cifs: Fix preauth hash corruption

smb311_update_preauth_hash() uses the shash in server->secmech without
appropriate locking, and this can lead to sessions corrupting each
other's preauth hashes.

The following script can easily trigger the problem:

	#!/bin/sh -e

	NMOUNTS=10
	for i in $(seq $NMOUNTS);
		mkdir -p /tmp/mnt$i
		umount /tmp/mnt$i 2>/dev/null || :
	done
	while :; do
		for i in $(seq $NMOUNTS); do
			mount -t cifs //192.168.0.1/test /tmp/mnt$i -o ... &
		done
		wait
		for i in $(seq $NMOUNTS); do
			umount /tmp/mnt$i
		done
	done

Usually within seconds this leads to one or more of the mounts failing
with the following errors, and a "Bad SMB2 signature for message" is
seen in the server logs:

 CIFS: VFS: \\192.168.0.1 failed to connect to IPC (rc=-13)
 CIFS: VFS: cifs_mount failed w/return code = -13

Fix it by holding the server mutex just like in the other places where
the shashes are used.

Fixes: 8bd68c6e ("CIFS: implement v3.11 preauth integrity")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
CC: <stable@vger.kernel.org>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/transport.c

@@ -1196,9 +1196,12 @@ compound_send_recv(const unsigned int xid, struct cifs_ses *ses,
 	/*
 	 * Compounding is never used during session establish.
 	 */
-	if ((ses->status == CifsNew) || (optype & CIFS_NEG_OP) || (optype & CIFS_SESS_OP))
+	if ((ses->status == CifsNew) || (optype & CIFS_NEG_OP) || (optype & CIFS_SESS_OP)) {
+		mutex_lock(&server->srv_mutex);
 		smb311_update_preauth_hash(ses, rqst[0].rq_iov,
 					   rqst[0].rq_nvec);
+		mutex_unlock(&server->srv_mutex);
+	}
 
 	for (i = 0; i < num_rqst; i++) {
 		rc = wait_for_response(server, midQ[i]);
@@ -1266,7 +1269,9 @@ compound_send_recv(const unsigned int xid, struct cifs_ses *ses,
 			.iov_base = resp_iov[0].iov_base,
 			.iov_len = resp_iov[0].iov_len
 		};
+		mutex_lock(&server->srv_mutex);
 		smb311_update_preauth_hash(ses, &iov, 1);
+		mutex_unlock(&server->srv_mutex);
 	}
 
 out: