Date:   Tue, 24 Jan 2023 13:11:22 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Andrey Konovalov <andreyknvl@gmail.com>
Cc:     andrey.konovalov@linux.dev, Marco Elver <elver@google.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        kasan-dev@googlegroups.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrey Konovalov <andreyknvl@google.com>,
        Peter Collingbourne <pcc@google.com>
Subject: Re: [PATCH mm] kasan: reset page tags properly with sampling
Message-Id: <20230124131122.fdbf6ae3069e6b0d05d14361@linux-foundation.org>
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        <20230124124504.2b21f0fde58af208a4f4e290@linux-foundation.org>
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On Tue, 24 Jan 2023 21:46:51 +0100 Andrey Konovalov <andreyknvl@gmail.com> wrote:

> On Tue, Jan 24, 2023 at 9:45 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > On Tue, 24 Jan 2023 21:35:26 +0100 andrey.konovalov@linux.dev wrote:
> >
> > > The implementation of page_alloc poisoning sampling assumed that
> > > tag_clear_highpage resets page tags for __GFP_ZEROTAGS allocations.
> > > However, this is no longer the case since commit 70c248aca9e7
> > > ("mm: kasan: Skip unpoisoning of user pages").
> > >
> > > This leads to kernel crashes when MTE-enabled userspace mappings are
> > > used with Hardware Tag-Based KASAN enabled.
> > >
> > > Reset page tags for __GFP_ZEROTAGS allocations in post_alloc_hook().
> > >
> > > Also clarify and fix related comments.
> >
> > I assume this is a fix against 44383cef54c0 ("kasan: allow sampling
> > page_alloc allocations for HW_TAGS") which is presently in mm-stable,
> > yes?
> 
> Correct. I assumed I shouldn't include a Fixes tag, as the patch is
> not in the mainline.

I think it's best to add the Fixes: if it's known.  If the patch was in
mm-unstable then I'd just fold the fix into the base patch, but a
Fixes: is still helpful because it tells people (especially me) which
patch needs the fix.

If the patch is in mm-stable then the SHA is stable and the Fixes: is
desirable for people who are backporting the base patch into earlier
kernels - hopefully when doing this they know to search the tree for
other patches which fix the patch which they are backporting.