ipc,shm: correct error return value in shmctl (SHM_UNLOCK)

Commit 2caacaa82a51 ("ipc,shm: shorten critical region for shmctl")
restructured the ipc shm to shorten critical region, but introduced a
path where the return value could be -EPERM, even if the operation
actually was performed.

Before the commit, the err return value was reset by the return value
from security_shm_shmctl() after the if (!ns_capable(...)) statement.

Now, we still exit the if statement with err set to -EPERM, and in the
case of SHM_UNLOCK, it is not reset at all, and used as the return value
from shmctl.

To fix this, we only set err when errors occur, leaving the fallthrough
case alone.

Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>
Cc: Davidlohr Bueso <davidlohr@hp.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: <stable@vger.kernel.org>	[3.12.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/ipc/shm.c b/ipc/shm.c
index 0bdf21c6814e..7a51443a51d6 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -977,12 +977,15 @@ SYSCALL_DEFINE3(shmctl, int, shmid, int, cmd, struct shmid_ds __user *, buf)
                 ipc_lock_object(&shp->shm_perm);
                 if (!ns_capable(ns->user_ns, CAP_IPC_LOCK)) {
                         kuid_t euid = current_euid();
-                        err = -EPERM;
                         if (!uid_eq(euid, shp->shm_perm.uid) &&
-                            !uid_eq(euid, shp->shm_perm.cuid))
+                            !uid_eq(euid, shp->shm_perm.cuid)) {
+                                err = -EPERM;
                                 goto out_unlock0;
-                        if (cmd == SHM_LOCK && !rlimit(RLIMIT_MEMLOCK))
+                        }
+                        if (cmd == SHM_LOCK && !rlimit(RLIMIT_MEMLOCK)) {
+                                err = -EPERM;
                                 goto out_unlock0;
+                        }
                 }
 
                 shm_file = shp->shm_file;