arm64: mte: Lock a page for MTE tag initialisation

Initialising the tags and setting PG_mte_tagged flag for a page can race
between multiple set_pte_at() on shared pages or setting the stage 2 pte
via user_mem_abort(). Introduce a new PG_mte_lock flag as PG_arch_3 and
set it before attempting page initialisation. Given that PG_mte_tagged
is never cleared for a page, consider setting this flag to mean page
unlocked and wait on this bit with acquire semantics if the page is
locked:

- try_page_mte_tagging() - lock the page for tagging, return true if it
  can be tagged, false if already tagged. No acquire semantics if it
  returns true (PG_mte_tagged not set) as there is no serialisation with
  a previous set_page_mte_tagged().

- set_page_mte_tagged() - set PG_mte_tagged with release semantics.

The two-bit locking is based on Peter Collingbourne's idea.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Peter Collingbourne <pcc@google.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221104011041.290951-6-pcc@google.com

1 files changed, 2 insertions, 0 deletions

diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
index 731d8a35701e..8dd5a8fe64b4 100644
--- a/arch/arm64/mm/copypage.c
+++ b/arch/arm64/mm/copypage.c
@@ -23,6 +23,8 @@ void copy_highpage(struct page *to, struct page *from)
 
 	if (system_supports_mte() && page_mte_tagged(from)) {
 		page_kasan_tag_reset(to);
+		/* It's a new page, shouldn't have been tagged yet */
+		WARN_ON_ONCE(!try_page_mte_tagging(to));
 		mte_copy_page_tags(kto, kfrom);
 		set_page_mte_tagged(to);
 	}