prctl: PR_SET_MM -- factor out mmap_sem when updating mm::exe_file

Instead of taking mm->mmap_sem inside prctl_set_mm_exe_file() move it out
and rename the helper to prctl_set_mm_exe_file_locked().  This will allow
to reuse this function in a next patch.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andrew Vagin <avagin@openvz.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Vasiliy Kulikov <segoon@openwall.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Julien Tinnes <jln@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/sys.c

@@ -1628,12 +1628,14 @@ SYSCALL_DEFINE1(umask, int, mask)
 	return mask;
 }
 
-static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
+static int prctl_set_mm_exe_file_locked(struct mm_struct *mm, unsigned int fd)
 {
 	struct fd exe;
 	struct inode *inode;
 	int err;
 
+	VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
+
 	exe = fdget(fd);
 	if (!exe.file)
 		return -EBADF;
@@ -1654,8 +1656,6 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 	if (err)
 		goto exit;
 
-	down_write(&mm->mmap_sem);
-
 	/*
 	 * Forbid mm->exe_file change if old file still mapped.
 	 */
@@ -1667,7 +1667,7 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 			if (vma->vm_file &&
 			    path_equal(&vma->vm_file->f_path,
 				       &mm->exe_file->f_path))
-				goto exit_unlock;
+				goto exit;
 	}
 
 	/*
@@ -1678,13 +1678,10 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 	 */
 	err = -EPERM;
 	if (test_and_set_bit(MMF_EXE_FILE_CHANGED, &mm->flags))
-		goto exit_unlock;
+		goto exit;
 
 	err = 0;
 	set_mm_exe_file(mm, exe.file);	/* this grabs a reference to exe.file */
-exit_unlock:
-	up_write(&mm->mmap_sem);
-
 exit:
 	fdput(exe);
 	return err;
@@ -1703,8 +1700,12 @@ static int prctl_set_mm(int opt, unsigned long addr,
 	if (!capable(CAP_SYS_RESOURCE))
 		return -EPERM;
 
-	if (opt == PR_SET_MM_EXE_FILE)
-		return prctl_set_mm_exe_file(mm, (unsigned int)addr);
+	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 (addr >= TASK_SIZE || addr < mmap_min_addr)
 		return -EINVAL;