CVE-2022-49961

In the Linux kernel, the following vulnerability has been resolved:

bpf: Do mark_chain_precision for ARG_CONST_ALLOC_SIZE_OR_ZERO

Precision markers need to be propagated whenever we have an ARG_CONST_* style argument, as the verifier cannot consider imprecise scalars to be equivalent for the purposes of states_equal check when such arguments refine the return value (in this case, set mem_size for PTR_TO_MEM). The resultant mem_size for the R0 is derived from the constant value, and if the verifier incorrectly prunes states considering them equivalent where such arguments exist (by seeing that both registers have reg->precise as false in regsafe), we can end up with invalid programs passing the verifier which can do access beyond what should have been the correct mem_size in that explored state.

To show a concrete example of the problem:

0000000000000000 <prog>: 0: r2 = *(u32 *)(r1 + 80) 1: r1 = *(u32 *)(r1 + 76) 2: r3 = r1 3: r3 += 4 4: if r3 > r2 goto +18 <LBB5_5> 5: w2 = 0 6: *(u32 *)(r1 + 0) = r2 7: r1 = *(u32 *)(r1 + 0) 8: r2 = 1 9: if w1 == 0 goto +1 <LBB5_3> 10: r2 = -1

0000000000000058 <LBB5_3>: 11: r1 = 0 ll 13: r3 = 0 14: call bpf_ringbuf_reserve 15: if r0 == 0 goto +7 <LBB5_5> 16: r1 = r0 17: r1 += 16777215 18: w2 = 0 19: *(u8 *)(r1 + 0) = r2 20: r1 = r0 21: r2 = 0 22: call bpf_ringbuf_submit

00000000000000b8 <LBB5_5>: 23: w0 = 0 24: exit

For the first case, the single line execution's exploration will prune the search at insn 14 for the branch insn 9's second leg as it will be verified first using r2 = -1 (UINT_MAX), while as w1 at insn 9 will always be 0 so at runtime we don't get error for being greater than UINT_MAX/4 from bpf_ringbuf_reserve. The...