From: Eduard Zingerman <eddyz87@xxxxxxxxx>
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@xxxxxxxxx>
---
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, "; ");
--
2.42.1
