning
303=======
304
305The verifier does not actually walk all possible paths through the program.  For
306each new branch to analyse, the verifier looks at all the states it's previously
307been in when at this instruction.  If any of them contain the current state as a
308subset, the branch is 'pruned' - that is, the fact that the previous state was
309accepted implies the current state would be as well.  For instance, if in the
310previous state, r1 held a packet-pointer, and in the current state, r1 holds a
311packet-pointer with a range as long or longer and at least as strict an
312alignment, then r1 is safe.  Similarly, if r2 was NOT_INIT before then it can't
313have been used by any path from that point, so any value in r2 (including
314another NOT_INIT) is safe.  The implementation is in the function regsafe().
315Pruning considers not only the registers but also the stack (and any spilled
316registers it may hold).  They must all be safe for the branch to be pruned.
317This is implemented in states_equal().
318
319Understanding eBPF verifier messages
320====================================
321
322The following are few examples of invalid eBPF programs and verifier error
323messages as seen in the log:
324
325Program with unreachable instructions::
326
327  static struct bpf_insn prog[] = {
328  BPF_EXIT_INSN(),
329  BPF_EXIT_INSN(),
330  };
331
332Error::
333
334  unreachable insn 1
335
336Program that reads uninitialized register::
337
338  BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),
339  BPF_EXIT_INSN(),
340
341Error::
342
343  0: (bf) r0 = r2
344  R2 !read_ok
345
346Program that doesn't initialize R0 before exiting::
347
348  BPF_MOV64_REG(BPF_REG_2, BPF_REG_1),
349  BPF_EXIT_INSN(),
350
351Error::
352
353  0: (bf) r2 = r1
354  1: (95) exit
355  R0 !read_ok
356
357Program that accesses stack out of bounds::
358
359    BPF_ST_MEM(BPF_DW, BPF_REG_10, 8, 0),
360    BPF_EXIT_INSN(),
361
362Error::
363
364    0: (7a) *(u64 *)(r10 +8) = 0
365    invalid stack off=8 size=8
366
367Program that doesn't initialize stack before passing its address into function::
368
369  BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
370  BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
371  BPF_LD_MAP_FD(BPF_REG_1, 0),
372  BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
373  BPF_EXIT_INSN(),
374
375Error::
376
377  0: (bf) r2 = r10
378  1: (07) r2 += -8
379  2: (b7) r1 = 0x0
380  3: (85) call 1
381  invalid indirect read from stack off -8+0 size 8
382
383Program that uses invalid map_fd=0 while calling to map_lookup_elem() function::
384
385  BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
386  BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
387  BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
388  BPF_LD_MAP_FD(BPF_REG_1, 0),
389  BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
390  BPF_EXIT_INSN(),
391
392Error::
393
394  0: (7a) *(u64 *)(r10 -8) = 0
395  1: (bf) r2 = r10
396  2: (07) r2 += -8
397  3: (b7) r1 = 0x0
398  4: (85) call 1
399  fd 0 is not pointing to valid bpf_map
400
401Program that doesn't check return value of map_lookup_elem() before accessing
402map element::
403
404  BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
405  BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
406  BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
407  BPF_LD_MAP_FD(BPF_REG_1, 0),
408  BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
409  BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0),
410  BPF_EXIT_INSN(),
411
412Error::
413
414  0: (7a) *(u64 *)(r10 -8) = 0
415  1: (bf) r2 = r10
416  2: (07) r2 += -8
417  3: (b7) r1 = 0x0
418  4: (85) call 1
419  5: (7a) *(u64 *)(r0 +0) = 0
420  R0 invalid mem access 'map_value_or_null'
421
422Program that correctly checks map_lookup_elem() returned value for NULL, but
423accesses the memory with incorrect alignment::
424
425  BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
426  BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
427  BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
428  BPF_LD_MAP_FD(BPF_REG_1, 0),
429  BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
430  BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 1),
431  BPF_ST_MEM(BPF_DW, BPF_REG_0, 4, 0),
432  BPF_EXIT_INSN(),
433
434Error::
435
436  0: (7a) *(u64 *)(r10 -8) = 0
437  1: (bf) r2 = r10
438  2: (07) r2 += -8
439  3: (b7) r1 = 1
440  4: (85) call 1
441  5: (15) if r0 == 0x0 goto pc+1
442   R0=map_ptr R10=fp
443  6: (7a) *(u64 *)(r0 +4) = 0
444  misaligned access off 4 size 8
445
446Program that correctly checks map_lookup_elem() returned value for NULL and
447accesses memory with correct alignment in one side of 'if' branch, but fails
448to do so in the other side of 'if' branch::
449
450  BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
451  BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
452  BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
453  BPF_LD_MAP_FD(BPF_REG_1, 0),
454  BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
455  BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 2),
456  BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0),
457  BPF_EXIT_INSN(),
458  BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 1),
459  BPF_EXIT_INSN(),
460
461Error::
462
463  0: (7a) *(u64 *)(r10 -8) = 0
464  1: (bf) r2 = r10
465  2: (07) r2 += -8
466  3: (b7) r1 = 1
467  4: (85) call 1
468  5: (15) if r0 == 0x0 goto pc+2
469   R0=map_ptr R10=fp
470  6: (7a) *(u64 *)(r0 +0) = 0
471  7: (95) exit
472
473  from 5 to 8: R0=imm0 R10=fp
474  8: (7a) *(u64 *)(r0 +0) = 1
475  R0 invalid mem access 'imm'
476
477Program that performs a socket lookup then sets the pointer to NULL without
478checking it::
479
480  BPF_MOV64_IMM(BPF_REG_2, 0),
481  BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_2, -8),
482  BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
483  BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
484  BPF_MOV64_IMM(BPF_REG_3, 4),
485  BPF_MOV64_IMM(BPF_REG_4, 0),
486  BPF_MOV64_IMM(BPF_REG_5, 0),
487  BPF_EMIT_CALL(BPF_FUNC_sk_lookup_tcp),
488  BPF_MOV64_IMM(BPF_REG_0, 0),
489  BPF_EXIT_INSN(),
490
491Error::
492
493  0: (b7) r2 = 0
494  1: (63) *(u32 *)(r10 -8) = r2
495  2: (bf) r2 = r10
496  3: (07) r2 += -8
497  4: (b7) r3 = 4
498  5: (b7) r4 = 0
499  6: (b7) r5 = 0
500  7: (85) call bpf_sk_lookup_tcp#65
501  8: (b7) r0 = 0
502  9: (95) exit
503  Unreleased reference id=1, alloc_insn=7
504
505Program that performs a socket lookup but does not NULL-check the returned
506value::
507
508  BPF_MOV64_IMM(BPF_REG_2, 0),
509  BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_2, -8),
510  BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
511  BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
512  BPF_MOV64_IMM(BPF_REG_3, 4),
513  BPF_MOV64_IMM(BPF_REG_4, 0),
514  BPF_MOV64_IMM(BPF_REG_5, 0),
515  BPF_EMIT_CALL(BPF_FUNC_sk_lookup_tcp),
516  BPF_EXIT_INSN(),
517
518Error::
519
520  0: (b7) r2 = 0
521  1: (63) *(u32 *)(r10 -8) = r2
522  2: (bf) r2 = r10
523  3: (07) r2 += -8
524  4: (b7) r3 = 4
525  5: (b7) r4 = 0
526  6: (b7) r5 = 0
527  7: (85) call bpf_sk_lookup_tcp#65
528  8: (95) exit
529  Unreleased reference id=1, alloc_insn=7
530