bpf: keep track of max number of bpf_loop callback iterations

commit bb124da69c47dd98d69361ec13244ece50bec63e upstream.

In some cases verifier can't infer convergence of the bpf_loop()
iteration. E

bpf: keep track of max number of bpf_loop callback iterations

commit bb124da69c47dd98d69361ec13244ece50bec63e upstream.

In some cases verifier can't infer convergence of the bpf_loop()
iteration. E.g. for the following program:

static int cb(__u32 idx, struct num_context* ctx)
{
ctx->i++;
return 0;
}

SEC("?raw_tp")
int prog(void *_)
{
struct num_context ctx = { .i = 0 };
__u8 choice_arr[2] = { 0, 1 };

bpf_loop(2, cb, &ctx, 0);
return choice_arr[ctx.i];
}

Each 'cb' simulation would eventually return to 'prog' and reach
'return choice_arr[ctx.i]' statement. At which point ctx.i would be
marked precise, thus forcing verifier to track multitude of separate
states with {.i=0}, {.i=1}, ... at bpf_loop() callback entry.

This commit allows "brute force" handling for such cases by limiting
number of callback body simulations using 'umax' value of the first
bpf_loop() parameter.

For this, extend bpf_func_state with 'callback_depth' field.
Increment this field when callback visiting state is pushed to states
traversal stack. For frame #N it's 'callback_depth' field counts how
many times callback with frame depth N+1 had been executed.
Use bpf_func_state specifically to allow independent tracking of
callback depths when multiple nested bpf_loop() calls are present.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20231121020701.26440-11-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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bpf: verify callbacks as if they are called unknown number of times

commit ab5cfac139ab8576fb54630d4cca23c3e690ee90 upstream.

Prior to this patch callbacks were handled as regular function calls,
e

bpf: verify callbacks as if they are called unknown number of times

commit ab5cfac139ab8576fb54630d4cca23c3e690ee90 upstream.

Prior to this patch callbacks were handled as regular function calls,
execution of callback body was modeled exactly once.
This patch updates callbacks handling logic as follows:
- introduces a function push_callback_call() that schedules callback
body verification in env->head stack;
- updates prepare_func_exit() to reschedule callback body verification
upon BPF_EXIT;
- as calls to bpf_*_iter_next(), calls to callback invoking functions
are marked as checkpoints;
- is_state_visited() is updated to stop callback based iteration when
some identical parent state is found.

Paths with callback function invoked zero times are now verified first,
which leads to necessity to modify some selftests:
- the following negative tests required adding release/unlock/drop
calls to avoid previously masked unrelated error reports:
- cb_refs.c:underflow_prog
- exceptions_fail.c:reject_rbtree_add_throw
- exceptions_fail.c:reject_with_cp_reference
- the following precision tracking selftests needed change in expected
log trace:
- verifier_subprog_precision.c:callback_result_precise
(note: r0 precision is no longer propagated inside callback and
I think this is a correct behavior)
- verifier_subprog_precision.c:parent_callee_saved_reg_precise_with_callback
- verifier_subprog_precision.c:parent_stack_slot_precise_with_callback

Reported-by: Andrew Werner <awerner32@gmail.com>
Closes: https://lore.kernel.org/bpf/CA+vRuzPChFNXmouzGG+wsy=6eMcfr1mFG0F3g7rbg-sedGKW3w@mail.gmail.com/
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20231121020701.26440-7-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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selftests/bpf: add precision propagation tests in the presence of subprogs

Add a bunch of tests validating verifier's precision backpropagation
logic in the presence of subprog calls and/or callback

selftests/bpf: add precision propagation tests in the presence of subprogs

Add a bunch of tests validating verifier's precision backpropagation
logic in the presence of subprog calls and/or callback-calling
helpers/kfuncs.

We validate the following conditions:
- subprog_result_precise: static subprog r0 result precision handling;
- global_subprog_result_precise: global subprog r0 precision
shortcutting, similar to BPF helper handling;
- callback_result_precise: similarly r0 marking precise for
callback-calling helpers;
- parent_callee_saved_reg_precise, parent_callee_saved_reg_precise_global:
propagation of precision for callee-saved registers bypassing
static/global subprogs;
- parent_callee_saved_reg_precise_with_callback: same as above, but in
the presence of callback-calling helper;
- parent_stack_slot_precise, parent_stack_slot_precise_global:
similar to above, but instead propagating precision of stack slot
(spilled SCALAR reg);
- parent_stack_slot_precise_with_callback: same as above, but in the
presence of callback-calling helper;
- subprog_arg_precise: propagation of precision of static subprog's
input argument back to caller;
- subprog_spill_into_parent_stack_slot_precise: negative test
validating that verifier currently can't support backtracking of stack
access with non-r10 register, we validate that we fallback to
forcing precision for all SCALARs.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230505043317.3629845-10-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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