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

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Date:   Mon, 21 Dec 2020 13:49:55 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-mm <linux-mm@...ck.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Pavel Emelyanov <xemul@...nvz.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        stable <stable@...r.kernel.org>,
        Minchan Kim <minchan@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

> On Dec 21, 2020, at 1:24 PM, Yu Zhao <yuzhao@...gle.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@...il.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).

Yes, the commit message is completely off. Will fix.

Thanks for your guidance and assistance.

> 
> Reviewed-by: Yu Zhao <yuzhao@...gle.com>
> 
> While we are all here, there is also clear_soft_dirty() that could
> use a similar fix…

Let me try to build a small reproducer for clear_soft_dirty() and then I’ll
send another patch for it too.

BTW: In general, I think that you are right, and that changing of PTEs
should not require taking mmap_lock for write. However, I am not sure
cow_user_page() is not the only one that poses a problem and whether a more
systematic solution is needed. If cow_user_pages() is the only problem, do
you think it is possible to do the copying while holding the PTL? It works
for normal-pages, but I am not sure whether special-pages pose special
problems.

Anyhow, this is an enhancement that we can try later.

Thanks again,
Nadav