sctp: validate cached peer INIT chunk length in COOKIE_ECHO processing

sctp: validate cached peer INIT chunk length in COOKIE_ECHO processing

When a listening SCTP server processes a COOKIE_ECHO chunk, the cached
peer INIT chunk embedded after the cookie is parsed and its parameters
are later walked by sctp_process_init() using sctp_walk_params().

However, the chunk header length of this cached INIT chunk was not
validated against the remaining buffer in the COOKIE_ECHO payload. If
the length field is inflated, the parameter walk can run beyond the
actual received data, leading to out-of-bounds reads and potential
memory corruption during later parameter handling (e.g. STATE_COOKIE
processing and kmemdup() copies).

Add a bounds check in sctp_unpack_cookie() to ensure the cached INIT
chunk length does not exceed the available data in the COOKIE_ECHO
buffer before it is used.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Brian Geffon <bgeffon@google.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/eb60825fa22d6f9e663c7d4dbb69f397b5d34d42.1780362366.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>