ipc, shm: guard against non-existant vma in shmdt(2)

When !CONFIG_MMU there's a chance we can derefence a NULL pointer when the
VM area isn't found - check the return value of find_vma().

Also, remove the redundant -EINVAL return: retval is set to the proper
return code and *only* changed to 0, when we actually unmap the segments.

Signed-off-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Cc: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 2 deletions

diff --git a/ipc/shm.c b/ipc/shm.c
index cb2cedaa8808..a0ed957cefc9 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -1288,8 +1288,7 @@ SYSCALL_DEFINE1(shmdt, char __user *, shmaddr)
 #else /* CONFIG_MMU */
         /* under NOMMU conditions, the exact address to be destroyed must be
          * given */
-        retval = -EINVAL;
-        if (vma->vm_start == addr && vma->vm_ops == &shm_vm_ops) {
+        if (vma && vma->vm_start == addr && vma->vm_ops == &shm_vm_ops) {
                 do_munmap(mm, vma->vm_start, vma->vm_end - vma->vm_start);
                 retval = 0;
         }