ipc: compute kern_ipc_perm.id under the ipc lock

ipc_addid() initializes kern_ipc_perm.id after having called
ipc_idr_alloc().

Thus a parallel semctl() or msgctl() that uses e.g.  MSG_STAT may use this
unitialized value as the return code.

The patch moves all accesses to kern_ipc_perm.id under the spin_lock().

The issues is related to the finding of
syzbot+2827ef6b3385deb07eaf@syzkaller.appspotmail.com: syzbot found an
issue with kern_ipc_perm.seq

Link: http://lkml.kernel.org/r/20180712185241.4017-2-manfred@colorfullife.com
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Reviewed-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 5 deletions

diff --git a/ipc/msg.c b/ipc/msg.c
index 203281198079..7cb89a9b24e2 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -492,7 +492,6 @@ static int msgctl_stat(struct ipc_namespace *ns, int msqid,
                          int cmd, struct msqid64_ds *p)
 {
         struct msg_queue *msq;
-        int id = 0;
         int err;
 
         memset(p, 0, sizeof(*p));
@@ -504,7 +503,6 @@ static int msgctl_stat(struct ipc_namespace *ns, int msqid,
                         err = PTR_ERR(msq);
                         goto out_unlock;
                 }
-                id = msq->q_perm.id;
         } else { /* IPC_STAT */
                 msq = msq_obtain_object_check(ns, msqid);
                 if (IS_ERR(msq)) {
@@ -549,10 +547,21 @@ static int msgctl_stat(struct ipc_namespace *ns, int msqid,
         p->msg_lspid  = pid_vnr(msq->q_lspid);
         p->msg_lrpid  = pid_vnr(msq->q_lrpid);
 
-        ipc_unlock_object(&msq->q_perm);
-        rcu_read_unlock();
-        return id;
+        if (cmd == IPC_STAT) {
+                /*
+                 * As defined in SUS:
+                 * Return 0 on success
+                 */
+                err = 0;
+        } else {
+                /*
+                 * MSG_STAT and MSG_STAT_ANY (both Linux specific)
+                 * Return the full id, including the sequence number
+                 */
+                err = msq->q_perm.id;
+        }
 
+        ipc_unlock_object(&msq->q_perm);
 out_unlock:
         rcu_read_unlock();
         return err;