[PATCH] nfsd: Fix vm overcommit crash

[Alan Cox <alan@redhat.com>]

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. While at it the
checks are switched to WARN_ON - we need to know if someone gets current->mm
wrong in the other calls but we don't need to crash.

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

Signed-off-by: Alan Cox <alan@redhat.com>
---

 include/linux/security.h |    1 +
 mm/mmap.c                |    3 ++-
 mm/nommu.c               |    3 ++-
 mm/shmem.c               |    4 ++--
 security/security.c      |    9 +++++++++
 5 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/include/linux/security.h b/include/linux/security.h
index f5c4a51..a2b8430 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);
diff --git a/mm/mmap.c b/mm/mmap.c
index 74f4d15..de14ac2 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 2696b24..7695dc8 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 d38d7e6..1677b3e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -162,7 +162,7 @@ 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;
+		security_vm_enough_memory_kern(VM_ACCT(size)): 0;
 }
 
 static inline void shmem_unacct_size(unsigned long flags, loff_t size)
@@ -180,7 +180,7 @@ 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));
+		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 255b085..c0acfa7 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);