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    Subject[PATCH, RFC 46/62] x86/mm: Keep reference counts on encrypted VMAs for MKTME
    Date
    From: Alison Schofield <alison.schofield@intel.com>

    The MKTME (Multi-Key Total Memory Encryption) Key Service needs
    a reference count on encrypted VMAs. This reference count is used
    to determine when a hardware encryption KeyID is no longer in use
    and can be freed and reassigned to another Userspace Key.

    The MKTME Key service does the percpu_ref_init and _kill, so
    these gets/puts on encrypted VMA's can be considered the
    intermediaries in the lifetime of the key.

    Increment/decrement the reference count during encrypt_mprotect()
    system call for initial or updated encryption on a VMA.

    Piggy back on the vm_area_dup/free() helpers. If the VMAs being
    duplicated, or freed are encrypted, adjust the reference count.

    Signed-off-by: Alison Schofield <alison.schofield@intel.com>
    Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    ---
    arch/x86/include/asm/mktme.h | 5 +++++
    arch/x86/mm/mktme.c | 37 ++++++++++++++++++++++++++++++++++--
    include/linux/mm.h | 2 ++
    kernel/fork.c | 2 ++
    4 files changed, 44 insertions(+), 2 deletions(-)

    diff --git a/arch/x86/include/asm/mktme.h b/arch/x86/include/asm/mktme.h
    index 0e6df07f1921..14da002d2e85 100644
    --- a/arch/x86/include/asm/mktme.h
    +++ b/arch/x86/include/asm/mktme.h
    @@ -16,6 +16,11 @@ extern int mktme_keyid_shift;
    extern void mprotect_set_encrypt(struct vm_area_struct *vma, int newkeyid,
    unsigned long start, unsigned long end);

    +/* MTKME encrypt_count for VMAs */
    +extern struct percpu_ref *encrypt_count;
    +extern void vma_get_encrypt_ref(struct vm_area_struct *vma);
    +extern void vma_put_encrypt_ref(struct vm_area_struct *vma);
    +
    DECLARE_STATIC_KEY_FALSE(mktme_enabled_key);
    static inline bool mktme_enabled(void)
    {
    diff --git a/arch/x86/mm/mktme.c b/arch/x86/mm/mktme.c
    index 91b49e88ca3f..df70651816a1 100644
    --- a/arch/x86/mm/mktme.c
    +++ b/arch/x86/mm/mktme.c
    @@ -66,11 +66,12 @@ void mprotect_set_encrypt(struct vm_area_struct *vma, int newkeyid,

    if (oldkeyid == newkeyid)
    return;
    -
    + vma_put_encrypt_ref(vma);
    newprot = pgprot_val(vma->vm_page_prot);
    newprot &= ~mktme_keyid_mask;
    newprot |= (unsigned long)newkeyid << mktme_keyid_shift;
    vma->vm_page_prot = __pgprot(newprot);
    + vma_get_encrypt_ref(vma);

    /*
    * The VMA doesn't have any inherited pages.
    @@ -79,6 +80,18 @@ void mprotect_set_encrypt(struct vm_area_struct *vma, int newkeyid,
    unlink_anon_vmas(vma);
    }

    +void vma_get_encrypt_ref(struct vm_area_struct *vma)
    +{
    + if (vma_keyid(vma))
    + percpu_ref_get(&encrypt_count[vma_keyid(vma)]);
    +}
    +
    +void vma_put_encrypt_ref(struct vm_area_struct *vma)
    +{
    + if (vma_keyid(vma))
    + percpu_ref_put(&encrypt_count[vma_keyid(vma)]);
    +}
    +
    /* Prepare page to be used for encryption. Called from page allocator. */
    void __prep_encrypted_page(struct page *page, int order, int keyid, bool zero)
    {
    @@ -102,6 +115,22 @@ void __prep_encrypted_page(struct page *page, int order, int keyid, bool zero)

    page++;
    }
    +
    + /*
    + * Make sure the KeyID cannot be freed until the last page that
    + * uses the KeyID is gone.
    + *
    + * This is required because the page may live longer than VMA it
    + * is mapped into (i.e. in get_user_pages() case) and having
    + * refcounting per-VMA is not enough.
    + *
    + * Taking a reference per-4K helps in case if the page will be
    + * split after the allocation. free_encrypted_page() will balance
    + * out the refcount even if the page was split and freed as bunch
    + * of 4K pages.
    + */
    +
    + percpu_ref_get_many(&encrypt_count[keyid], 1 << order);
    }

    /*
    @@ -110,7 +139,9 @@ void __prep_encrypted_page(struct page *page, int order, int keyid, bool zero)
    */
    void free_encrypted_page(struct page *page, int order)
    {
    - int i;
    + int i, keyid;
    +
    + keyid = page_keyid(page);

    /*
    * The hardware/CPU does not enforce coherency between mappings
    @@ -125,6 +156,8 @@ void free_encrypted_page(struct page *page, int order)
    lookup_page_ext(page)->keyid = 0;
    page++;
    }
    +
    + percpu_ref_put_many(&encrypt_count[keyid], 1 << order);
    }

    static int sync_direct_mapping_pte(unsigned long keyid,
    diff --git a/include/linux/mm.h b/include/linux/mm.h
    index a7f52d053826..00c0fd70816b 100644
    --- a/include/linux/mm.h
    +++ b/include/linux/mm.h
    @@ -2831,6 +2831,8 @@ static inline void mprotect_set_encrypt(struct vm_area_struct *vma,
    int newkeyid,
    unsigned long start,
    unsigned long end) {}
    +static inline void vma_get_encrypt_ref(struct vm_area_struct *vma) {}
    +static inline void vma_put_encrypt_ref(struct vm_area_struct *vma) {}
    #endif /* CONFIG_X86_INTEL_MKTME */
    #endif /* __KERNEL__ */
    #endif /* _LINUX_MM_H */
    diff --git a/kernel/fork.c b/kernel/fork.c
    index 9dcd18aa210b..f0e35ed76f5a 100644
    --- a/kernel/fork.c
    +++ b/kernel/fork.c
    @@ -342,12 +342,14 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
    if (new) {
    *new = *orig;
    INIT_LIST_HEAD(&new->anon_vma_chain);
    + vma_get_encrypt_ref(new);
    }
    return new;
    }

    void vm_area_free(struct vm_area_struct *vma)
    {
    + vma_put_encrypt_ref(vma);
    kmem_cache_free(vm_area_cachep, vma);
    }

    --
    2.20.1
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