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

[Peter Zijlstra]

On Mon, Jan 04, 2021 at 02:24:38PM -0500, Andrea Arcangeli wrote:
> 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).
> 
> 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.

Lets assume CPU0 does a read-lock, W -> RO with deferred flush.

> > Isn't this all rather similar to the problem that resulted in the
> > tlb_flush_pending mess?
> > 
> > I still think that's all fundamentally buggered, the much saner solution
> > (IMO) would've been to make things wait for the pending flush, instead
> 
> How do intend you wait in PT lock while the writer also has to take PT
> lock repeatedly before it can do wake_up_var?
> 
> If you release the PT lock before calling wait_tlb_flush_pending it
> all falls apart again.

I suppose you can check for pending, if found, release lock, wait for 0,
and re-take the fault?

> This I guess explains why a local pte/hugepmd smp local invlpg is the
> only working solution for this issue, similarly to how it's done in rmap.

In that case a local invalidate on CPU1 simply doesn't help anything.

CPU1 needs to do a global invalidate or wait for the in-progress one to
complete, such that CPU2 is sure to not have a W entry left before CPU1
goes and copies the page.