[PATCH] arm64: mte: Ensure the cleared tags are visible before setting the PTE

[Catalin Marinas]

As an optimisation, only pages mapped with PROT_MTE in user space have
the MTE tags zeroed. This is done lazily at the set_pte_at() time via
mte_sync_tags(). However, this function is missing a barrier and another
CPU may see the PTE updated before the zeroed tags are visible. Add an
smp_wmb() barrier if the page tags have been updated.

Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
Fixes: 34bfeea4a9e9 ("arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE")
Cc: <stable at vger.kernel.org> # 5.10.x
Reported-by: Vladimir Murzin <vladimir.murzin at arm.com>
Cc: Will Deacon <will at kernel.org>
---
 arch/arm64/kernel/mte.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 78b3e0f8e997..07dabd52377d 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -34,18 +34,18 @@ DEFINE_STATIC_KEY_FALSE(mte_async_or_asymm_mode);
 EXPORT_SYMBOL_GPL(mte_async_or_asymm_mode);
 #endif
 
-static void mte_sync_page_tags(struct page *page, pte_t old_pte,
+static bool mte_sync_page_tags(struct page *page, pte_t old_pte,
 			       bool check_swap, bool pte_is_tagged)
 {
 	if (check_swap && is_swap_pte(old_pte)) {
 		swp_entry_t entry = pte_to_swp_entry(old_pte);
 
 		if (!non_swap_entry(entry) && mte_restore_tags(entry, page))
-			return;
+			return true;
 	}
 
 	if (!pte_is_tagged)
-		return;
+		return false;
 
 	page_kasan_tag_reset(page);
 	/*
@@ -57,6 +57,7 @@ static void mte_sync_page_tags(struct page *page, pte_t old_pte,
 	 */
 	smp_wmb();
 	mte_clear_page_tags(page_address(page));
+	return true;
 }
 
 void mte_sync_tags(pte_t old_pte, pte_t pte)
@@ -65,6 +66,7 @@ void mte_sync_tags(pte_t old_pte, pte_t pte)
 	long i, nr_pages = compound_nr(page);
 	bool check_swap = nr_pages == 1;
 	bool pte_is_tagged = pte_tagged(pte);
+	bool updated = false;
 
 	/* Early out if there's nothing to do */
 	if (!check_swap && !pte_is_tagged)
@@ -72,10 +74,15 @@ void mte_sync_tags(pte_t old_pte, pte_t pte)
 
 	/* if PG_mte_tagged is set, tags have already been initialised */
 	for (i = 0; i < nr_pages; i++, page++) {
-		if (!test_and_set_bit(PG_mte_tagged, &page->flags))
-			mte_sync_page_tags(page, old_pte, check_swap,
-					   pte_is_tagged);
+		if (!test_and_set_bit(PG_mte_tagged, &page->flags) &&
+		    mte_sync_page_tags(page, old_pte, check_swap,
+				       pte_is_tagged))
+			updated = true;
 	}
+
+	/* ensure the tags are visible before the PTE is set */
+	if (updated)
+		smp_wmb();
 }
 
 int memcmp_pages(struct page *page1, struct page *page2)