[PATCH] vfs: vfs_tmpfile: ensure O_EXCL flag is enforced

[Peter Griffin]

If O_EXCL is *not* specified, then linkat() can be
used to link the temporary file into the filesystem.
If O_EXCL is specified then linkat() should fail (-1).

After commit 863f144f12ad ("vfs: open inside ->tmpfile()")
the O_EXCL flag is no longer honored by the vfs layer for
tmpfile, which means the file can be linked even if O_EXCL
flag is specified, which is a change in behaviour for
userspace!

The open flags was previously passed as a parameter, so it
was uneffected by the changes to file->f_flags caused by
finish_open(). This patch fixes the issue by storing
file->f_flags in a local variable so the O_EXCL test
logic is restored.

This regression was detected by Android CTS Bionic fcntl()
tests running on android-mainline [1].

[1] https://android.googlesource.com/platform/bionic/+/
    refs/heads/master/tests/fcntl_test.cpp#352

Fixes: 863f144f12ad ("vfs: open inside ->tmpfile()")
To: lkml <linux-kernel@xxxxxxxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Will McVicker <willmcvicker@xxxxxxxxxx>
Cc: Peter Griffin <gpeter@xxxxxxxxxx>
Signed-off-by: Peter Griffin <peter.griffin@xxxxxxxxxx>
---
 fs/namei.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index 578c2110df02..9155ecb547ce 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3591,6 +3591,7 @@ static int vfs_tmpfile(struct user_namespace *mnt_userns,
 	struct inode *dir = d_inode(parentpath->dentry);
 	struct inode *inode;
 	int error;
+	int open_flag = file->f_flags;
 
 	/* we want directory to be writable */
 	error = inode_permission(mnt_userns, dir, MAY_WRITE | MAY_EXEC);
@@ -3613,7 +3614,7 @@ static int vfs_tmpfile(struct user_namespace *mnt_userns,
 	if (error)
 		return error;
 	inode = file_inode(file);
-	if (!(file->f_flags & O_EXCL)) {
+	if (!(open_flag & O_EXCL)) {
 		spin_lock(&inode->i_lock);
 		inode->i_state |= I_LINKABLE;
 		spin_unlock(&inode->i_lock);
-- 
2.34.1