On 17/05/2021 17:14, Marc Zyngier wrote:

On Mon, 17 May 2021 13:32:34 +0100,
Steven Price [off-list ref] wrote:

quoted

A KVM guest could store tags in a page even if the VMM hasn't mapped
the page with PROT_MTE. So when restoring pages from swap we will
need to check to see if there are any saved tags even if !pte_tagged().

However don't check pages for which pte_access_permitted() returns false
as these will not have been swapped out.

Signed-off-by: Steven Price <steven.price@arm.com>
---
 arch/arm64/include/asm/pgtable.h |  9 +++++++--
 arch/arm64/kernel/mte.c          | 16 ++++++++++++++--
 2 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 0b10204e72fc..275178a810c1 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h

@@ -314,8 +314,13 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
 	if (pte_present(pte) && pte_user_exec(pte) && !pte_special(pte))
 		__sync_icache_dcache(pte);
 
-	if (system_supports_mte() &&
-	    pte_present(pte) && pte_tagged(pte) && !pte_special(pte))
+	/*
+	 * If the PTE would provide user space access to the tags associated
+	 * with it then ensure that the MTE tags are synchronised.  Exec-only
+	 * mappings don't expose tags (instruction fetches don't check tags).

I'm not sure I understand this comment. Of course, execution doesn't
match tags. But the memory could still have tags associated with
it. Does this mean such a page would lose its tags is swapped out?

Hmm, I probably should have reread that - the context of the comment is
lost.

I added the comment when changing to pte_access_permitted(), and the
comment on pte_access_permitted() explains a potential gotcha:

 * p??_access_permitted() is true for valid user mappings (PTE_USER
 * bit set, subject to the write permission check). For execute-only
 * mappings, like PROT_EXEC with EPAN (both PTE_USER and PTE_UXN bits
 * not set) must return false. PROT_NONE mappings do not have the
 * PTE_VALID bit set.

So execute-only mappings return false even though that is effectively a
type of user access. However, because MTE checks are not performed by
the PE for instruction fetches this doesn't matter. I'll update the
comment, how about:

/*
 * If the PTE would provide user space access to the tags associated
 * with it then ensure that the MTE tags are synchronised.  Although
 * pte_access_permitted() returns false for exec only mappings, they
 * don't expose tags (instruction fetches don't check tags).
 */

Thanks,

Steve

Thanks,

	M.

quoted

+	 */
+	if (system_supports_mte() && pte_present(pte) &&
+	    pte_access_permitted(pte, false) && !pte_special(pte))
 		mte_sync_tags(ptep, pte);
 
 	__check_racy_pte_update(mm, ptep, pte);

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index c88e778c2fa9..a604818c52c1 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c

@@ -33,11 +33,15 @@ DEFINE_STATIC_KEY_FALSE(mte_async_mode);
 EXPORT_SYMBOL_GPL(mte_async_mode);
 #endif
 
-static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
+static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap,
+			       bool pte_is_tagged)
 {
 	unsigned long flags;
 	pte_t old_pte = READ_ONCE(*ptep);
 
+	if (!is_swap_pte(old_pte) && !pte_is_tagged)
+		return;
+
 	spin_lock_irqsave(&tag_sync_lock, flags);
 
 	/* Recheck with the lock held */

@@ -53,6 +57,9 @@ static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
 		}
 	}
 
+	if (!pte_is_tagged)
+		goto out;
+
 	page_kasan_tag_reset(page);
 	/*
 	 * We need smp_wmb() in between setting the flags and clearing the

@@ -76,10 +83,15 @@ void mte_sync_tags(pte_t *ptep, pte_t pte)
 	bool check_swap = nr_pages == 1;
 	bool pte_is_tagged = pte_tagged(pte);
 
+	/* Early out if there's nothing to do */
+	if (!check_swap && !pte_is_tagged)
+		return;
+
 	/* if PG_mte_tagged is set, tags have already been initialised */
 	for (i = 0; i < nr_pages; i++, page++) {
 		if (!test_bit(PG_mte_tagged, &page->flags))
-			mte_sync_page_tags(page, ptep, check_swap);
+			mte_sync_page_tags(page, ptep, check_swap,
+					   pte_is_tagged);
 	}
 }
 

-- 
2.20.1

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