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Jann Horn authored
The BPF verifier tried to track values based on 32-bit comparisons by (ab)using the tnum state via 581738a6 ("bpf: Provide better register bounds after jmp32 instructions"). The idea is that after a check like this: if ((u32)r0 > 3) exit We can't meaningfully constrain the arithmetic-range-based tracking, but we can update the tnum state to (value=0,mask=0xffff'ffff'0000'0003). However, the implementation from 581738a6 didn't compute the tnum constraint based on the fixed operand, but instead derives it from the arithmetic-range-based tracking. This means that after the following sequence of operations: if (r0 >= 0x1'0000'0001) exit if ((u32)r0 > 7) exit The verifier assumed that the lower half of r0 is in the range (0, 0) and apply the tnum constraint (value=0,mask=0xffff'ffff'0000'0000) thus causing the overall tnum to be (value=0,mask=0x1'0000'0000), which was incorrect. Provide a fixed implementation. Fixes: 581738a6 ("bpf: Provide better register bounds after jmp32 instructions") Signed-off-by:
Jann Horn <jannh@google.com> Signed-off-by:
Daniel Borkmann <daniel@iogearbox.net> Signed-off-by:
Alexei Starovoitov <ast@kernel.org> Link: https://lore.kernel.org/bpf/20200330160324.15259-3-daniel@iogearbox.net
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