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

[Linus Torvalds]

On Mon, Dec 21, 2020 at 12:21 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>
> Well, unfortunately we have places that use optimizations like
>
>   inc_tlb_flush_pending()
>     lock page table
>       pte_wrprotect
>   flush_tlb_range()
>   dec_tlb_flush_pending()
>
> which complicate things.

My point is, none of that  matters.

Because the software side that does the actual page table
modifications do not depend on the TLB at all.

They depend on the page table lock, and the pte in memory.

So the "pending flush" simply shoudln't be an issue. It's about the
actual hardware usage.

But what DOES matter for the software accesses is that you can't
modify protections without holding the proper lock.

And userfaultfd seems to do exactly that, breaking the whole "load pte
early, then check that it didn't change".

(Which we do in other places too, not just COW - it's basically _the_
pattern for page table updates).

               Linus