2 files changed, 41 insertions, 25 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 259202637531..0d23e76a0c61 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -711,15 +711,22 @@ EXPORT_SYMBOL_GPL(mmput);
  *
  * This changes mm's executable file (shown as symlink /proc/[pid]/exe).
  *
- * Main users are mmput(), sys_execve() and sys_prctl(PR_SET_MM_MAP/EXE_FILE).
- * Callers prevent concurrent invocations: in mmput() nobody alive left,
- * in execve task is single-threaded, prctl holds mmap_sem exclusively.
+ * Main users are mmput() and sys_execve(). Callers prevent concurrent
+ * invocations: in mmput() nobody alive left, in execve task is single
+ * threaded. sys_prctl(PR_SET_MM_MAP/EXE_FILE) also needs to set the
+ * mm->exe_file, but does so without using set_mm_exe_file() in order
+ * to do avoid the need for any locks.
  */
 void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
 {
-	struct file *old_exe_file = rcu_dereference_protected(mm->exe_file,
-			!atomic_read(&mm->mm_users) || current->in_execve ||
-			lockdep_is_held(&mm->mmap_sem));
+	struct file *old_exe_file;
+
+	/*
+	 * It is safe to dereference the exe_file without RCU as
+	 * this function is only called if nobody else can access
+	 * this mm -- see comment above for justification.
+	 */
+	old_exe_file = rcu_dereference_raw(mm->exe_file);
 
 	if (new_exe_file)
 		get_file(new_exe_file);
diff --git a/kernel/sys.c b/kernel/sys.c
index 3be344902316..a4e372b798a5 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1649,14 +1649,13 @@ SYSCALL_DEFINE1(umask, int, mask)
 	return mask;
 }
 
-static int prctl_set_mm_exe_file_locked(struct mm_struct *mm, unsigned int fd)
+static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 {
 	struct fd exe;
+	struct file *old_exe, *exe_file;
 	struct inode *inode;
 	int err;
 
-	VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);
-
 	exe = fdget(fd);
 	if (!exe.file)
 		return -EBADF;
@@ -1680,15 +1679,22 @@ static int prctl_set_mm_exe_file_locked(struct mm_struct *mm, unsigned int fd)
 	/*
 	 * Forbid mm->exe_file change if old file still mapped.
 	 */
+	exe_file = get_mm_exe_file(mm);
 	err = -EBUSY;
-	if (mm->exe_file) {
+	if (exe_file) {
 		struct vm_area_struct *vma;
 
-		for (vma = mm->mmap; vma; vma = vma->vm_next)
-			if (vma->vm_file &&
-			    path_equal(&vma->vm_file->f_path,
-				       &mm->exe_file->f_path))
-				goto exit;
+		down_read(&mm->mmap_sem);
+		for (vma = mm->mmap; vma; vma = vma->vm_next) {
+			if (!vma->vm_file)
+				continue;
+			if (path_equal(&vma->vm_file->f_path,
+				       &exe_file->f_path))
+				goto exit_err;
+		}
+
+		up_read(&mm->mmap_sem);
+		fput(exe_file);
 	}
 
 	/*
@@ -1702,10 +1708,18 @@ static int prctl_set_mm_exe_file_locked(struct mm_struct *mm, unsigned int fd)
 		goto exit;
 
 	err = 0;
-	set_mm_exe_file(mm, exe.file);	/* this grabs a reference to exe.file */
+	/* set the new file, lockless */
+	get_file(exe.file);
+	old_exe = xchg(&mm->exe_file, exe.file);
+	if (old_exe)
+		fput(old_exe);
 exit:
 	fdput(exe);
 	return err;
+exit_err:
+	up_read(&mm->mmap_sem);
+	fput(exe_file);
+	goto exit;
 }
 
 #ifdef CONFIG_CHECKPOINT_RESTORE
@@ -1840,10 +1854,9 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
 		user_auxv[AT_VECTOR_SIZE - 1] = AT_NULL;
 	}
 
-	down_write(&mm->mmap_sem);
 	if (prctl_map.exe_fd != (u32)-1)
-		error = prctl_set_mm_exe_file_locked(mm, prctl_map.exe_fd);
-	downgrade_write(&mm->mmap_sem);
+		error = prctl_set_mm_exe_file(mm, prctl_map.exe_fd);
+	down_read(&mm->mmap_sem);
 	if (error)
 		goto out;
 
@@ -1909,12 +1922,8 @@ static int prctl_set_mm(int opt, unsigned long addr,
 	if (!capable(CAP_SYS_RESOURCE))
 		return -EPERM;
 
-	if (opt == PR_SET_MM_EXE_FILE) {
-		down_write(&mm->mmap_sem);
-		error = prctl_set_mm_exe_file_locked(mm, (unsigned int)addr);
-		up_write(&mm->mmap_sem);
-		return error;
-	}
+	if (opt == PR_SET_MM_EXE_FILE)
+		return prctl_set_mm_exe_file(mm, (unsigned int)addr);
 
 	if (addr >= TASK_SIZE || addr < mmap_min_addr)
 		return -EINVAL;