``` commit 16cbc4520eb13d2065d145c76b97fb93035d81e8 Author: KaFai Wan kafai.wan@linux.dev
bpf: Fix tnum_overlap to check for zero mask intersection
This patch adds an early return check in tnum_overlap() to detect non-overlapping tnums when their masks have no overlapping bits. The fix addresses a syzbot report where the verifier failed to recognize that two tnums couldn't represent the same value.
Reported-by: syzbot+c950cc277150935cc0b5@syzkaller.appspotmail.com
diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c index f8e70e9c3..1a75b7c9a 100644 --- a/kernel/bpf/tnum.c +++ b/kernel/bpf/tnum.c @@ -163,6 +163,8 @@ bool tnum_overlap(struct tnum a, struct tnum b) { u64 mu;
- if (a.mask && b.mask && !(a.mask & b.mask))
return false;
^^^^^
Does this logic correctly detect non-overlapping tnums? Consider this counter-example:
a = (value=0x5, mask=0xA) // binary: value=0101, mask=1010 b = (value=0xA, mask=0x5) // binary: value=1010, mask=0101
The masks are disjoint: (0xA & 0x5) == 0, so the new check returns false (no overlap).
However, these tnums actually represent overlapping sets: a represents: {5, 7, 13, 15} // 0b0101, 0b0111, 0b1101, 0b1111 b represents: {10, 11, 14, 15} // 0b1010, 0b1011, 0b1110, 0b1111
Both sets contain 15, so they do overlap. Can this cause incorrect verifier behavior when is_branch_taken() calls tnum_overlap() to determine if JEQ/JNE branches are reachable?
mu = ~a.mask & ~b.mask; return (a.value & mu) == (b.value & mu); }
```
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In-Reply-To-Subject: `bpf: Fix tnum_overlap to check for zero mask intersection` CI run summary: https://github.com/kernel-patches/bpf/actions/runs/18880108453