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/fault.c b/arch/arm64/mm/fault.c
index 629e886ceec4..b8b299d1736a 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -933,6 +933,8 @@ struct page *alloc_zeroed_user_highpage_movable(struct vm_area_struct *vma,
 
 void tag_clear_highpage(struct page *page)
 {
+	/* Newly allocated page, shouldn't have been tagged yet */
+	WARN_ON_ONCE(!try_page_mte_tagging(page));
 	mte_zero_clear_page_tags(page_address(page));
 	set_page_mte_tagged(page);
 }