Re: [PATCH 2/2] mm: Fix a huge pud insertion race during faulting

[Kirill A. Shutemov]

On Fri, Nov 15, 2019 at 11:58:00AM -0800, Andrew Morton wrote:
> On Fri, 15 Nov 2019 12:58:08 +0100 Thomas Hellström (VMware) <thomas_os@xxxxxxxxxxxx> wrote:
> 
> > A huge pud page can theoretically be faulted in racing with pmd_alloc()
> > in __handle_mm_fault(). That will lead to pmd_alloc() returning an
> > invalid pmd pointer. Fix this by adding a pud_trans_unstable() function
> > similar to pmd_trans_unstable() and check whether the pud is really stable
> > before using the pmd pointer.
> > 
> > Race:
> > Thread 1:             Thread 2:                 Comment
> > create_huge_pud()                               Fallback - not taken.
> > 		      create_huge_pud()         Taken.
> > pmd_alloc()                                     Returns an invalid pointer.
> 
> What are the user-visible runtime effects of this change?

Data corruption: kernel writes to a huge page thing it's page table.

> Is a -stable backport warranted?

I believe it is.

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov