linux-kernel - Re: [PATCH v3 1/2] fork: extend clone3() to support CLONE_SET

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Message-ID: <20190807154828.GD24112@redhat.com>
Date:   Wed, 7 Aug 2019 17:48:29 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Adrian Reber <areber@...hat.com>
Cc:     Christian Brauner <christian@...uner.io>,
        Eric Biederman <ebiederm@...ssion.com>,
        Pavel Emelianov <xemul@...tuozzo.com>,
        Jann Horn <jannh@...gle.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        linux-kernel@...r.kernel.org, Andrei Vagin <avagin@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@...il.com>
Subject: Re: [PATCH v3 1/2] fork: extend clone3() to support CLONE_SET_TID

On 08/06, Adrian Reber wrote:
>
> @@ -2530,12 +2530,14 @@ 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)))
> +	/* The struct needs to be at least the size of the original struct. */
> +	if (size < (sizeof(struct clone_args) - sizeof(__aligned_u64)))
>  		return -EINVAL;

slightly off-topic, but with or without this patch I do not understand
-EINVAL. Can't we replace this check with

	if (size < sizeof(struct clone_args))
		memset((void*)&args + size, sizeof(struct clone_args) - size, 0);

?

this way we can new members at the end of clone_args and this matches
the "if (size > sizeof(struct clone_args))" block below which promises
that whatever we add into clone_args a zero value should work.


And if we do this

> +	if (size == sizeof(struct clone_args)) {
> +		/* Only check permissions if set_tid is actually set. */
> +		if (args.set_tid &&
> +			!ns_capable(pid_ns->user_ns, CAP_SYS_ADMIN))
> +			return -EPERM;
> +		kargs->set_tid = args.set_tid;
> +	}

we can move this check into clone3_args_valid() or even copy_process()

	if (kargs->set_tid) {
		if (!ns_capable(...))
			return -EPERM;
	}


Either way,

> @@ -2585,6 +2595,10 @@ static bool clone3_args_valid(const struct kernel_clone_args *kargs)
>  	if (kargs->flags & ~CLONE_LEGACY_FLAGS)
>  		return false;
>
> +	/* Fail if set_tid is invalid */
> +	if (kargs->set_tid < 0)
> +		return false;

I think it would be more clean to do this along with ns_capable() check,
or along with the "set_tid >= pid_max" check in alloc_pid().

I won't insist, but I do not really like the fact we check set_tid 3 times
in copy_clone_args_from_user(), clone3_args_valid(), and alloc_pid().

Oleg.