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

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

> On Jan 16, 2021, at 8:41 PM, Yu Zhao <yuzhao@google.com> wrote:
> 
> On Tue, Jan 12, 2021 at 09:43:38PM +0000, Will Deacon wrote:
>> On Tue, Jan 12, 2021 at 12:38:34PM -0800, Nadav Amit wrote:
>>>> On Jan 12, 2021, at 11:56 AM, Yu Zhao <yuzhao@google.com> wrote:
>>>> On Tue, Jan 12, 2021 at 11:15:43AM -0800, Nadav Amit wrote:
>>>>> 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).
>>>> 
>>>> I don't want to discourage you but I don't think this would end up
>>>> well. PPC doesn't necessarily follow one-page-struct-per-table rule,
>>>> and I've run into problems with this before while trying to do
>>>> something similar.
>>> 
>>> Discourage, discourage. Better now than later.
>>> 
>>> It will be relatively easy to extend the scheme to be per-VMA instead of
>>> per-table for architectures that prefer it this way. It does require
>>> TLB-generation tracking though, which Andy only implemented for x86, so I
>>> will focus on x86-64 right now.
>> 
>> Can you remind me of what we're missing on arm64 in this area, please? I'm
>> happy to help get this up and running once you have something I can build
>> on.
> 
> I noticed arm/arm64 don't support ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH.
> Would it be something worth pursuing? Arm has been using mm_cpumask,
> so it might not be too difficult I guess?

[ +Mel Gorman who implemented ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH ]

IIUC, there are at least two bugs in x86 implementation.

First, there is a missing memory barrier in tlbbatch_add_mm() between
inc_mm_tlb_gen() and the read of mm_cpumask().

Second, try_to_unmap_flush() clears flush_required after flushing. Another
thread can call set_tlb_ubc_flush_pending() after the flush and before
flush_required is cleared, and the indication that a TLB flush is pending
can be lost.

I am working on addressing these issues among others, but, as you already
saw, I am a bit slow.

On a different but related topic: Another thing that I noticed that Arm does
not do is batching TLB flushes across VMAs. Since Arm does not have its own
tlb_end_vma(), it uses the default tlb_end_vma(), which flushes each VMA
separately. Peter Zijlstra’s comment says that there are advantages in
flushing each VMA separately, but I am not sure it is better or intentional
(especially since x86 does not do so).

I am trying to remove the arch-specific tlb_end_vma() and have a config
option to control this behavior.

Again, sorry for being slow. I hope to send an RFC soon.