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

["Thomas Hellström (VMware)" <thomas_os@shipmail.org>]

On 11/18/19 11:22 AM, Kirill A. Shutemov wrote:
> 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@shipmail.org> 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.

Note that this was caught during a code audit rather than a real 
experienced problem. It looks to me like the only implementation that 
currently creates huge pud pagetable entries is dev_dax_huge_fault() 
which doesn't appear to care much about private (COW) mappings or 
write-tracking which is, I believe, a prerequisite for create_huge_pud() 
falling back on thread 1, but not in thread 2.

This means (assuming that's intentional) that a stable backport 
shouldn't be needed.

For the WIP huge page support for graphics memory we'll be allowing both 
COW mappings and write-tracking, though, but that's still some time away.

In any case, I think this patch needs -rc testing to catch potential 
pud_devmap issues before submitted to stable.

Thanks,

Thomas

>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>