Re: [PATCH v2 2/6] mm/hugetlb: Fix uffd-wp bit lost when unsharing happens

[Peter Xu]

Hi, Andrew,

On Mon, Apr 17, 2023 at 04:48:22PM -0700, Andrew Morton wrote:
> On Mon, 17 Apr 2023 15:53:13 -0400 Peter Xu <peterx@xxxxxxxxxx> wrote:
> 
> > When we try to unshare a pinned page for a private hugetlb, uffd-wp bit can
> > get lost during unsharing.  Fix it by carrying it over.
> > 
> > This should be very rare, only if an unsharing happened on a private
> > hugetlb page with uffd-wp protected (e.g. in a child which shares the same
> > page with parent with UFFD_FEATURE_EVENT_FORK enabled).
> 
> What are the user-visible consequences of the bug?

When above condition met, one can lose uffd-wp bit on the privately mapped
hugetlb page.  It allows the page to be writable even if it should still be
wr-protected.  I assume it can mean data loss.

However it's very hard to trigger. When I wrote the reproducer (provided in
the last patch) I needed to use the newest gup_test cmd introduced by David
to trigger it because I don't even know another way to do a proper RO
longerm pin.

Besides that, it needs a bunch of other conditions all met:

        (1) hugetlb being mapped privately,
        (2) userfaultfd registered with WP and EVENT_FORK,
        (3) the user app fork()s, then,
        (4) RO longterm pin onto a wr-protected anonymous page.

If it's not impossible to hit in production I'd say extremely rare.

> 
> > Cc: linux-stable <stable@xxxxxxxxxxxxxxx>
> 
> When proposing a backport, it's better to present the patch as a
> standalone thing, against current -linus.  I'll then queue it in
> mm-hotfixes and shall send it upstream during this -rc cycle.
> 
> As presented, this patch won't go upstream until after 6.3 is released,
> and as it comes later in time, more backporting effort might be needed.
> 
> I can rework things if this fix is reasonably urgent (the "user-visible
> consequences" info is the guide).  If not urgent, we can leave things
> as they are.

IMHO it's not urgent so suitable for mm-unstable (current base of this set;
sorry if I forgot to mention it explicitly).  I'll post (and remember to
post) patches on top of mm-stable if they're urgent, or e.g. bugs
introduced in current release.

I copied stable for the pure logic of fixing a bug in old kernels.  The
consequence of hitting the bug is very bad but chance to hit is very low.

Thanks,

-- 
Peter Xu