bpf: Fix tnum_overlap to check for zero mask first #10103
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Syzbot reported a kernel warning due to a range invariant violation in the BPF verifier. The issue occurs when tnum_overlap() fails to detect that two tnums don't have any overlapping bits. The problematic BPF program: 0: call bpf_get_prandom_u32 1: r6 = r0 2: r6 &= 0xFFFFFFFFFFFFFFF0 3: r7 = r0 4: r7 &= 0x07 5: r7 -= 0xFF 6: if r6 == r7 goto <exit> After instruction 5, R7 has the range: R7: u64=[0xffffffffffffff01, 0xffffffffffffff08] var_off=(0xffffffffffffff00; 0xf) R6 and R7 don't overlap since they have no agreeing bits. However, is_branch_taken() fails to recognize this, causing the verifier to refine register bounds and end up with inconsistent bounds: 6: if r6 == r7 goto <exit> R6: u64=[0xffffffffffffff01, 0xffffffffffffff00] var_off=(0xffffffffffffff00, 0x0) R7: u64=[0xffffffffffffff01, 0xffffffffffffff00] var_off=(0xffffffffffffff00, 0x0) The root cause is that tnum_overlap() doesn't properly handle the case where the masks have no overlapping bits. Fix this by adding an early check for zero mask intersection in tnum_overlap(). Reported-by: [email protected] Fixes: f41345f ("bpf: Use tnums for JEQ/JNE is_branch_taken logic") Signed-off-by: KaFai Wan <[email protected]>
This patch adds coverage for the warning detected by syzkaller and fixed in the previous patch. Without the previous patch, this test fails with: verifier bug: REG INVARIANTS VIOLATION (true_reg1): range bounds violation u64=[0xffffffffffffff01, 0xffffffffffffff00] s64=[0xffffffffffffff01, 0xffffffffffffff00] u32=[0xffffff01, 0xffffff00] s32=[0xffffff00, 0xffffff00] var_off=(0xffffffffffffff00, 0x0) verifier bug: REG INVARIANTS VIOLATION (true_reg2): range bounds violation u64=[0xffffffffffffff01, 0xffffffffffffff00] s64=[0xffffffffffffff01, 0xffffffffffffff00] u32=[0xffffff01, 0xffffff00] s32=[0xffffff01, 0xffffff00] var_off=(0xffffffffffffff00, 0x0) Signed-off-by: KaFai Wan <[email protected]>
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Pull request for series with
subject: bpf: Fix tnum_overlap to check for zero mask first
version: 1
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1015928