linux-kernel - Re: [PATCH v2] fork: check exit_signal passed in clone3() call

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Message-ID: <20190911145446.vkcqy2shldi5ibb5@wittgenstein>
Date:   Wed, 11 Sep 2019 16:54:47 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Eugene Syromiatnikov <esyr@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        Eric Biederman <ebiederm@...ssion.com>
Subject: Re: [PATCH v2] fork: check exit_signal passed in clone3() call

On Wed, Sep 11, 2019 at 03:32:13PM +0100, Eugene Syromiatnikov wrote:
> On Wed, Sep 11, 2019 at 04:16:36PM +0200, Christian Brauner wrote:
> > On Wed, Sep 11, 2019 at 03:52:36PM +0200, Christian Brauner wrote:
> > > On Wed, Sep 11, 2019 at 06:48:52AM -0700, Andrew Morton wrote:
> > > > What are the user-visible runtime effects of this bug?
> 
> The userspace can set -1 as an exit_signal, and that will break process
> signalling and reaping.
> 
> > > > Relatedly, should this fix be backported into -stable kernels?  If so, why?
> > > 
> > > No, as I said in my other mail clone3() is not in any released kernel
> > > yet. clone3() is going to be released in v5.3.
> > 
> > Sigh, I spoke to soon... Hm, this is placed in _do_fork(). There's a
> > chance that this might be visible in legacy clone if anyone passes in an
> > invalid signal greater than NSIG right now somehow, they'd now get
> > EINVAL if I'm seeing this right.
> > 
> > So an alternative might be to only fix this in clone3() only right now
> > and get this patch into 5.3 to not release clone3() with this bug from
> > legacy clone duplicated.
> > And we defer the actual legacy clone fix until after next merge window
> > having it stew in linux-next for a couple of rcs. Distros often pull in
> > rcs so if anyone notices a regression for legacy clone we'll know about
> > it... valid_signal() checks at process exit time when the parent is
> > supposed to be notifed will catch faulty signals anyway so it's not that
> > big of a deal.
> 
> As the patch is written, only copy_clone_args_from_user is touched (which
> is used only by clone3 and not legacy clone), and the check added

Great!

> replicates legacy clone behaviour: userspace can set 0..CSIGNAL
> as an exit_signal.   Having ability to set exit_signal in NSIG..CSIGNAL

Hm. The way I see it for clone3() it would make sense to only have <
NSIG right away. valid_signal() won't let through anything else
anyway... Since clone3() isn't out yet it doesn't make sense to
replicate the (buggy) behavior of legacy clone, right?

Christian