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    Subject[PATCH v17 1/6] arm64: mte: Sync tags for pages where PTE is untagged
    Date
    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 | 18 +++++++++++++-----
    3 files changed, 34 insertions(+), 10 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 125a10e413e9..69b3fde8759e 100644
    --- 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);
    }
    }

    --
    2.20.1
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