nfsd: fix vm overcommit crash fix #2

The previous patch from Alan Cox ("nfsd: fix vm overcommit crash",
commit 731572d39fcd3498702eda4600db4c43d51e0b26) fixed the problem where
knfsd crashes on exported shmemfs objects and strict overcommit is set.

But the patch forgot supporting the case when CONFIG_SECURITY is
disabled.

This patch copies a part of his fix which is mainly for detecting a bug
earlier.

Acked-by: James Morris <jmorris@namei.org>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Junjiro R. Okajima <hooanon05@yahoo.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/security.h

@@ -1818,17 +1818,21 @@ static inline int security_settime(struct timespec *ts, struct timezone *tz)
 
 static inline int security_vm_enough_memory(long pages)
 {
+	WARN_ON(current->mm == NULL);
 	return cap_vm_enough_memory(current->mm, pages);
 }
 
-static inline int security_vm_enough_memory_kern(long pages)
+static inline int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
 {
-	return cap_vm_enough_memory(current->mm, pages);
+	WARN_ON(mm == NULL);
+	return cap_vm_enough_memory(mm, pages);
 }
 
-static inline int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
+static inline int security_vm_enough_memory_kern(long pages)
 {
-	return cap_vm_enough_memory(mm, pages);
+	/* If current->mm is a kernel thread then we will pass NULL,
+	   for this specific case that is fine */
+	return cap_vm_enough_memory(current->mm, pages);
 }
 
 static inline int security_bprm_alloc(struct linux_binprm *bprm)