From: Eduard Zingerman eddyz87@gmail.com
Current infinite loops detection mechanism is speculative: - first, states_maybe_looping() check is done which simply does memcmp for R1-R10 in current frame; - second, states_equal(..., exact=false) is called. With exact=false states_equal() would compare scalars for equality only if in old state scalar has precision mark.
Such logic might be problematic if compiler makes some unlucky stack spill/fill decisions. An artificial example of a false positive looks as follows:
r0 = ... unknown scalar ... r0 &= 0xff; *(u64 *)(r10 - 8) = r0; r0 = 0; loop: r0 = *(u64 *)(r10 - 8); if r0 > 10 goto exit_; r0 += 1; *(u64 *)(r10 - 8) = r0; r0 = 0; goto loop;
This commit updates call to states_equal to use exact=true, forcing all scalar comparisons to be exact.
Signed-off-by: Eduard Zingerman eddyz87@gmail.com --- kernel/bpf/verifier.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index f13008d27f35..89f8c527ed3c 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -17008,7 +17008,7 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx) } /* attempt to detect infinite loop to avoid unnecessary doomed work */ if (states_maybe_looping(&sl->state, cur) && - states_equal(env, &sl->state, cur, false) && + states_equal(env, &sl->state, cur, true) && !iter_active_depths_differ(&sl->state, cur) && sl->state.callback_unroll_depth == cur->callback_unroll_depth) { verbose_linfo(env, insn_idx, "; ");