Re: [PATCH mm] kasan: reset page tags properly with sampling

[Andrew Morton]

On Tue, 24 Jan 2023 21:46:51 +0100 Andrey Konovalov <andreyknvl@xxxxxxxxx> wrote:

> On Tue, Jan 24, 2023 at 9:45 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Tue, 24 Jan 2023 21:35:26 +0100 andrey.konovalov@xxxxxxxxx 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.