linux-kernel - Re: [PATCH v4 3/3] prctl: Allow ptrace capable processes to change /proc/self/exe

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Date:   Thu, 2 Jul 2020 18:00:10 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     Adrian Reber <areber@...hat.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Pavel Emelyanov <ovzxemul@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Andrei Vagin <avagin@...il.com>,
        Nicolas Viennot <Nicolas.Viennot@...sigma.com>,
        Michał Cłapiński <mclapinski@...gle.com>,
        Kamil Yurtsever <kyurtsever@...gle.com>,
        Dirk Petersen <dipeit@...il.com>,
        Christine Flood <chf@...hat.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@...il.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Arnd Bergmann <arnd@...db.de>,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, selinux@...r.kernel.org,
        Eric Paris <eparis@...isplace.org>,
        Jann Horn <jannh@...gle.com>, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v4 3/3] prctl: Allow ptrace capable processes to change /proc/self/exe

On Thu, Jul 2, 2020 at 5:16 PM Serge E. Hallyn <serge@...lyn.com> wrote:
> On Wed, Jul 01, 2020 at 08:49:06AM +0200, Adrian Reber wrote:
> > From: Nicolas Viennot <Nicolas.Viennot@...sigma.com>
> >
> > Previously, the current process could only change the /proc/self/exe
> > link with local CAP_SYS_ADMIN.
> > This commit relaxes this restriction by permitting such change with
> > CAP_CHECKPOINT_RESTORE, and the ability to use ptrace.
> >
> > With access to ptrace facilities, a process can do the following: fork a
> > child, execve() the target executable, and have the child use ptrace()
> > to replace the memory content of the current process. This technique
> > makes it possible to masquerade an arbitrary program as any executable,
> > even setuid ones.
> >
> > Signed-off-by: Nicolas Viennot <Nicolas.Viennot@...sigma.com>
> > Signed-off-by: Adrian Reber <areber@...hat.com>
>
> This is scary.  But I believe it is safe.
>
> Reviewed-by: Serge Hallyn <serge@...lyn.com>
>
> I am a bit curious about the implications of the selinux patch.
> IIUC you are using the permission of the tracing process to
> execute the file without transition, so this is a way to work
> around the policy which might prevent the tracee from doing so.
> Given that SELinux wants to be MAC, I'm not *quite* sure that's
> considered kosher.  You also are skipping the PROCESS__PTRACE
> to SECCLASS_PROCESS check which selinux_bprm_set_creds does later
> on.  Again I'm just not quite sure what's considered normal there
> these days.
>
> Paul, do you have input there?

I agree, the SELinux hook looks wrong.  Building on what Christian
said, this looks more like a ptrace operation than an exec operation.

-- 
paul moore
www.paul-moore.com