Re: [PATCH RESEND v3] mm/userfaultfd: fix memory corruption due to writeprotect

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

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> On Mar 3, 2021, at 11:03 AM, Peter Xu <peterx@redhat.com> wrote:
> 
> On Wed, Mar 03, 2021 at 01:57:02AM -0800, Nadav Amit wrote:
>> From: Nadav Amit <namit@vmware.com>
>> 
>> Userfaultfd self-test fails occasionally, indicating a memory
>> corruption.
> 
> It's failing very constantly now for me after I got it run on a 40 cores
> system...  While indeed not easy to fail on my laptop.
> 

It fails rather constantly for me, but since nobody else reproduced it,
I was afraid to say otherwise ;-)

> 
>> Fixes: 292924b26024 ("userfaultfd: wp: apply _PAGE_UFFD_WP bit")
>> Signed-off-by: Nadav Amit <namit@vmware.com>
>> 
>> ---
>> v2->v3:
>> * Do not acquire mmap_lock for write, flush conditionally instead [Yu]
>> * Change the fixes tag to the patch that made the race apparent [Yu]
> 
> Did you forget about this one?  It would still be good to point to 09854ba94c6a
> just to show that 5.7/5.8 stable branches shouldn't need this patch as they're
> not prone to the tlb data curruption.  Maybe also cc stable with 5.9+?

The fixes tag is wrong, as you say. I will fix it and cc stable with 5.9+.

> 
>> * Removing patch to avoid write-protect on uffd unprotect. More
>>  comprehensive solution to follow (and avoid the TLB flush as well).
>> ---
>> mm/memory.c | 7 +++++++
>> 1 file changed, 7 insertions(+)
>> 
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 9e8576a83147..06da04f98936 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -3092,6 +3092,13 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>> 		return handle_userfault(vmf, VM_UFFD_WP);
>> 	}
>> 
>> +	/*
>> +	 * Userfaultfd write-protect can defer flushes. Ensure the TLB
>> +	 * is flushed in this case before copying.
>> +	 */
>> +	if (userfaultfd_wp(vmf->vma) && mm_tlb_flush_pending(vmf->vma->vm_mm))
>> +		flush_tlb_page(vmf->vma, vmf->address);
>> +
>> 	vmf->page = vm_normal_page(vma, vmf->address, vmf->orig_pte);
>> 	if (!vmf->page) {
>> 		/*
>> --
>> 2.25.1
>> 
> 
> Thanks for being consistent on fixing this problem.
> 
> Maybe it's even better to put that into a "unlikely" to reduce the affect of
> normal do_wp_page as much as possible?  But I'll leave it to others.
> 
> If with the fixes tag modified:
> 
> Reviewed-by: Peter Xu <peterx@redhat.com>
> Tested-by: Peter Xu <peterx@redhat.com>

Thanks, I will send v4 later today.

Regards,
Nadav


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