linux-kernel - Re: [PATCH 1/1] userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT

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Message-ID: <CAKOZuet1Hrdd7U4VVBmXNCkE6xwiUPERRHjP=A3bX6B9A4BQRQ@mail.gmail.com>
Date:   Tue, 5 Nov 2019 08:39:26 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Nick Kralevich <nnk@...gle.com>,
        Nosh Minwalla <nosh@...gle.com>,
        Pavel Emelyanov <ovzxemul@...il.com>,
        Tim Murray <timmurray@...gle.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 1/1] userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT_FORK

On Tue, Nov 5, 2019 at 8:33 AM Andrea Arcangeli <aarcange@...hat.com> wrote:
>
> On Tue, Nov 05, 2019 at 08:06:49AM -0800, Daniel Colascione wrote:
> > Sure, but the same argument applies to all the other permission checks
> > that we do at open time, not at ioctl time. For better or for worse,
> > the DAC-ish model used in most places is that access checks happen at
> > file object creation time and anyone who has the FD can perform those
> > operations later. Confusing the model by doing *some* permission
> > checks at open time and *some* permission checks at usage time makes
> > the system harder to understand.
>
> The only case that requires change is if userland requested the
> UFFD_FEATURE_EVENT_FORK feature (which AFIK only CRIU does) and that
> request is done in the UFFDIO_API call not during the syscall.
>
> Doing the check in the syscall would then break all non privileged
> users like if we'd set /proc/sys/vm/unprivileged_userfaultfd to
> zero.

I'm not suggesting that we fail userfaultfd(2) without CAP_SYS_PTRACE.
That would, as you point out, break things. I'm talking about
recording *whether* we had CAP_SYS_PTRACE in an internal flag in the
uffd context when we create the thing --- and then, at ioctl time,
checking that flag, not the caller's CAP_SYS_PTRACE, to see whether
UFFD_FEATURE_EVENT_FORK should be made available. This way, the
security check hinges on whether the caller *at create time* was
privileged.