Date: Thu, 10 Aug 2017 03:58:28 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Cc: paulmck@linux.vnet.ibm.com, peterz@infradead.org,
        akpm@linux-foundation.org, ak@linux.intel.com, mhocko@kernel.org,
        dave@stgolabs.net, jack@suse.cz, Matthew Wilcox <willy@infradead.org>,
        benh@kernel.crashing.org, mpe@ellerman.id.au, paulus@samba.org,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        hpa@zytor.com, Will Deacon <will.deacon@arm.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        haren@linux.vnet.ibm.com, khandual@linux.vnet.ibm.com,
        npiggin@gmail.com, bsingharora@gmail.com,
        Tim Chen <tim.c.chen@linux.intel.com>, linuxppc-dev@lists.ozlabs.org,
        x86@kernel.org
Subject: Re: [PATCH 05/16] mm: Protect VMA modifications using VMA sequence
 count
Message-ID: <20170810005828.qmw3p7d676hjwkss@node.shutemov.name>
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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.

Nothing (apart from common sense) prevents compiler from generating this
kind of pattern.

-- 
 Kirill A. Shutemov