There is a wrong case of link() on overlay:
$ mkdir /lower /fuse /merge
$ mount -t fuse /fuse
$ mkdir /fuse/upper /fuse/work
$ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,\
workdir=work
$ touch /merge/file
$ chown bin.bin /merge/file // the file's caller becomes "bin"
$ ln /merge/file /merge/lnkfile
Then we will get an error(EACCES) because fuse daemon checks the link()'s
caller is "bin", it denied this request.
In the changing history of ovl_link(), there are two key commits:
The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
overrides the cred's fsuid/fsgid using the new inode. The new inode's
owner is initialized by inode_init_owner(), and inode->fsuid is
assigned to the current user. So the override fsuid becomes the
current user. We know link() is actually modifying the directory, so
the caller must have the MAY_WRITE permission on the directory. The
current caller may should have this permission. This is acceptable
to use the caller's fsuid.
The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
which removed the inode creation in ovl_link(). This commit move
inode_init_owner() into ovl_create_object(), so the ovl_link() just
give the old inode to ovl_create_or_link(). Then the override fsuid
becomes the old inode's fsuid, neither the caller nor the overlay's
mounter! So this is incorrect.
Fix this bug by using ovl mounter's fsuid/fsgid to do underlying
fs's link().
v1: https://lore.kernel.org/all/20220817102952.xnvesg3a7rbv576x@wittgenstein/T
v2: https://lore.kernel.org/lkml/20220825130552.29587-1-zhangtianci.1997@xxxxxxxxxxxxx/t
Signed-off-by: Zhang Tianci <zhangtianci.1997@xxxxxxxxxxxxx>
Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@xxxxxxxxxxxxx>
---
fs/overlayfs/dir.c | 46 ++++++++++++++++++++++++++++++----------------
1 file changed, 30 insertions(+), 16 deletions(-)
diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 6b03457f72bb..c3032cef391e 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -592,28 +592,42 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
goto out_revert_creds;
}
- err = -ENOMEM;
- override_cred = prepare_creds();
- if (override_cred) {
+ if (!attr->hardlink) {
+ err = -ENOMEM;
+ override_cred = prepare_creds();
+ if (!override_cred)
+ goto out_revert_creds;
+ /*
+ * In the creation cases(create, mkdir, mknod, symlink),
+ * ovl should transfer current's fs{u,g}id to underlying
+ * fs. Because underlying fs want to initialize its new
+ * inode owner using current's fs{u,g}id. And in this
+ * case, the @inode is a new inode that is initialized
+ * in inode_init_owner() to current's fs{u,g}id. So use
+ * the inode's i_{u,g}id to override the cred's fs{u,g}id.
+ *
+ * But in the other hardlink case, ovl_link() does not
+ * create a new inode, so just use the ovl mounter's
+ * fs{u,g}id.
+ */
override_cred->fsuid = inode->i_uid;
override_cred->fsgid = inode->i_gid;
- if (!attr->hardlink) {
- err = security_dentry_create_files_as(dentry,
- attr->mode, &dentry->d_name, old_cred,
- override_cred);
- if (err) {
- put_cred(override_cred);
- goto out_revert_creds;
- }
+ err = security_dentry_create_files_as(dentry,
+ attr->mode, &dentry->d_name, old_cred,
+ override_cred);
+ if (err) {
+ put_cred(override_cred);
+ goto out_revert_creds;
}
put_cred(override_creds(override_cred));
put_cred(override_cred);
-
- if (!ovl_dentry_is_whiteout(dentry))
- err = ovl_create_upper(dentry, inode, attr);
- else
- err = ovl_create_over_whiteout(dentry, inode, attr);
}
+
+ if (!ovl_dentry_is_whiteout(dentry))
+ err = ovl_create_upper(dentry, inode, attr);
+ else
+ err = ovl_create_over_whiteout(dentry, inode, attr);
+
out_revert_creds:
revert_creds(old_cred);
return err;
--
2.32.1 (Apple Git-133)
