linux-kernel - RE: [PATCH v5 5/6] prctl: Allow checkpoint/restore capable processes to change exe link

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Message-ID: <ba55d8b160e541429dc0c823d3240eb3@EXMBDFT11.ad.twosigma.com>
Date:   Wed, 15 Jul 2020 15:49:57 +0000
From:   Nicolas Viennot <Nicolas.Viennot@...sigma.com>
To:     Christian Brauner <christian.brauner@...ntu.com>,
        Adrian Reber <areber@...hat.com>
CC:     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>,
        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>,
        Serge Hallyn <serge@...lyn.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Arnd Bergmann <arnd@...db.de>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "selinux@...r.kernel.org" <selinux@...r.kernel.org>,
        Eric Paris <eparis@...isplace.org>,
        Jann Horn <jannh@...gle.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: RE: [PATCH v5 5/6] prctl: Allow checkpoint/restore capable processes
 to change exe link

> On Wed, Jul 15, 2020 at 04:49:53PM +0200, Adrian Reber wrote:
> > From: Nicolas Viennot <Nicolas.Viennot@...sigma.com>
> > 
> > Allow CAP_CHECKPOINT_RESTORE capable users to change /proc/self/exe.
> > 
> > This commit also changes the permission error code from -EINVAL to 
> > -EPERM for consistency with the rest of the prctl() syscall when 
> > checking capabilities.
> I agree that EINVAL seems weird here but this is a potentially user visible change. Might be nice to have the EINVAL->EPERM change be an additional patch on top after this one so we can revert it in case it breaks someone (unlikely though). I can split this out myself though so no need to resend for that alone.
> What I would also prefer is to have some history in the commit message tbh. The reason is that when we started discussing that specific change I had to hunt down the history of changing /proc/self/exe and had to dig up and read through ancient threads on lore to come up with the explanation why this is placed under a capability. The commit message should then also mention that there are other ways to change the /proc/self/exe link that don't require capabilities and that /proc/self/exe itself is not something userspace should rely on for security. Mainly so that in a few months/years we can read through that commit message and go "Weird, but ok.". :)
> But maybe I can just rewrite this myself so you don't have to go through the trouble. This is really not pedantry it's just that it's a lot of work digging up the reasons for a piece of code existing when it's really not obvious. :)

Hello Christian,

I agree.
Thank you for suggesting doing the work, but you've done plenty already. So we'll come back to you with:
1) A separate commit for EINVAL->EPERM
2) A full history of discussions in the commit message related to /proc/self/exe capability check

Thanks,
Nico