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@xxxxxxxxxx>
---
fs/cifs/misc.c | 37 ++++++++++++++++++++++++-------------
1 files changed, 24 insertions(+), 13 deletions(-)
diff --git a/fs/cifs/misc.c b/fs/cifs/misc.c
index 77dc732..2a930a7 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;
}
--
1.7.3.4
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