Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

[Nadav Amit <nadav.amit@gmail.com>]

> On Jan 12, 2021, at 11:02 AM, Laurent Dufour <ldufour@linux.vnet.ibm.com> wrote:
> 
> Le 12/01/2021 à 17:57, Peter Zijlstra a écrit :
>> On Tue, Jan 12, 2021 at 04:47:17PM +0100, Laurent Dufour wrote:
>>> Le 12/01/2021 à 12:43, Vinayak Menon a écrit :
>>>> Possibility of race against other PTE modifiers
>>>> 
>>>> 1) Fork - We have seen a case of SPF racing with fork marking PTEs RO and that
>>>> is described and fixed here https://lore.kernel.org/patchwork/patch/1062672/
>> Right, that's exactly the kind of thing I was worried about.
>>>> 2) mprotect - change_protection in mprotect which does the deferred flush is
>>>> marked under vm_write_begin/vm_write_end, thus SPF bails out on faults
>>>> on those VMAs.
>> Sure, mprotect also changes vm_flags, so it really needs that anyway.
>>>> 3) userfaultfd - mwriteprotect_range is not protected unlike in (2) above.
>>>> But SPF does not take UFFD faults.
>>>> 4) hugetlb - hugetlb_change_protection - called from mprotect and covered by
>>>> (2) above.
>>>> 5) Concurrent faults - SPF does not handle all faults. Only anon page faults.
>> What happened to shared/file-backed stuff? ISTR I had that working.
> 
> File-backed mappings are not processed in a speculative way, there were options to manage some of them depending on the underlying file system but that's still not done.
> 
> Shared anonymous mapping, are also not yet handled in a speculative way (vm_ops is not null).
> 
>>>> Of which do_anonymous_page and do_swap_page are NONE/NON-PRESENT->PRESENT
>>>> transitions without tlb flush. And I hope do_wp_page with RO->RW is fine as well.
>> The tricky one is demotion, specifically write to non-write.
>>>> I could not see a case where speculative path cannot see a PTE update done via
>>>> a fault on another CPU.
>> One you didn't mention is the NUMA balancing scanning crud; although I
>> think that's fine, loosing a PTE update there is harmless. But I've not
>> thought overly hard on it.
> 
> That's a good point, I need to double check on that side.
> 
>>> You explained it fine. Indeed SPF is handling deferred TLB invalidation by
>>> marking the VMA through vm_write_begin/end(), as for the fork case you
>>> mentioned. Once the PTL is held, and the VMA's seqcount is checked, the PTE
>>> values read are valid.
>> That should indeed work, but are we really sure we covered them all?
>> Should we invest in better TLBI APIs to make sure we can't get this
>> wrong?
> 
> That may be a good option to identify deferred TLB invalidation but I've no clue on what this API would look like.

I will send an RFC soon for per-table deferred TLB flushes tracking.
The basic idea is to save a generation in the page-struct that tracks
when deferred PTE change took place, and track whenever a TLB flush
completed. In addition, other users - such as mprotect - would use
the tlb_gather interface.

Unfortunately, due to limited space in page-struct this would only
be possible for 64-bit (and my implementation is only for x86-64).

It would still require to do the copying while holding the PTL though.