Hello Miklos,
Gentle ping...
Thanks,
Tianci
On Thu, Sep 1, 2022 at 4:29 PM Zhang Tianci
<zhangtianci.1997@xxxxxxxxxxxxx> wrote:
>
> 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)
>
