[patch 25/60] arm64: mte: reset the page tag in page->flags

[Andrew Morton <akpm@linux-foundation.org>]

From: Vincenzo Frascino <vincenzo.frascino@arm.com>
Subject: arm64: mte: reset the page tag in page->flags

The hardware tag-based KASAN for compatibility with the other modes stores
the tag associated to a page in page->flags.  Due to this the kernel
faults on access when it allocates a page with an initial tag and the user
changes the tags.

Reset the tag associated by the kernel to a page in all the meaningful
places to prevent kernel faults on access.

Note: An alternative to this approach could be to modify page_to_virt(). 
This though could end up being racy, in fact if a CPU checks the
PG_mte_tagged bit and decides that the page is not tagged but another CPU
maps the same with PROT_MTE and becomes tagged the subsequent kernel
access would fail.

Link: https://lkml.kernel.org/r/9073d4e973747a6f78d5bdd7ebe17f290d087096.1606161801.git.andreyknvl@google.com
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Branislav Rankov <Branislav.Rankov@arm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Marco Elver <elver@google.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 arch/arm64/kernel/hibernate.c |    5 +++++
 arch/arm64/kernel/mte.c       |    9 +++++++++
 arch/arm64/mm/copypage.c      |    9 +++++++++
 arch/arm64/mm/mteswap.c       |    9 +++++++++
 4 files changed, 32 insertions(+)

--- a/arch/arm64/kernel/hibernate.c~arm64-mte-reset-the-page-tag-in-page-flags
+++ a/arch/arm64/kernel/hibernate.c
@@ -371,6 +371,11 @@ static void swsusp_mte_restore_tags(void
 		unsigned long pfn = xa_state.xa_index;
 		struct page *page = pfn_to_online_page(pfn);
 
+		/*
+		 * It is not required to invoke page_kasan_tag_reset(page)
+		 * at this point since the tags stored in page->flags are
+		 * already restored.
+		 */
 		mte_restore_page_tags(page_address(page), tags);
 
 		mte_free_tag_storage(tags);
--- a/arch/arm64/kernel/mte.c~arm64-mte-reset-the-page-tag-in-page-flags
+++ a/arch/arm64/kernel/mte.c
@@ -34,6 +34,15 @@ static void mte_sync_page_tags(struct pa
 			return;
 	}
 
+	page_kasan_tag_reset(page);
+	/*
+	 * We need smp_wmb() in between setting the flags and clearing the
+	 * tags because if another thread reads page->flags and builds a
+	 * tagged address out of it, there is an actual dependency to the
+	 * memory access, but on the current thread we do not guarantee that
+	 * the new page->flags are visible before the tags were updated.
+	 */
+	smp_wmb();
 	mte_clear_page_tags(page_address(page));
 }
 
--- a/arch/arm64/mm/copypage.c~arm64-mte-reset-the-page-tag-in-page-flags
+++ a/arch/arm64/mm/copypage.c
@@ -23,6 +23,15 @@ void copy_highpage(struct page *to, stru
 
 	if (system_supports_mte() && test_bit(PG_mte_tagged, &from->flags)) {
 		set_bit(PG_mte_tagged, &to->flags);
+		page_kasan_tag_reset(to);
+		/*
+		 * We need smp_wmb() in between setting the flags and clearing the
+		 * tags because if another thread reads page->flags and builds a
+		 * tagged address out of it, there is an actual dependency to the
+		 * memory access, but on the current thread we do not guarantee that
+		 * the new page->flags are visible before the tags were updated.
+		 */
+		smp_wmb();
 		mte_copy_page_tags(kto, kfrom);
 	}
 }
--- a/arch/arm64/mm/mteswap.c~arm64-mte-reset-the-page-tag-in-page-flags
+++ a/arch/arm64/mm/mteswap.c
@@ -53,6 +53,15 @@ bool mte_restore_tags(swp_entry_t entry,
 	if (!tags)
 		return false;
 
+	page_kasan_tag_reset(page);
+	/*
+	 * We need smp_wmb() in between setting the flags and clearing the
+	 * tags because if another thread reads page->flags and builds a
+	 * tagged address out of it, there is an actual dependency to the
+	 * memory access, but on the current thread we do not guarantee that
+	 * the new page->flags are visible before the tags were updated.
+	 */
+	smp_wmb();
 	mte_restore_page_tags(page_address(page), tags);
 
 	return true;
_