[PATCH v12 3/8] arm64: mte: Sync tags for pages where PTE is untagged

[Steven Price]

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).
+        */
+       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