Re: [fuse-devel] [PATCH v4 4/5] fuse: Support privileged xattrs only with a mount option

From: Eric W. Biederman
Date: Tue Oct 14 2014 - 16:01:51 EST

Michael j Theall <mtheall@xxxxxxxxxx> writes:

> Seth Forshee <seth.forshee@xxxxxxxxxxxxx> wrote on 10/14/2014 09:25:55 AM:
>> From: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>
>> To: Miklos Szeredi <miklos@xxxxxxxxxx>
>> Cc: fuse-devel@xxxxxxxxxxxxxxxxxxxxx, "Serge H. Hallyn"
>> <serge.hallyn@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Seth
>> Forshee <seth.forshee@xxxxxxxxxxxxx>, "Eric W. Biederman"
>> <ebiederm@xxxxxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx
>> Date: 10/14/2014 09:27 AM
>> Subject: [fuse-devel] [PATCH v4 4/5] fuse: Support privileged xattrs
>> only with a mount option
>> Allowing unprivileged users to provide arbitrary xattrs via fuse
>> mounts bypasses the normal restrictions on setting xattrs. Such
>> mounts should be restricted to reading and writing xattrs in the
>> user.* namespace.
> Can you explain how the normal restrictions on setting xattrs are
> bypassed?

If the fuse server is not run by root. Which is a large part of the
point of fuse.

> My filesystem still needs security.* and system.*, and it looks like
> xattr_permission already prevents non-privileged users from accessing
> trusted.*

If the filesystem is mounted with nosuid (typical of a non-privileged
mount of fuse) then the security.* attributes are ignored.

>> It's difficult though to tell whether a mount is being performed
>> on behalf of an unprivileged user since fuse mounts are ususally
>> done via a suid root helper. Thus a new mount option,
>> privileged_xattrs, is added to indicated that xattrs from other
>> namespaces are allowed. This option can only be supplied by
>> system-wide root; supplying the option as an unprivileged user
>> will cause the mount to fail.
> I can't say I'm convinced that this is the right direction to head.

With respect to defaults we could keep the current default if you
have the global CAP_SYS_ADMIN privilege when the mount takes place
and then avoid breaking anything.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at