[PATCH 3/3] arm64: kasan: Revert "arm64: mte: reset the page tag in page->flags"

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

This reverts commit e5b8d9218951e59df986f627ec93569a0d22149b.

On a system with MTE and KASAN_HW_TAGS enabled, when a page is allocated
kasan_unpoison_pages() sets a random tag and saves it in page->flags.
page_to_virt() re-creates the correct tagged pointer.

If such page is mapped in user-space with PROT_MTE, the architecture
code will set the tag to 0 and a subsequent page_to_virt() dereference
will fault. The reverted commit aimed to fix this by resetting the tag
in page->flags so that it is 0xff (match-all, not faulting). However,
setting the tags and flags can race with another CPU reading the flags
(page_to_virt()) and barriers can't help:

P0 (mte_sync_page_tags):	P1 (memcpy from virt_to_page):
				  Rflags!=0xff
  Wflags=0xff
  DMB (doesn't help)
  Wtags=0
				  Rtags=0   // fault

Since clearing the flags in the arch code doesn't help, revert the patch
altogether. In addition, remove the page_kasan_tag_reset() call in
tag_clear_highpage() since the core kasan code should take care of
resetting the page tag.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Peter Collingbourne <pcc@google.com>
---
 arch/arm64/kernel/hibernate.c | 5 -----
 arch/arm64/kernel/mte.c       | 9 ---------
 arch/arm64/mm/copypage.c      | 9 ---------
 arch/arm64/mm/fault.c         | 1 -
 arch/arm64/mm/mteswap.c       | 9 ---------
 5 files changed, 33 deletions(-)

diff --git a/arch/arm64/kernel/hibernate.c b/arch/arm64/kernel/hibernate.c
index 6328308be272..7754ef328657 100644
--- a/arch/arm64/kernel/hibernate.c
+++ b/arch/arm64/kernel/hibernate.c
@@ -300,11 +300,6 @@ 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);
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 78b3e0f8e997..90994aca54f3 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -47,15 +47,6 @@ static void mte_sync_page_tags(struct page *page, pte_t old_pte,
 	if (!pte_is_tagged)
 		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));
 }
 
diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
index b5447e53cd73..70a71f38b6a9 100644
--- a/arch/arm64/mm/copypage.c
+++ b/arch/arm64/mm/copypage.c
@@ -23,15 +23,6 @@ void copy_highpage(struct page *to, struct page *from)
 
 	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);
 	}
 }
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 77341b160aca..f2f21cd6d43f 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -926,6 +926,5 @@ struct page *alloc_zeroed_user_highpage_movable(struct vm_area_struct *vma,
 void tag_clear_highpage(struct page *page)
 {
 	mte_zero_clear_page_tags(page_address(page));
-	page_kasan_tag_reset(page);
 	set_bit(PG_mte_tagged, &page->flags);
 }
diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
index a9e50e930484..4334dec93bd4 100644
--- a/arch/arm64/mm/mteswap.c
+++ b/arch/arm64/mm/mteswap.c
@@ -53,15 +53,6 @@ bool mte_restore_tags(swp_entry_t entry, struct page *page)
 	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;