cifs: force a reconnect if there are too many MIDs in flight

Currently, we allow the pending_mid_q to grow without bound with
SIGKILL'ed processes. This could eventually be a DoS'able problem. An
unprivileged user could a process that does a long-running call and then
SIGKILL it.

If he can also intercept the NT_CANCEL calls or the replies from the
server, then the pending_mid_q could grow very large, possibly even to
2^16 entries which might leave GetNextMid in an infinite loop. Fix this
by imposing a hard limit of 32k calls per server. If we cross that
limit, set the tcpStatus to CifsNeedReconnect to force cifsd to
eventually reconnect the socket and clean out the pending_mid_q.

While we're at it, clean up the function a bit and eliminate an
unnecessary NULL pointer check.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

1 files changed, 24 insertions, 13 deletions

diff --git a/fs/cifs/misc.c b/fs/cifs/misc.c
index 72e99ece78cf..24f0a9d97ad8 100644
--- a/fs/cifs/misc.c
+++ b/fs/cifs/misc.c
@@ -236,10 +236,7 @@ __u16 GetNextMid(struct TCP_Server_Info *server)
 {
         __u16 mid = 0;
         __u16 last_mid;
-        int   collision;
-
-        if (server == NULL)
-                return mid;
+        bool collision;
 
         spin_lock(&GlobalMid_Lock);
         last_mid = server->CurrentMid; /* we do not want to loop forever */
@@ -252,24 +249,38 @@ __u16 GetNextMid(struct TCP_Server_Info *server)
         (and it would also have to have been a request that
          did not time out) */
         while (server->CurrentMid != last_mid) {
-                struct list_head *tmp;
                 struct mid_q_entry *mid_entry;
+                unsigned int num_mids;
 
-                collision = 0;
+                collision = false;
                 if (server->CurrentMid == 0)
                         server->CurrentMid++;
 
-                list_for_each(tmp, &server->pending_mid_q) {
-                        mid_entry = list_entry(tmp, struct mid_q_entry, qhead);
-
-                        if ((mid_entry->mid == server->CurrentMid) &&
-                            (mid_entry->midState == MID_REQUEST_SUBMITTED)) {
+                num_mids = 0;
+                list_for_each_entry(mid_entry, &server->pending_mid_q, qhead) {
+                        ++num_mids;
+                        if (mid_entry->mid == server->CurrentMid &&
+                            mid_entry->midState == MID_REQUEST_SUBMITTED) {
                                 /* This mid is in use, try a different one */
-                                collision = 1;
+                                collision = true;
                                 break;
                         }
                 }
-                if (collision == 0) {
+
+                /*
+                 * if we have more than 32k mids in the list, then something
+                 * is very wrong. Possibly a local user is trying to DoS the
+                 * box by issuing long-running calls and SIGKILL'ing them. If
+                 * we get to 2^16 mids then we're in big trouble as this
+                 * function could loop forever.
+                 *
+                 * Go ahead and assign out the mid in this situation, but force
+                 * an eventual reconnect to clean out the pending_mid_q.
+                 */
+                if (num_mids > 32768)
+                        server->tcpStatus = CifsNeedReconnect;
+
+                if (!collision) {
                         mid = server->CurrentMid;
                         break;
                 }