Seth Forshee <seth.forshee@xxxxxxxxxxxxx> writes: > On Tue, Oct 14, 2014 at 01:01:02PM -0700, Eric W. Biederman wrote: >> 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. > > So the server could for example return trusted.* xattrs which were not > set by a privileged user. > >> > 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. > > That I wasn't aware of. In fact I still haven't found where this > restriction is implemented. My memory may be have been incomplete. What I was thinking of is security/commoncap.c the MNT_NOSUID check in get_file_caps. Upon inspection that appears limited to file capabilities, and while there are a few other MNT_NOSUID checks under security the feel far from complete. Sigh. This deserves a hard look because if MNT_NOSUID is not sufficient than it may be possible for me to insert a usb stick with an extN filesystem with the right labels having it auto-mounted nosuid and subvert the security of something like selinux. > Nonetheless, a userns mount could be done without nosuid (though that > mount will also be unaccessible outside of that namespace). > >> >> 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. > > Except that unprivileged mounts are normally done by a suid root helper, > which is why I've required both global CAP_SYS_ADMIN and a mount option > to get the current default behavior. If nosuid is sufficient that may break existing setups for no good reason. Shrug. I won't have much time for a bit but I figured I would highlight the potential security hole in existing setups. So someone with time this week can look at that. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
