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

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Message-ID: <20190913090732.lzw4dpfj5j72ivy2@wittgenstein>
Date:   Fri, 13 Sep 2019 11:07:33 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Eugene Syromiatnikov <esyr@...hat.com>,
        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 03:52:36PM +0200, Christian Brauner wrote:
> On Wed, Sep 11, 2019 at 06:48:52AM -0700, Andrew Morton wrote:
> > On Tue, 10 Sep 2019 18:58:52 +0100 Eugene Syromiatnikov <esyr@...hat.com> wrote:
> > 
> > > Previously, higher 32 bits of exit_signal fields were lost when
> > > copied to the kernel args structure (that uses int as a type for the
> > > respective field).  Moreover, as Oleg has noted[1], exit_signal is used
> > > unchecked, so it has to be checked for sanity before use; for the legacy
> > > syscalls, applying CSIGNAL mask guarantees that it is at least non-negative;
> > > however, there's no such thing is done in clone3() code path, and that can
> > > break at least thread_group_leader.
> > > 
> > > Checking user-passed exit_signal against ~CSIGNAL mask solves both
> > > of these problems.
> > > 
> > > [1] https://lkml.org/lkml/2019/9/10/467
> > > 
> > > * kernel/fork.c (copy_clone_args_from_user): Fail with -EINVAL if
> > > args.exit_signal has bits set outside CSIGNAL mask.
> > > (_do_fork): Note that exit_signal is expected to be checked for the
> > > sanity by the caller.
> > > 
> > > Fixes: 7f192e3cd316 ("fork: add clone3")
> > 
> > What are the user-visible runtime effects of this bug?
> > 
> > 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.

Applied yesteday. This is now fixed and included in mainline.

Thanks!
Christian