From: Zhang Tianci zhangtianci.1997@bytedance.com
commit 5b0db51215e895a361bc63132caa7cca36a53d6a upstream.
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().
Link: https://lore.kernel.org/all/20220817102952.xnvesg3a7rbv576x@wittgenstein/T Link: https://lore.kernel.org/lkml/20220825130552.29587-1-zhangtianci.1997@bytedan... Signed-off-by: Zhang Tianci zhangtianci.1997@bytedance.com Signed-off-by: Jiachen Zhang zhangjiachen.jaycee@bytedance.com Reviewed-by: Christian Brauner (Microsoft) brauner@kernel.org Fixes: 51f7e52dc943 ("ovl: share inode for hard link") Cc: stable@vger.kernel.org # v4.8 Signed-off-by: Miklos Szeredi mszeredi@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/overlayfs/dir.c | 46 ++++++++++++++++++++++++++++++---------------- 1 file changed, 30 insertions(+), 16 deletions(-)
--- a/fs/overlayfs/dir.c +++ b/fs/overlayfs/dir.c @@ -586,28 +586,42 @@ static int ovl_create_or_link(struct den 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;