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

[Kirill A. Shutemov]

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