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

[Will Deacon]

On Tue, Dec 22, 2020 at 11:20:21AM -0800, Nadav Amit wrote:
> > On Dec 22, 2020, at 10:30 AM, Yu Zhao <yuzhao@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxxx>
> >>> 
> >>> 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@xxxxxxxxxx/
> > 
> 
> 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.

> 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.

Will