ovl: during copy up, switch to mounter's creds early

Now, we have the notion that copy up of a file is done with the creds
of mounter of overlay filesystem (as opposed to task). Right now before
we switch creds, we do some vfs_getattr() operations in the context of
task and that itself can fail. We should do that getattr() using the
creds of mounter instead.

So this patch switches to mounter's creds early during copy up process so
that even vfs_getattr() is done with mounter's creds.

Do not call revert_creds() unless we have already called
ovl_override_creds(). [Reported by Arnd Bergmann]

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 8 insertions, 5 deletions

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index c75625c1efa3..ce5d7dfaf769 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -19,6 +19,7 @@ static int ovl_copy_up_truncate(struct dentry *dentry)
 	struct dentry *parent;
 	struct kstat stat;
 	struct path lowerpath;
+	const struct cred *old_cred;
 
 	parent = dget_parent(dentry);
 	err = ovl_copy_up(parent);
@@ -26,12 +27,14 @@ static int ovl_copy_up_truncate(struct dentry *dentry)
 		goto out_dput_parent;
 
 	ovl_path_lower(dentry, &lowerpath);
-	err = vfs_getattr(&lowerpath, &stat);
-	if (err)
-		goto out_dput_parent;
 
-	stat.size = 0;
-	err = ovl_copy_up_one(parent, dentry, &lowerpath, &stat);
+	old_cred = ovl_override_creds(dentry->d_sb);
+	err = vfs_getattr(&lowerpath, &stat);
+	if (!err) {
+		stat.size = 0;
+		err = ovl_copy_up_one(parent, dentry, &lowerpath, &stat);
+	}
+	revert_creds(old_cred);
 
 out_dput_parent:
 	dput(parent);