linux-kernel - Re: [PATCH v3 2/2] powerpc/mm: Tracking vDSO remap

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Message-ID: <5513DBE1.4070404@linux.vnet.ibm.com>
Date:	Thu, 26 Mar 2015 11:13:53 +0100
From:	Laurent Dufour <ldufour@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
CC:	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Jeff Dike <jdike@...toit.com>,
	Richard Weinberger <richard@....at>,
	Guan Xuetao <gxt@...c.pku.edu.cn>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Arnd Bergmann <arnd@...db.de>, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	user-mode-linux-devel@...ts.sourceforge.net,
	user-mode-linux-user@...ts.sourceforge.net,
	linux-arch@...r.kernel.org, linux-mm@...ck.org, cov@...eaurora.org,
	criu@...nvz.org
Subject: Re: [PATCH v3 2/2] powerpc/mm: Tracking vDSO remap

On 26/03/2015 10:48, Ingo Molnar wrote:
> 
> * Benjamin Herrenschmidt <benh@...nel.crashing.org> wrote:
> 
>>>> +#define __HAVE_ARCH_REMAP
>>>> +static inline void arch_remap(struct mm_struct *mm,
>>>> +			      unsigned long old_start, unsigned long old_end,
>>>> +			      unsigned long new_start, unsigned long new_end)
>>>> +{
>>>> +	/*
>>>> +	 * mremap() doesn't allow moving multiple vmas so we can limit the
>>>> +	 * check to old_start == vdso_base.
>>>> +	 */
>>>> +	if (old_start == mm->context.vdso_base)
>>>> +		mm->context.vdso_base = new_start;
>>>> +}
>>>
>>> mremap() doesn't allow moving multiple vmas, but it allows the 
>>> movement of multi-page vmas and it also allows partial mremap()s, 
>>> where it will split up a vma.
>>>
>>> In particular, what happens if an mremap() is done with 
>>> old_start == vdso_base, but a shorter end than the end of the vDSO? 
>>> (i.e. a partial mremap() with fewer pages than the vDSO size)
>>
>> Is there a way to forbid splitting ? Does x86 deal with that case at 
>> all or it doesn't have to for some other reason ?
> 
> So we use _install_special_mapping() - maybe PowerPC does that too? 
> That adds VM_DONTEXPAND which ought to prevent some - but not all - of 
> the VM API weirdnesses.

The same is done on PowerPC. So calling mremap() to extend the vDSO is
failing but splitting it or unmapping a part of it is allowed but lead
to an unusable vDSO.

> On x86 we'll just dump core if someone unmaps the vdso.

On PowerPC, you'll get the same result.

Should we prevent the user to break its vDSO ?

Thanks,
Laurent.

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