nfsd: fix vm overcommit crash

Junjiro R.  Okajima reported a problem where knfsd crashes if you are
using it to export shmemfs objects and run strict overcommit.  In this
situation the current->mm based modifier to the overcommit goes through a
NULL pointer.

We could simply check for NULL and skip the modifier but we've caught
other real bugs in the past from mm being NULL here - cases where we did
need a valid mm set up (eg the exec bug about a year ago).

To preserve the checks and get the logic we want shuffle the checking
around and add a new helper to the vm_ security wrappers

Also fix a current->mm reference in nommu that should use the passed mm

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

5 files changed, 23 insertions, 6 deletions

diff --git a/include/linux/security.h b/include/linux/security.h
index f5c4a51eb42e..c13f1cec9abb 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -1585,6 +1585,7 @@ int security_syslog(int type);
 int security_settime(struct timespec *ts, struct timezone *tz);
 int security_vm_enough_memory(long pages);
 int security_vm_enough_memory_mm(struct mm_struct *mm, long pages);
+int security_vm_enough_memory_kern(long pages);
 int security_bprm_alloc(struct linux_binprm *bprm);
 void security_bprm_free(struct linux_binprm *bprm);
 void security_bprm_apply_creds(struct linux_binprm *bprm, int unsafe);
@@ -1820,6 +1821,11 @@ static inline int security_vm_enough_memory(long pages)
         return cap_vm_enough_memory(current->mm, pages);
 }
 
+static inline int security_vm_enough_memory_kern(long pages)
+{
+        return cap_vm_enough_memory(current->mm, pages);
+}
+
 static inline int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
 {
         return cap_vm_enough_memory(mm, pages);
diff --git a/mm/mmap.c b/mm/mmap.c
index 74f4d158022e..de14ac21e5b5 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -175,7 +175,8 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
 
         /* Don't let a single process grow too big:
            leave 3% of the size of this process for other processes */
-        allowed -= mm->total_vm / 32;
+        if (mm)
+                allowed -= mm->total_vm / 32;
 
         /*
          * cast `allowed' as a signed long because vm_committed_space
diff --git a/mm/nommu.c b/mm/nommu.c
index 2696b24f2bb3..7695dc850785 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1454,7 +1454,8 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
 
         /* Don't let a single process grow too big:
            leave 3% of the size of this process for other processes */
-        allowed -= current->mm->total_vm / 32;
+        if (mm)
+                allowed -= mm->total_vm / 32;
 
         /*
          * cast `allowed' as a signed long because vm_committed_space
diff --git a/mm/shmem.c b/mm/shmem.c
index d38d7e61fcd0..0ed075215e5f 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -161,8 +161,8 @@ static inline struct shmem_sb_info *SHMEM_SB(struct super_block *sb)
  */
 static inline int shmem_acct_size(unsigned long flags, loff_t size)
 {
-        return (flags & VM_ACCOUNT)?
-                security_vm_enough_memory(VM_ACCT(size)): 0;
+        return (flags & VM_ACCOUNT) ?
+                security_vm_enough_memory_kern(VM_ACCT(size)) : 0;
 }
 
 static inline void shmem_unacct_size(unsigned long flags, loff_t size)
@@ -179,8 +179,8 @@ static inline void shmem_unacct_size(unsigned long flags, loff_t size)
  */
 static inline int shmem_acct_block(unsigned long flags)
 {
-        return (flags & VM_ACCOUNT)?
-                0: security_vm_enough_memory(VM_ACCT(PAGE_CACHE_SIZE));
+        return (flags & VM_ACCOUNT) ?
+                0 : security_vm_enough_memory_kern(VM_ACCT(PAGE_CACHE_SIZE));
 }
 
 static inline void shmem_unacct_blocks(unsigned long flags, long pages)
diff --git a/security/security.c b/security/security.c
index 255b08559b2b..c0acfa7177e5 100644
--- a/security/security.c
+++ b/security/security.c
@@ -198,14 +198,23 @@ int security_settime(struct timespec *ts, struct timezone *tz)
 
 int security_vm_enough_memory(long pages)
 {
+        WARN_ON(current->mm == NULL);
         return security_ops->vm_enough_memory(current->mm, pages);
 }
 
 int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
 {
+        WARN_ON(mm == NULL);
         return security_ops->vm_enough_memory(mm, pages);
 }
 
+int security_vm_enough_memory_kern(long pages)
+{
+        /* If current->mm is a kernel thread then we will pass NULL,
+           for this specific case that is fine */
+        return security_ops->vm_enough_memory(current->mm, pages);
+}
+
 int security_bprm_alloc(struct linux_binprm *bprm)
 {
         return security_ops->bprm_alloc_security(bprm);