arm64: mte: Sync tags for pages where PTE is untagged

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.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210621111716.37157-2-steven.price@arm.com

diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index bc88a1c..347ef38 100644 (file)
--- a/arch/arm64/include/asm/mte.h
+++ b/arch/arm64/include/asm/mte.h
@@ -37,7 +37,7 @@ void mte_free_tag_storage(char *storage);
 /* track which pages have valid allocation tags */
 #define PG_mte_tagged  PG_arch_2
 
-void mte_sync_tags(pte_t *ptep, pte_t pte);
+void mte_sync_tags(pte_t old_pte, pte_t pte);
 void mte_copy_page_tags(void *kto, const void *kfrom);
 void mte_thread_init_user(void);
 void mte_thread_switch(struct task_struct *next);
@@ -53,7 +53,7 @@ int mte_ptrace_copy_tags(struct task_struct *child, long request,
 /* unused if !CONFIG_ARM64_MTE, silence the compiler */
 #define PG_mte_tagged  0
 
-static inline void mte_sync_tags(pte_t *ptep, pte_t pte)
+static inline void mte_sync_tags(pte_t old_pte, pte_t pte)
 {
 }
 static inline void mte_copy_page_tags(void *kto, const void *kfrom)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 0b10204..db54021 100644 (file)
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -314,9 +314,25 @@ 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))
-               mte_sync_tags(ptep, pte);
+       /*
+        * 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).
+        */
+       if (system_supports_mte() && pte_access_permitted(pte, false) &&
+           !pte_special(pte)) {
+               pte_t old_pte = READ_ONCE(*ptep);
+               /*
+                * We only need to synchronise if the new PTE has tags enabled
+                * or if swapping in (in which case another mapping may have
+                * set tags in the past even if this PTE isn't tagged).
+                * (!pte_none() && !pte_present()) is an open coded version of
+                * is_swap_pte()
+                */
+               if (pte_tagged(pte) || (!pte_none(old_pte) && !pte_present(old_pte)))
+                       mte_sync_tags(old_pte, pte);
+       }
 
        __check_racy_pte_update(mm, ptep, pte);
 

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 125a10e..69b3fde 100644 (file)
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -32,10 +32,9 @@ 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 old_pte,
+                              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);
 
@@ -43,6 +42,9 @@ static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
                        return;
        }
 
+       if (!pte_is_tagged)
+               return;
+
        page_kasan_tag_reset(page);
        /*
         * We need smp_wmb() in between setting the flags and clearing the
@@ -55,16 +57,22 @@ static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
        mte_clear_page_tags(page_address(page));
 }
 
-void mte_sync_tags(pte_t *ptep, pte_t pte)
+void mte_sync_tags(pte_t old_pte, 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);
+
+       /* 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_and_set_bit(PG_mte_tagged, &page->flags))
-                       mte_sync_page_tags(page, ptep, check_swap);
+                       mte_sync_page_tags(page, old_pte, check_swap,
+                                          pte_is_tagged);
        }
 }