[PATCH v8 1/5] 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 which are !pte_valid_user() as these will
not have been swapped out.

Signed-off-by: Steven Price <steven.price@arm.com>
---
 arch/arm64/include/asm/pgtable.h |  2 +-
 arch/arm64/kernel/mte.c          | 16 ++++++++++++----
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 501562793ce2..27416d52f6a9 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -312,7 +312,7 @@ static inline void set_pte_at(struct mm_struct *mm, 
unsigned long addr,
                __sync_icache_dcache(pte);
 
        if (system_supports_mte() &&
-           pte_present(pte) && pte_tagged(pte) && !pte_special(pte))
+           pte_present(pte) && pte_valid_user(pte) && !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 dc9ada64feed..961031190227 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -25,17 +25,23 @@
 
 u64 gcr_kernel_excl __ro_after_init;
 
-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)
 {
        pte_t old_pte = READ_ONCE(*ptep);
 
        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))
+               if (!non_swap_entry(entry) && mte_restore_tags(entry, page)) {
+                       set_bit(PG_mte_tagged, &page->flags);
                        return;
+               }
        }
 
+       if (!pte_is_tagged || test_and_set_bit(PG_mte_tagged, &page->flags))
+               return;
+
        page_kasan_tag_reset(page);
        /*
         * We need smp_wmb() in between setting the flags and clearing the
@@ -53,11 +59,13 @@ void mte_sync_tags(pte_t *ptep, pte_t pte)
        struct page *page = pte_page(pte);
        long i, nr_pages = compound_nr(page);
        bool check_swap = nr_pages == 1;
+       bool pte_is_tagged = pte_tagged(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, ptep, check_swap);
+               if (!test_bit(PG_mte_tagged, &page->flags))
+                       mte_sync_page_tags(page, ptep, check_swap,
+                                          pte_is_tagged);
        }
 }
 
-- 
2.20.1