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 <[email protected]>
Cc: Sedat Dilek <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Manfred Spraul <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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;
 	}