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bpf: account for current allocated stack depth in widen_imprecise_scalars()

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The usage pattern for widen_imprecise_scalars() looks as follows:

    prev_st = find_prev_entry(env, ...);
    queued_st = push_stack(...);
    widen_imprecise_scalars(env, prev_st, queued_st);

Where prev_st is an ancestor of the queued_st in the explored states
tree. This ancestor is not guaranteed to have same allocated stack
depth as queued_st. E.g. in the following case:

    def main():
      for i in 1..2:
        foo(i)        // same callsite, differnt param

    def foo(i):
      if i == 1:
        use 128 bytes of stack
      iterator based loop

Here, for a second 'foo' call prev_st->allocated_stack is 128,
while queued_st->allocated_stack is much smaller.
widen_imprecise_scalars() needs to take this into account and avoid
accessing bpf_verifier_state->frame[*]->stack out of bounds.

Fixes: 2793a8b015 ("bpf: exact states comparison for iterator convergence checks")
Reported-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251114025730.772723-1-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Eduard Zingerman
2025-11-13 18:57:29 -08:00
committed by Alexei Starovoitov
parent 4ef9274362
commit b0c8e6d3d8
1 changed files with 4 additions and 2 deletions
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6
kernel/bpf/verifier.c
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@@ -8866,7 +8866,7 @@ static int widen_imprecise_scalars(struct bpf_verifier_env *env,
struct bpf_verifier_state *cur)
{
struct bpf_func_state *fold, *fcur;
int i, fr;
int i, fr, num_slots;
reset_idmap_scratch(env);
for (fr = old->curframe; fr >= 0; fr--) {
@@ -8879,7 +8879,9 @@ static int widen_imprecise_scalars(struct bpf_verifier_env *env,
&fcur->regs[i],
&env->idmap_scratch);
for (i = 0; i < fold->allocated_stack / BPF_REG_SIZE; i++) {
num_slots = min(fold->allocated_stack / BPF_REG_SIZE,
fcur->allocated_stack / BPF_REG_SIZE);
for (i = 0; i < num_slots; i++) {
if (!is_spilled_reg(&fold->stack[i]) ||
!is_spilled_reg(&fcur->stack[i]))
continue;