[PATCH] arm64: Reset KASAN tag in copy_highpage with HW tags only

[Peter Collingbourne]

On Tue, Feb 14, 2023 at 9:54 AM Catalin Marinas <catalin.marinas at arm.com> wrote:
>
> On Mon, Feb 13, 2023 at 05:52:14PM -0800, Peter Collingbourne wrote:
> > During page migration, the copy_highpage function is used to copy the
> > page data to the target page. If the source page is a userspace page
> > with MTE tags, the KASAN tag of the target page must have the match-all
> > tag in order to avoid tag check faults during subsequent accesses to the
> > page by the kernel. However, the target page may have been allocated in
> > a number of ways, some of which will use the KASAN allocator and will
> > therefore end up setting the KASAN tag to a non-match-all tag. Therefore,
> > update the target page's KASAN tag to match the source page.
> >
> > We ended up unintentionally fixing this issue as a result of a bad
> > merge conflict resolution between commit e059853d14ca ("arm64: mte:
> > Fix/clarify the PG_mte_tagged semantics") and commit 20794545c146 ("arm64:
> > kasan: Revert "arm64: mte: reset the page tag in page->flags""), which
> > preserved a tag reset for PG_mte_tagged pages which was considered to be
> > unnecessary at the time. Because SW tags KASAN uses separate tag storage,
> > update the code to only reset the tags when HW tags KASAN is enabled.
>
> Does KASAN_SW_TAGS work together with MTE?

Yes, it works fine. One of my usual kernel patch tests runs an
MTE-utilizing userspace program under a kernel with KASAN_SW_TAGS.

> In theory they should but I
> wonder whether we have other places calling page_kasan_tag_reset()
> without the kasan_hw_tags_enabled() check.

It's unclear to me whether any of the other references are
specifically related to KASAN_HW_TAGS or not. Because KASAN_SW_TAGS
also uses all-ones as a match-all tag, I wouldn't expect calling
page_kasan_tag_reset() to cause any problems aside from false
negatives.

> > Signed-off-by: Peter Collingbourne <pcc at google.com>
> > Link: https://linux-review.googlesource.com/id/If303d8a709438d3ff5af5fd85706505830f52e0c
> > Reported-by: "Kuan-Ying Lee (李冠穎)" <Kuan-Ying.Lee at mediatek.com>
> > Cc: <stable at vger.kernel.org> # 6.1
>
> What are we trying to fix? The removal of page_kasan_tag_reset() in
> copy_highpage()?

Yes.

> If yes, I think we should use:
>
> Fixes: 20794545c146 ("arm64: kasan: Revert "arm64: mte: reset the page tag in page->flags"")
> Cc: <stable at vger.kernel.org> # 6.0.x

I agree with the Fixes tag, but are you sure that 6.0.y is still
supported as a stable kernel release? kernel.org only lists 6.1, and I
don't see any updates to Greg's linux-6.0.y branch since January 12.

I'm having some email trouble at the moment so I can't send a v2, so
please feel free to add the Fixes tag yourself.

Peter