keys: Check the authorization payload in process keyring searches

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

keys: Check the authorization payload in process keyring searches

An authorization payload and its saved credentials must remain valid
while search_process_keyrings_rcu() searches the requestor's keyrings.
The function validates the authorization key, then separately loads
payload.data[0] and unconditionally reads rka->cred. Validation does
not prevent request_key_auth_revoke() from clearing the payload.

A request-key helper can assume authority and fork a descendant that
inherits the authorization key. If the descendant misses in its own
keyrings, it falls back to the saved requestor credentials. Successful
target instantiation invalidates the authorization key without clearing
the payload. After the helper exits, UMH_WAIT_PROC returns to the
requester, which calls complete_request_key() and revokes that key.

With the descendant paused after validation, the paths can interleave
as follows:

1. The descendant enters search_process_keyrings_rcu() under RCU and
key_validate() succeeds.
2. The helper instantiates the target, invalidates the authorization
key, and exits.
3. The requester resumes from UMH_WAIT_PROC, calls
complete_request_key(), and revokes the authorization key.
request_key_auth_revoke() sets its payload pointer to NULL.
4. The descendant loads NULL and dereferences rka->cred.

This NULL dereference can oops the kernel. RCU delays payload disposal
but does not prevent the pointer from being cleared.

Load the payload with dereference_key_rcu() and require a non-NULL
result before validating the authorization key and searching its saved
credentials. A non-NULL payload and its credentials remain alive for
the caller's RCU read-side section; a cleared payload skips the fallback
search and follows the existing error selection.

KASAN report as below:

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000005: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
RIP: 0010:search_process_keyrings_rcu+0x32b/0x790