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

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

> On Dec 23, 2020, at 8:23 AM, Will Deacon <will@kernel.org> wrote:
> 
> On Tue, Dec 22, 2020 at 11:20:21AM -0800, Nadav Amit wrote:
>>> On Dec 22, 2020, at 10:30 AM, Yu Zhao <yuzhao@google.com> wrote:
>>> 
>>> On Tue, Dec 22, 2020 at 04:40:32AM -0800, Nadav Amit wrote:
>>>>> On Dec 21, 2020, at 1:24 PM, Yu Zhao <yuzhao@google.com> wrote:
>>>>> 
>>>>> On Mon, Dec 21, 2020 at 12:26:22PM -0800, Linus Torvalds wrote:
>>>>>> On Mon, Dec 21, 2020 at 12:23 PM Nadav Amit <nadav.amit@gmail.com> wrote:
>>>>>>> Using mmap_write_lock() was my initial fix and there was a strong pushback
>>>>>>> on this approach due to its potential impact on performance.
>>>>>> 
>>>>>> From whom?
>>>>>> 
>>>>>> Somebody who doesn't understand that correctness is more important
>>>>>> than performance? And that userfaultfd is not the most important part
>>>>>> of the system?
>>>>>> 
>>>>>> The fact is, userfaultfd is CLEARLY BUGGY.
>>>>>> 
>>>>>>        Linus
>>>>> 
>>>>> Fair enough.
>>>>> 
>>>>> Nadav, for your patch (you might want to update the commit message).
>>>>> 
>>>>> Reviewed-by: Yu Zhao <yuzhao@google.com>
>>>>> 
>>>>> While we are all here, there is also clear_soft_dirty() that could
>>>>> use a similar fix…
>>>> 
>>>> Just an update as for why I have still not sent v2: I fixed
>>>> clear_soft_dirty(), created a reproducer, and the reproducer kept failing.
>>>> 
>>>> So after some debugging, it appears that clear_refs_write() does not flush
>>>> the TLB. It indeed calls tlb_finish_mmu() but since 0758cd830494
>>>> ("asm-generic/tlb: avoid potential double flush”), tlb_finish_mmu() does not
>>>> flush the TLB since there is clear_refs_write() does not call to
>>>> __tlb_adjust_range() (unless there are nested TLBs are pending).
>>> 
>>> Sorry Nadav, I assumed you knew this existing problem fixed by:
>>> https://patchwork.kernel.org/project/linux-mm/cover/20201210121110.10094-1-will@kernel.org/
>> 
>> Thanks, Yu! For some reason I assumed it was already upstreamed and did not
>> look back (yet if I was cc’d on v2…)
> 
> I'll repost in the new year, as it was a bit tight for the merge window.
> I've made a note to put you on cc.

No worries. I just like to complain. I read v1 but forgot.

> 
>> Yet, something still goes bad. Debugging.
> 
> Did you figure this out? I tried to read the whole thread, but it's a bit
> of a rollercoaster.

Yes, it was embarrassing bug of mine (not in any code sent). The
soft-dirty code is entangled and the deep nesting of the code
is unnecessary and confusing.

I tried not to change much to ease backporting and merging with
your pending patch, but some merging will be needed.