Re: [PATCH v5 2/4] fuse: Support fuse filesystems outside of init_user_ns

From: Andy Lutomirski
Date: Fri Nov 21 2014 - 13:38:49 EST

On Fri, Nov 21, 2014 at 10:14 AM, Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
> - Tweak the file capability code to look at s_user_ns and treat it
> properly.

> - Tweak the security checks to allow setting file capabilities and
> other security xattrs if we have the appropriate capabilities in
> s_user_ns.

Thinking about this some more, what do you mean by tweaking the file
capability code to look at s_user_ns and treat it properly?

I think that the semantics should be that cap_inode_setxattr should
check ns_capable wrt s_user_ns, but that the fscap *consumer* should
check the mount as in my may_suid patch (and maybe also check
s_user_ns). There is legacy code that starts a FUSE server as global
root, mounts the thing in a mount namespace belonging to an
unprivileged user ns, and (I think) hands the /dev/fuse fd to that
unprivileged code.

Without the mount ns check, that FUSE server can take over the system.
With the mount ns check, it's safe.


> When those bits are done we can tweak the fuse patches to also set
> s_user_ns.
> As for MNT_NO_SUID if fuse wants to enforce that in some way. I don't
> particularly care, but I don't think that makes sense as a vfs property.
> Eric

Andy Lutomirski
AMA Capital Management, LLC
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