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Date:   Tue, 10 Sep 2019 14:44:41 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Eugene Syromiatnikov <esyr@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Christian Brauner <christian@brauner.io>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        "Dmitry V. Levin" <ldv@altlinux.org>,
        Eric Biederman <ebiederm@xmission.com>
Subject: Re: [PATCH] fork: fail on non-zero higher 32 bits of args.exit_signal
Message-ID: <20190910124440.GA25647@redhat.com>
References: <20190910115711.GA3755@asgard.redhat.com>
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On 09/10, Eugene Syromiatnikov wrote:
>
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2562,6 +2562,9 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
>  	if (copy_from_user(&args, uargs, size))
>  		return -EFAULT;
>  
> +	if (unlikely(((unsigned int)args.exit_signal) != args.exit_signal))
> +		return -EINVAL;

Hmm. Unless I am totally confused you found a serious bug...

Without CLONE_THREAD/CLONE_PARENT copy_process() blindly does

	p->exit_signal = args->exit_signal;

the valid_signal(sig) check in do_notify_parent() mostly saves us, but we
must not allow child->exit_signal < 0, if nothing else this breaks
thread_group_leader().

And afaics this patch doesn't fix this? I think we need the valid_signal()
check...

Oleg.