linux-kernel - Re: [PATCH v1] fsnotify: Pass correct offset to fsnotify_mmap

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Message-ID: <CAOQ4uxh+Mho71c93FNqcw=crw2H3yEs-uecWf4b6JMKYDTBCWQ@mail.gmail.com>
Date: Mon, 6 Oct 2025 16:40:00 +0200
From: Amir Goldstein <amir73il@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: Ryan Roberts <ryan.roberts@....com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>, 
	Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org, Jan Kara <jack@...e.com>
Subject: Re: [PATCH v1] fsnotify: Pass correct offset to fsnotify_mmap_perm()

On Mon, Oct 6, 2025 at 3:53 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 06.10.25 14:14, Ryan Roberts wrote:
> > On 06/10/2025 12:36, David Hildenbrand wrote:
> >> On 03.10.25 17:52, Ryan Roberts wrote:
> >>> fsnotify_mmap_perm() requires a byte offset for the file about to be
> >>> mmap'ed. But it is called from vm_mmap_pgoff(), which has a page offset.
> >>> Previously the conversion was done incorrectly so let's fix it, being
> >>> careful not to overflow on 32-bit platforms.
> >>>
> >>> Discovered during code review.
> >>>
> >>> Cc: <stable@...r.kernel.org>
> >>> Fixes: 066e053fe208 ("fsnotify: add pre-content hooks on mmap()")
> >>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> >>> ---
> >>> Applies against today's mm-unstable (aa05a436eca8).
> >>>
> >>
> >> Curious: is there some easy way to write a reproducer? Did you look into that?
> >
> > I didn't; this was just a drive-by discovery.
> >
> > It looks like there are some fanotify tests in the filesystems selftests; I
> > guess they could be extended to add a regression test?
> >
> > But FWIW, I think the kernel is just passing the ofset/length info off to user
> > space and isn't acting on it itself. So there is no kernel vulnerability here.
>
> Right, I'm rather wondering if this could have been caught earlier and
> how we could have caught it earlier :)

Ha! you would have thought we either have no test for it or we test
only mmap with offset 0.

But we have LTP test fanotify24 which does mmap with offset page_sz*100
and indeed it prints the info and info says offset 0, only we do not verify the
offset info in this test...

Will be fixed.

Thanks Ryan for being alert!
Amir.