eCryptfs: fix permission denied with ecryptfs_xattr mount option when create readonly file

When the ecryptfs_xattr mount option is turned on, the ecryptfs
metadata will be written to xattr via vfs_setxattr, which will
check the WRITE permissions.

However, this will cause denial of permission when creating a
file withoug write permission.

So fix this by calling __vfs_setxattr directly to skip permission
check.
Signed-off-by: Robbie Ko <robbieko@synology.com>
[tyhicks: Copy up lower inode attributes when successful]
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>

fs/ecryptfs/crypto.c

@@ -37,6 +37,7 @@
 #include <linux/slab.h>
 #include <asm/unaligned.h>
 #include <linux/kernel.h>
+#include <linux/xattr.h>
 #include "ecryptfs_kernel.h"
 
 #define DECRYPT		0
@@ -1131,9 +1132,21 @@ ecryptfs_write_metadata_to_xattr(struct dentry *ecryptfs_dentry,
 				 char *page_virt, size_t size)
 {
 	int rc;
+	struct dentry *lower_dentry = ecryptfs_dentry_to_lower(ecryptfs_dentry);
+	struct inode *lower_inode = d_inode(lower_dentry);
 
-	rc = ecryptfs_setxattr(ecryptfs_dentry, ecryptfs_inode,
-			       ECRYPTFS_XATTR_NAME, page_virt, size, 0);
+	if (!(lower_inode->i_opflags & IOP_XATTR)) {
+		rc = -EOPNOTSUPP;
+		goto out;
+	}
+
+	inode_lock(lower_inode);
+	rc = __vfs_setxattr(lower_dentry, lower_inode, ECRYPTFS_XATTR_NAME,
+			    page_virt, size, 0);
+	if (!rc && ecryptfs_inode)
+		fsstack_copy_attr_all(ecryptfs_inode, lower_inode);
+	inode_unlock(lower_inode);
+out:
 	return rc;
 }