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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>
This commit is contained in:
Jeff Layton 2011-01-29 07:02:28 -05:00 committed by Steve French
parent d804d41d16
commit 92a4e0f016
1 changed files with 24 additions and 13 deletions
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37
fs/cifs/misc.c
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@ -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;
}