Re: [PATCH v1 1/2] mm: Clear uffd-wp PTE/PMD state on mremap()

[Peter Xu]

On Wed, Jan 15, 2025 at 04:58:06PM +0000, Ryan Roberts wrote:
> Hi Peter, David,

Hey, Ryan,

> 
> On 07/01/2025 14:47, Ryan Roberts wrote:
> > When mremap()ing a memory region previously registered with userfaultfd
> > as write-protected but without UFFD_FEATURE_EVENT_REMAP, an
> > inconsistency in flag clearing leads to a mismatch between the vma flags
> > (which have uffd-wp cleared) and the pte/pmd flags (which do not have
> > uffd-wp cleared). This mismatch causes a subsequent mprotect(PROT_WRITE)
> > to trigger a warning in page_table_check_pte_flags() due to setting the
> > pte to writable while uffd-wp is still set.
> > 
> > Fix this by always explicitly clearing the uffd-wp pte/pmd flags on any
> > such mremap() so that the values are consistent with the existing
> > clearing of VM_UFFD_WP. Be careful to clear the logical flag regardless
> > of its physical form; a PTE bit, a swap PTE bit, or a PTE marker. Cover
> > PTE, huge PMD and hugetlb paths.
> 
> I just noticed that Andrew sent this to Linus and it's now in his tree; I'm
> suddenly very nervous that it doesn't have any acks. I don't suppose you would
> be able to do a quick review to calm the nerves??

Heh, I fully trusted you, and I appreciated your help too. I'll need to run
for 1-2 hours, but I'll read it this afternoon.

Side note: no review is as good as tests on reliability POV if that was the
concern, but I'll try my best.

Thanks,

-- 
Peter Xu