vfs: vfs_tmpfile: ensure O_EXCL flag is enforced

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 863f144f ("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: 863f144f ("vfs: open inside ->tmpfile()")
Acked-by: NMiklos Szeredi <mszeredi@redhat.com>
Tested-by: NWill McVicker <willmcvicker@google.com>
Signed-off-by: NPeter Griffin <peter.griffin@linaro.org>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

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);