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

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Message-ID: <9221000a-161b-46ea-a065-ee339837aacb@lucifer.local>
Date: Wed, 15 Jan 2025 17:30:20 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Peter Xu <peterx@...hat.com>
Cc: Ryan Roberts <ryan.roberts@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Muchun Song <muchun.song@...ux.dev>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Shuah Khan <shuah@...nel.org>, David Hildenbrand <david@...hat.com>,
        Mikołaj Lenczewski <miko.lenczewski@....com>,
        Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH v1 1/2] mm: Clear uffd-wp PTE/PMD state on mremap()

On Wed, Jan 15, 2025 at 12:21:15PM -0500, Peter Xu wrote:
> 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.

Things go all inception though when part of the review _are_ the tests ;)
Though of course there are also all existing uffd tests and the bots that
add a bit of weight.

This isn't really my area so will defer to Peter on the review side.

I sort of favour putting hotfixes in quick, but this one has gone in
quicker than some reviewed hotfixes which we left in unstable... however
towards the end of a cycle I think Andrew is stuck between a rock and a
hard place in deciding how to handle these.

So I'm guessing the heuristic is 'allow to simmer in unstable if time
permits in cycle', if known 'good egg' + no objection + towards end of
cycle + hotfix - send.

I do wonder whether we should require review on hotfixes generally. But
then of course that creates rock + hard place decision for Andrew as to
whether it gets deferred to the next cycle + stable backports...

Maybe one to discuss at LSF?

>
> Thanks,
>
> --
> Peter Xu
>