sctp: fix OOB write to userspace in sctp_getsockopt_peer_auth_chunks

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

sctp: fix OOB write to userspace in sctp_getsockopt_peer_auth_chunks

sctp_getsockopt_peer_auth_chunks() checks that the caller's optval

buffer is large enough for the peer AUTH chunk list with

if (len < num_chunks)

return -EINVAL;

but then writes num_chunks bytes to p->gauth_chunks, which lives

at offset offsetof(struct sctp_authchunks, gauth_chunks) == 8

inside optval. The check is missing the sizeof(struct

sctp_authchunks) = 8-byte header. When the caller supplies

len == num_chunks (for any num_chunks > 0) the test passes but

copy_to_user() writes sizeof(struct sctp_authchunks) = 8 bytes

past the declared buffer.

The sibling function sctp_getsockopt_local_auth_chunks() at the

next line already has the correct check:

if (len < sizeof(struct sctp_authchunks) + num_chunks)

return -EINVAL;

Align the peer variant with its sibling.

Reproducer confirms on v7.0-13-generic: an unprivileged userspace

caller that opens a loopback SCTP association with AUTH enabled,

queries num_chunks with a short optval, then issues the real

getsockopt with len == num_chunks and sentinel bytes painted past

the buffer observes those sentinel bytes overwritten with the

peer's AUTH chunk type. The bytes written are under the peer's

control but land in the caller's own userspace; this is not a

kernel memory corruption, but it is a kernel-side contract

violation that can silently corrupt adjacent userspace data.