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

[Nadav Amit <nadav.amit@gmail.com>]

> On Dec 19, 2020, at 1:34 PM, Nadav Amit <nadav.amit@gmail.com> wrote:
> 
> [ cc’ing some more people who have experience with similar problems ]
> 
>> On Dec 19, 2020, at 11:15 AM, Andrea Arcangeli <aarcange@redhat.com> wrote:
>> 
>> Hello,
>> 
>> On Fri, Dec 18, 2020 at 08:30:06PM -0800, Nadav Amit wrote:
>>> Analyzing this problem indicates that there is a real bug since
>>> mmap_lock is only taken for read in mwriteprotect_range(). This might
>> 
>> Never having to take the mmap_sem for writing, and in turn never
>> blocking, in order to modify the pagetables is quite an important
>> feature in uffd that justifies uffd instead of mprotect. It's not the
>> most important reason to use uffd, but it'd be nice if that guarantee
>> would remain also for the UFFDIO_WRITEPROTECT API, not only for the
>> other pgtable manipulations.
>> 
>>> Consider the following scenario with 3 CPUs (cpu2 is not shown):
>>> 
>>> cpu0				cpu1
>>> ----				----
>>> userfaultfd_writeprotect()
>>> [ write-protecting ]
>>> mwriteprotect_range()
>>> mmap_read_lock()
>>> change_protection()
>>> change_protection_range()
>>>  ...
>>>  change_pte_range()
>>>  [ defer TLB flushes]
>>> 				userfaultfd_writeprotect()
>>> 				 mmap_read_lock()
>>> 				 change_protection()
>>> 				 [ write-unprotect ]
>>> 				 ...
>>> 				  [ unprotect PTE logically ]
>>> 				...
>>> 				[ page-fault]
>>> 				...
>>> 				wp_page_copy()
>>> 				[ set new writable page in PTE]
>> 
>> Can't we check mm_tlb_flush_pending(vma->vm_mm) if MM_CP_UFFD_WP_ALL
>> is set and do an explicit (potentially spurious) tlb flush before
>> write-unprotect?
> 
> There is a concrete scenario that I actually encountered and then there is a
> general problem.
> 
> In general, the kernel code assumes that PTEs that are read from the
> page-tables are coherent across all the TLBs, excluding permission promotion
> (i.e., the PTE may have higher permissions in the page-tables than those
> that are cached in the TLBs).
> 
> We therefore need to both: (a) protect change_protection_range() from the
> changes of others who might defer TLB flushes without taking mmap_sem for
> write (e.g., try_to_unmap_one()); and (b) to protect others (e.g.,
> page-fault handlers) from concurrent changes of change_protection().
> 
> We have already encountered several similar bugs, and debugging such issues
> s time consuming and these bugs impact is substantial (memory corruption,
> security). So I think we should only stick to general solutions.
> 
> So perhaps your the approach of your proposed solution is feasible, but it
> would have to be applied all over the place: we will need to add a check for
> mm_tlb_flush_pending() and conditionally flush the TLB in every case in
> which PTEs are read and there might be an assumption that the
> access-permission reflect what the TLBs hold. This includes page-fault
> handlers, but also NUMA migration code in change_protection(), softdirty
> cleanup in clear_refs_write() and maybe others.
> 
> [ I have in mind another solution, such as keeping in each page-table a 
> “table-generation” which is the mm-generation at the time of the change,
> and only flush if “table-generation”==“mm-generation”, but it requires
> some thought on how to avoid adding new memory barriers. ]
> 
> IOW: I think the change that you suggest is insufficient, and a proper
> solution is too intrusive for “stable".
> 
> As for performance, I can add another patch later to remove the TLB flush
> that is unnecessarily performed during change_protection_range() that does
> permission promotion. I know that your concern is about the “protect” case
> but I cannot think of a good immediate solution that avoids taking mmap_lock
> for write.
> 
> Thoughts?

On a second thought (i.e., I don’t know what I was thinking), doing so —
checking mm_tlb_flush_pending() on every PTE read which is potentially
dangerous and flushing if needed - can lead to huge amount of TLB flushes
and shootodowns as the counter might be elevated for considerable amount of
time.

So this solution seems to me as a no-go.