CVE-2024-47702

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

bpf: Fail verification for sign-extension of packet data/data_end/data_meta

syzbot reported a kernel crash due to commit 1f1e864b6555 (bpf: Handle sign-extenstin ctx member accesses). The reason is due to sign-extension of 32-bit load for packet data/data_end/data_meta uapi field.

The original code looks like: r2 = *(s32 )(r1 + 76) / load __sk_buff->data */ r3 = *(u32 )(r1 + 80) / load __sk_buff->data_end */ r0 = r2 r0 += 8 if r3 > r0 goto +1 … Note that __sk_buff->data load has 32-bit sign extension.

After verification and convert_ctx_accesses(), the final asm code looks like: r2 = *(u64 *)(r1 +208) r2 = (s32)r2 r3 = *(u64 *)(r1 +80) r0 = r2 r0 += 8 if r3 > r0 goto pc+1 … Note that r2 = (s32)r2 may make the kernel __sk_buff->data address invalid which may cause runtime failure.

Currently, in C code, typically we have void *data = (void *)(long)skb->data; void *data_end = (void *)(long)skb->data_end; … and it will generate r2 = *(u64 *)(r1 +208) r3 = *(u64 *)(r1 +80) r0 = r2 r0 += 8 if r3 > r0 goto pc+1

If we allow sign-extension, void *data = (void *)(long)(int)skb->data; void *data_end = (void *)(long)skb->data_end; … the generated code looks like r2 = *(u64 *)(r1 +208) r2 «= 32 r2 s»= 32 r3 = *(u64 *)(r1 +80) r0 = r2 r0 += 8 if r3 > r0 goto pc+1 and this will cause verification failure since r2 «= 32 is not allowed as r2 is a packet pointer.

To fix this issue for case r2 = *(s32 )(r1 + 76) / load __sk_buff->data */ this patch added additional checking in is_valid_access() callback function for packet data/data_end/data_meta access. If those accesses are with sign-extenstion, the verification will fail.

[1] https://lore.kernel.org/bpf/000000000000c90eee061d236d37@google.com/

Affected Software

References

• https://git.kernel.org/stable/c/92de36080c93296ef9005690705cba260b9bd68a
• https://git.kernel.org/stable/c/f09757fe97a225ae505886eac572e4cbfba96537
• https://git.kernel.org/stable/c/f1620c93a1ec950d87ef327a565d3907736d3340