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Date:   Wed, 11 Sep 2019 15:31:20 +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 v2] fork: check exit_signal passed in clone3() call
Message-ID: <20190911133119.GA17580@redhat.com>
References: <20190910175852.GA15572@asgard.redhat.com>
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On 09/10, Eugene Syromiatnikov wrote:
>
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2338,6 +2338,8 @@ struct mm_struct *copy_init_mm(void)
>   *
>   * It copies the process, and if successful kick-starts
>   * it and waits for it to finish using the VM if required.
> + *
> + * args->exit_signal is expected to be checked for sanity by the caller.

not sure this comment is really useful but it doesn't hurt

>  long _do_fork(struct kernel_clone_args *args)
>  {
> @@ -2562,6 +2564,16 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
>  	if (copy_from_user(&args, uargs, size))
>  		return -EFAULT;
>  
> +	/*
> +	 * exit_signal is confined to CSIGNAL mask in legacy syscalls,
> +	 * so it is used unchecked deeper in syscall handling routines;
> +	 * moreover, copying to struct kernel_clone_args.exit_signals
> +	 * trims higher 32 bits, so it is has to be checked that they
> +	 * are zero.
> +	 */
> +	if (unlikely(args.exit_signal & ~((u64)CSIGNAL)))
> +		return -EINVAL;

OK, agreed. As you pointed out, this doesn't guarantee valid_signal(exit_signal).
But we do no really care as long as it is non-negative, it acts as exit_signal==0.

I have no idea if we want to deny exit_signal >= _NSIG in clone3(), this was always
allowed...

I think this needs the "CC: stable" tag.

Acked-by: Oleg Nesterov <oleg@redhat.com>