linux-kernel - Re: [PATCH 1/2] unshare: Unsharing a thread does not require unsharing a vm

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Date:	Wed, 12 Aug 2015 19:48:47 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Ricky Zhou <rickyz@...omium.org>,
	Julien Tinnes <jln@...gle.com>
Subject: Re: [PATCH 1/2] unshare: Unsharing a thread does not require
	unsharing a vm

On 08/11, Eric W. Biederman wrote:
>
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1866,13 +1866,17 @@ static int check_unshare_flags(unsigned long unshare_flags)
>  				CLONE_NEWUSER|CLONE_NEWPID))
>  		return -EINVAL;
>  	/*
> -	 * Not implemented, but pretend it works if there is nothing to
> -	 * unshare. Note that unsharing CLONE_THREAD or CLONE_SIGHAND
> -	 * needs to unshare vm.
> +	 * Not implemented, but pretend it works if there is nothing
> +	 * to unshare.  Note that unsharing the address space or the
> +	 * signal handlers also need to unshare the signal queues (aka
> +	 * CLONE_THREAD).
>  	 */
>  	if (unshare_flags & (CLONE_THREAD | CLONE_SIGHAND | CLONE_VM)) {
> -		/* FIXME: get_task_mm() increments ->mm_users */
> -		if (atomic_read(&current->mm->mm_users) > 1)
> +		if (!thread_group_empty(current))
> +			return -EINVAL;
> +	}
> +	if (unshare_flags & CLONE_VM) {
> +		if (!current_is_single_threaded())
>  			return -EINVAL;
>  	}

OK, but then you can remove "| CLONE_VM" from the previous check...


> @@ -1941,16 +1945,16 @@ SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
>  	if (unshare_flags & CLONE_NEWUSER)
>  		unshare_flags |= CLONE_THREAD | CLONE_FS;
>  	/*
> -	 * If unsharing a thread from a thread group, must also unshare vm.
> -	 */
> -	if (unshare_flags & CLONE_THREAD)
> -		unshare_flags |= CLONE_VM;

OK,

>  	/*
> +	 * If unsharing a signal handlers, must also unshare the signal queues.
> +	 */
> +	if (unshare_flags & CLONE_SIGHAND)
> +		unshare_flags |= CLONE_THREAD;

This looks unnecessary, check_unshare_flags() checks "THREAD | SIGHAND".
And to me the comment looks misleading although I won't argue.

And in fact this doesn't look exactly right, or I am totally confused.
Shouldn't we do

	if (unshare_flags & CLONE_SIGHAND)
		unshare_flags |= CLONE_VM;

? Or change check_unshare_flags()...

Otherwise suppose that a single threaded process does clone(VM | SIGHAND)
and (say) child does sys_unshare(SIGHAND). This will wrongly succeed afaics.

Oleg.

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