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

[Catalin Marinas <catalin.marinas@arm.com>]

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

> Signed-off-by: Peter Collingbourne <pcc@google.com>
> Link: https://linux-review.googlesource.com/id/If303d8a709438d3ff5af5fd85706505830f52e0c
> Reported-by: "Kuan-Ying Lee (李冠穎)" <Kuan-Ying.Lee@mediatek.com>
> Cc: <stable@vger.kernel.org> # 6.1

What are we trying to fix? The removal of page_kasan_tag_reset() in
copy_highpage()? If yes, I think we should use:

Fixes: 20794545c146 ("arm64: kasan: Revert "arm64: mte: reset the page tag in page->flags"")
Cc: <stable@vger.kernel.org> # 6.0.x

-- 
Catalin