[PATCH v15 2/7] arm64: mte: Sync tags for pages where PTE is untagged

Steven Price
2021-06-14T09:05:52Z

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>
---
arch/arm64/include/asm/mte.h | 4 ++--
arch/arm64/include/asm/pgtable.h | 22 +++++++++++++++++++---
arch/arm64/kernel/mte.c | 17 +++++++++++++----
3 files changed, 34 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index bc88a1ced0d7..347ef38a35f7 100644
--- 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 0b10204e72fc..db5402168841 100644
--- 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 a3583a7fd400..ae0a3c68fece 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -33,10 +33,10 @@ 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)
{
unsigned long flags;
- pte_t old_pte = READ_ONCE(*ptep);

spin_lock_irqsave(&tag_sync_lock, flags);

@@ -53,6 +53,9 @@ 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
@@ -69,16 +72,22 @@ static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
spin_unlock_irqrestore(&tag_sync_lock, flags);
}

-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_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);
}
}

--
2.20.1