Re: [PATCH 0/5] 4.14 backports of fixes for "CoW after fork() issue"

[Vlastimil Babka]

On 4/21/21 10:01 PM, Suren Baghdasaryan wrote:
> On Wed, Apr 7, 2021 at 2:53 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>>
>> On Wed, Apr 7, 2021 at 12:23 PM Linus Torvalds
>> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>> >
>> > On Wed, Apr 7, 2021 at 11:47 AM Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>> > >
>> > > So, we fixed it, but we don't know why.
>> > >
>> > > Peter Xu's patchset that fixed it is here:
>> > > https://lore.kernel.org/lkml/20200821234958.7896-1-peterx@xxxxxxxxxx/
>> >
>> > Yeah, that's the part that ends up being really painful to backport
>> > (with all the subsequent fixes too), so the 4.14 people would prefer
>> > to avoid it.
>> >
>> > But I think that if it's a "requires dax pmem and ptrace on top", it
>> > may simply be a non-issue for those users. Although who knows - maybe
>> > that ends up being a real issue on Android..
>>
>> A lot to digest, so I need to do some reading now. Thanks everyone!
> 
> After a delay due to vacation I prepared backports of 17839856fd58
> ("gup: document and work around "COW can break either way" issue") for
> 4.14 and 4.19 kernels. As Linus pointed out, uffd-wp was introduced
> later in 5.7, so is not an issue for 4.x kernels. The issue with THPs
> is still unresolved, so with or without this patch it's still there
> (Android is not affected by this since we do not use THPs with older
> kernels).

Which THP issue do you mean here? The race that was part of the same Project
zero report and was solved by a different patch adding some locking? Or the
vmsplice info leak but applied to THP's? Because if it's the latter then I
believe 17839856fd58 did solve that too. It was the later switch of approach to
rely just on page_count() that left THP side unfixed.

> Andrea pointed out that there are other issues and to properly fix
> them his COR approach is needed. However it has not been accepted yet,
> so I can't really backport it. I'll be happy to do that though if it
> is accepted in the future.
> 
> Peter, you mentioned https://lkml.org/lkml/2020/8/10/439 patch to
> distinguish real writes vs enforced COW read requests, however I also
> see that you had a later version of this patch here:
> https://lore.kernel.org/patchwork/patch/1286506/. Which one should I
> backport? Or is it not needed in the absence of uffd-wp support in the
> earlier kernels?
> Thanks,
> Suren.
> 
>>
>> >
>> >             Linus
>