ovl: consistent behavior for immutable/append-only inodes

When a lower file has immutable/append-only fileattr flags, the behavior of
overlayfs post copy up is inconsistent.

Immediattely after copy up, ovl inode still has the S_IMMUTABLE/S_APPEND
inode flags copied from lower inode, so vfs code still treats the ovl inode
as immutable/append-only.  After ovl inode evict or mount cycle, the ovl
inode does not have these inode flags anymore.

We cannot copy up the immutable and append-only fileattr flags, because
immutable/append-only inodes cannot be linked and because overlayfs will
not be able to set overlay.* xattr on the upper inodes.

Instead, if any of the fileattr flags of interest exist on the lower inode,
we store them in overlay.protattr xattr on the upper inode and we read the
flags from xattr on lookup and on fileattr_get().

This gives consistent behavior post copy up regardless of inode eviction
from cache.

When user sets new fileattr flags, we update or remove the overlay.protattr
xattr.

Storing immutable/append-only fileattr flags in an xattr instead of upper
fileattr also solves other non-standard behavior issues - overlayfs can now
copy up children of "ovl-immutable" directories and lower aliases of
"ovl-immutable" hardlinks.

Reported-by: Chengguang Xu <cgxu519@mykernel.net>
Link: https://lore.kernel.org/linux-unionfs/20201226104618.239739-1-cgxu519@mykernel.net/
Link: https://lore.kernel.org/linux-unionfs/20210210190334.1212210-5-amir73il@gmail.com/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 15 insertions, 2 deletions

diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
index daf2afa603d3..9d8ebf0e7237 100644
--- a/fs/overlayfs/copy_up.c
+++ b/fs/overlayfs/copy_up.c
@@ -131,7 +131,8 @@ out:
 	return error;
 }
 
-static int ovl_copy_fileattr(struct path *old, struct path *new)
+static int ovl_copy_fileattr(struct inode *inode, struct path *old,
+			     struct path *new)
 {
 	struct fileattr oldfa = { .flags_valid = true };
 	struct fileattr newfa = { .flags_valid = true };
@@ -145,6 +146,18 @@ static int ovl_copy_fileattr(struct path *old, struct path *new)
 	if (err)
 		return err;
 
+	/*
+	 * We cannot set immutable and append-only flags on upper inode,
+	 * because we would not be able to link upper inode to upper dir
+	 * not set overlay private xattr on upper inode.
+	 * Store these flags in overlay.protattr xattr instead.
+	 */
+	if (oldfa.flags & OVL_PROT_FS_FLAGS_MASK) {
+		err = ovl_set_protattr(inode, new->dentry, &oldfa);
+		if (err)
+			return err;
+	}
+
 	BUILD_BUG_ON(OVL_COPY_FS_FLAGS_MASK & ~FS_COMMON_FL);
 	newfa.flags &= ~OVL_COPY_FS_FLAGS_MASK;
 	newfa.flags |= (oldfa.flags & OVL_COPY_FS_FLAGS_MASK);
@@ -550,7 +563,7 @@ static int ovl_copy_up_inode(struct ovl_copy_up_ctx *c, struct dentry *temp)
 		 * Copy the fileattr inode flags that are the source of already
 		 * copied i_flags
 		 */
-		err = ovl_copy_fileattr(&c->lowerpath, &upperpath);
+		err = ovl_copy_fileattr(inode, &c->lowerpath, &upperpath);
 		if (err)
 			return err;
 	}