CVE-2024-47702

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

bpf: Fail verification for sign-extension of packet data/dataend/datameta

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/dataend/datameta uapi field.

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

After verification and convertctxaccesses(), 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 _skbuff->data address invalid which may cause runtime failure.

Currently, in C code, typically we have void *data = (void *)(long)skb->data; void *dataend = (void *)(long)skb->dataend; ... 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 *dataend = (void *)(long)skb->dataend; ... 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 _skbuff->data */ this patch added additional checking in isvalidaccess() callback function for packet data/dataend/datameta access. If those accesses are with sign-extenstion, the verification will fail.

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