linux-kernel - Re: [PATCH 05/16] mm: Protect VMA modifications using VMA sequence count

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Date:   Thu, 10 Aug 2017 16:43:25 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>
Cc:     paulmck@...ux.vnet.ibm.com, peterz@...radead.org,
        akpm@...ux-foundation.org, ak@...ux.intel.com, mhocko@...nel.org,
        dave@...olabs.net, jack@...e.cz,
        Matthew Wilcox <willy@...radead.org>, benh@...nel.crashing.org,
        mpe@...erman.id.au, paulus@...ba.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, khandual@...ux.vnet.ibm.com,
        npiggin@...il.com, bsingharora@...il.com,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH 05/16] mm: Protect VMA modifications using VMA sequence
 count

On Thu, Aug 10, 2017 at 10:27:50AM +0200, Laurent Dufour wrote:
> On 10/08/2017 02:58, Kirill A. Shutemov wrote:
> > On Wed, Aug 09, 2017 at 12:43:33PM +0200, Laurent Dufour wrote:
> >> On 09/08/2017 12:12, Kirill A. Shutemov wrote:
> >>> On Tue, Aug 08, 2017 at 04:35:38PM +0200, Laurent Dufour wrote:
> >>>> The VMA sequence count has been introduced to allow fast detection of
> >>>> VMA modification when running a page fault handler without holding
> >>>> the mmap_sem.
> >>>>
> >>>> This patch provides protection agains the VMA modification done in :
> >>>> 	- madvise()
> >>>> 	- mremap()
> >>>> 	- mpol_rebind_policy()
> >>>> 	- vma_replace_policy()
> >>>> 	- change_prot_numa()
> >>>> 	- mlock(), munlock()
> >>>> 	- mprotect()
> >>>> 	- mmap_region()
> >>>> 	- collapse_huge_page()
> >>>
> >>> I don't thinks it's anywhere near complete list of places where we touch
> >>> vm_flags. What is your plan for the rest?
> >>
> >> The goal is only to protect places where change to the VMA is impacting the
> >> page fault handling. If you think I missed one, please advise.
> > 
> > That's very fragile approach. We rely here too much on specific compiler behaviour.
> > 
> > Any write access to vm_flags can, in theory, be translated to several
> > write accesses. For instance with setting vm_flags to 0 in the middle,
> > which would result in sigfault on page fault to the vma.
> 
> Indeed, just setting vm_flags to 0 will not result in sigfault, the real
> job is done when the pte are updated and the bits allowing access are
> cleared. Access to the pte is controlled by the pte lock.
> Page fault handler is triggered based on the pte bits, not the content of
> vm_flags and the speculative page fault is checking for the vma again once
> the pte lock is held. So there is no concurrency when dealing with the pte
> bits.

Suppose we are getting page fault to readable VMA, pte is clear at the
time of page fault. In this case we need to consult vm_flags to check if
the vma is read-accessible.

If by the time of check vm_flags happend to be '0' we would get SIGSEGV as
the vma appears to be non-readable.

Where is my logic faulty?

> Regarding the compiler behaviour, there are memory barriers and locking
> which should prevent that.

Which locks barriers are you talking about?

We need at least READ_ONCE/WRITE_ONCE to access vm_flags everywhere.

-- 
 Kirill A. Shutemov