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Subject: Re: [PATCH v2 1/2] fork: extend clone3() to support CLONE_SET_TID
To:     Adrian Reber <areber@redhat.com>,
        Christian Brauner <christian@brauner.io>,
        Eric Biederman <ebiederm@xmission.com>,
        Pavel Emelianov <xemul@virtuozzo.com>,
        Jann Horn <jannh@google.com>, Oleg Nesterov <oleg@redhat.com>
Cc:     linux-kernel@vger.kernel.org, Andrei Vagin <avagin@gmail.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@gmail.com>
References: <20190731161223.2928-1-areber@redhat.com>
From:   Dmitry Safonov <0x7f454c46@gmail.com>
Message-ID: <417a9682-c7fc-fec4-3510-81a8aa7cd0af@gmail.com>
Date:   Wed, 31 Jul 2019 17:49:58 +0100
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Hi Adrian,

On 7/31/19 5:12 PM, Adrian Reber wrote:
[..]
> @@ -2530,14 +2530,12 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
>  					      struct clone_args __user *uargs,
>  					      size_t size)
>  {
> +	struct pid_namespace *pid_ns = task_active_pid_ns(current);
>  	struct clone_args args;
>  
>  	if (unlikely(size > PAGE_SIZE))
>  		return -E2BIG;
>  
> -	if (unlikely(size < sizeof(struct clone_args)))
> -		return -EINVAL;
> -

It might be better to validate it still somehow, but I don't insist.

[..]
> @@ -2578,11 +2580,16 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
>  
>  static bool clone3_args_valid(const struct kernel_clone_args *kargs)
>  {
> -	/*
> -	 * All lower bits of the flag word are taken.
> -	 * Verify that no other unknown flags are passed along.
> -	 */
> -	if (kargs->flags & ~CLONE_LEGACY_FLAGS)
> +	/* Verify that no other unknown flags are passed along. */
> +	if (kargs->flags & ~(CLONE_LEGACY_FLAGS | CLONE_SET_TID))
> +		return false;
> +
> +	/* Fail if set_tid is set without CLONE_SET_TID */
> +	if (kargs->set_tid && !(kargs->flags & CLONE_SET_TID))
> +		return false;
> +
> +	/* Also fail if set_tid is invalid */
> +	if ((kargs->set_tid <= 0) && (kargs->flags & CLONE_SET_TID))
>  		return false;

Sorry for not mentioning it on v1, but I've noticed it only now:
you check kargs->set_tid even with the legacy-sized kernel_clone_args,
which is probably some random value on a task's stack?

[..]

Thanks much,
          Dmitry