Re: [RFC PATCH v2 1/2] mm/userfaultfd: fix memory corruption due to writeprotect

[Nadav Amit <namit@vmware.com>]

> On Jan 4, 2021, at 11:24 AM, Andrea Arcangeli <aarcange@redhat.com> wrote:
> 
> Hello,
> 
> On Mon, Jan 04, 2021 at 01:22:27PM +0100, Peter Zijlstra wrote:
>> On Fri, Dec 25, 2020 at 01:25:28AM -0800, Nadav Amit wrote:
>> 
>>> The scenario that happens in selftests/vm/userfaultfd is as follows:
>>> 
>>> cpu0				cpu1			cpu2
>>> ----				----			----
>>> 							[ Writable PTE
>>> 							  cached in TLB ]
>>> userfaultfd_writeprotect()
>>> [ write-*unprotect* ]
>>> mwriteprotect_range()
>>> mmap_read_lock()
>>> change_protection()
>>> 
>>> change_protection_range()
>>> ...
>>> change_pte_range()
>>> [ *clear* “write”-bit ]
>>> [ defer TLB flushes ]
>>> 				[ page-fault ]
>>> 				...
>>> 				wp_page_copy()
>>> 				 cow_user_page()
>>> 				  [ copy page ]
>>> 							[ write to old
>>> 							  page ]
>>> 				...
>>> 				 set_pte_at_notify()
>> 
>> Yuck!
> 
> Note, the above was posted before we figured out the details so it
> wasn't showing the real deferred tlb flush that caused problems (the
> one showed on the left causes zero issues).

Actually it was posted after (note that this is v2). The aforementioned
scenario that Peter regards to is the one that I actually encountered (not
the second scenario that is “theoretical”). This scenario that Peter regards
is indeed more “stupid” in the sense that we should just not write-protect
the PTE on userfaultfd write-unprotect.

Let me know if I made any mistake in the description.

> The problematic one not pictured is the one of the wrprotect that has
> to be running in another CPU which is also isn't picture above. More
> accurate traces are posted later in the thread.

I think I included this scenario as well in the commit log (of v2). Let me
know if I screwed up and the description is not clear.