linux-kernel - Re: [PATCH] userfaultfd, capability: introduce CAP

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Date:   Fri, 25 Feb 2022 10:58:26 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Casey Schaufler <casey@...aufler-ca.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Serge Hallyn <serge@...lyn.com>,
        Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jeremy Kerr <jk@...econstruct.com.au>,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        Suren Baghdasaryan <surenb@...gle.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>
Subject: Re: [PATCH] userfaultfd, capability: introduce CAP_USERFAULTFD

On Thu, Feb 24, 2022 at 04:39:44PM -0800, Casey Schaufler wrote:
> What I'd want to see is multiple users where the use of CAP_USERFAULTD
> is independent of the use of CAP_SYS_PTRACE. That is, the programs would
> never require CAP_SYS_PTRACE. There should be demonstrated real value.
> Not just that a compromised program with CAP_SYS_PTRACE can do bad things,
> but that the programs with CAP_USERFAULTDD are somehow susceptible to
> being exploited to doing those bad things. Hypothetical users are just
> that, and often don't materialize.

I kind of have the same question indeed..

The use case we're talking about is VM migration, and the in-question
subject is literally the migration process or thread.  Isn't that a trusted
piece of software already?

Then the question is why the extra capability (in CAP_PTRACE but not in
CAP_UFFD) could bring much risk to the system.  Axel, did I miss something
important?

Thanks,

-- 
Peter Xu