[PATCH] ipc: Fix error pointer dereference

[Ethan Tidmore <ethantidmore06@gmail.com>]

The function shm_lock() can return an error pointer and is not checked
for one. Add check for error pointer.

Detected by Smatch:
ipc/shm.c:1678 do_shmat() error:
'shp' dereferencing possible ERR_PTR()

Fixes: 00c2bf85d8feb ("ipc: get rid of ipc_lock_down()")
Signed-off-by: Ethan Tidmore <ethantidmore06@gmail.com>
---
 ipc/shm.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/ipc/shm.c b/ipc/shm.c
index e8c7d1924c50..d4554f4e7063 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -1675,12 +1675,15 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg,
 out_nattch:
 	down_write(&shm_ids(ns).rwsem);
 	shp = shm_lock(ns, shmid);
-	shp->shm_nattch--;
+	if (!IS_ERR(shp)) {
+		shp->shm_nattch--;
+
+		if (shm_may_destroy(shp))
+			shm_destroy(ns, shp);
+		else
+			shm_unlock(shp);
+	}
 
-	if (shm_may_destroy(shp))
-		shm_destroy(ns, shp);
-	else
-		shm_unlock(shp);
 	up_write(&shm_ids(ns).rwsem);
 	return err;
 
-- 
2.53.0