Re: [PATCH v5 1/3] overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh

From: Mark Salyzyn
Date: Tue Aug 28 2018 - 13:44:16 EST

On 08/28/2018 10:34 AM, Amir Goldstein wrote:
On Tue, Aug 28, 2018 at 7:53 PM Mark Salyzyn <salyzyn@xxxxxxxxxxx> wrote:
Assumption never checked, should fail if the mounter creds are not

Signed-off-by: Mark Salyzyn <salyzyn@xxxxxxxxxxx>
Cc: Miklos Szeredi <miklos@xxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Amir Goldstein <amir73il@xxxxxxxxx>
Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Stephen Smalley <sds@xxxxxxxxxxxxx>
Cc: linux-unionfs@xxxxxxxxxxxxxxx
Cc: linux-doc@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx

- dependency of "overlayfs: override_creds=off option bypass creator_cred"
fs/overlayfs/namei.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index c993dd8db739..84982b6525fb 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -193,6 +193,11 @@ struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt,
if (!uuid_equal(&fh->uuid, &mnt->mnt_sb->s_uuid))
return NULL;

+ if (!capable(CAP_DAC_READ_SEARCH)) {
+ origin = ERR_PTR(-EPERM);
+ goto out;
Which branch is this works based on?
I don't see any out label in current code.

<sigh> I can only truly test this on 4.14 (android's current top of tree) and on Hikey with that. Lack of due diligence for Top of Linux.

+ }
bytes = (fh->len - offsetof(struct ovl_fh, fid));
real = exportfs_decode_fh(mnt, (struct fid *)fh->fid,
bytes >> 2, (int)fh->type,
Please add same test in ovl_can_decode_fh().

Problem: none of the ovl_export_operations functions override creds.
I guess things are working now because nfsd is privileged enough.
IOW, the capability check you added doesn't check mounter creds
when coming from nfs export ops - I guess that is not what you want
although you probably don'r enable nfs export.
NFS export/import blocked on Android devices.