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Date:   Mon, 30 Oct 2023 09:16:20 -0700
From:   Dionna Amalie Glaze <dionnaglaze@...gle.com>
To:     Nikunj A Dadhania <nikunj@....com>
Cc:     linux-kernel@...r.kernel.org, thomas.lendacky@....com,
        x86@...nel.org, kvm@...r.kernel.org, bp@...en8.de,
        mingo@...hat.com, tglx@...utronix.de, dave.hansen@...ux.intel.com,
        pgonda@...gle.com, seanjc@...gle.com, pbonzini@...hat.com
Subject: Re: [PATCH v5 05/14] virt: sev-guest: Add vmpck_id to snp_guest_dev struct

On Sun, Oct 29, 2023 at 11:38 PM Nikunj A Dadhania <nikunj@....com> wrote:
>
> Drop vmpck and os_area_msg_seqno pointers so that secret page layout
> does not need to be exposed to the sev-guest driver after the rework.
> Instead, add helper APIs to access vmpck and os_area_msg_seqno when
> needed.
>
> Also, change function is_vmpck_empty() to snp_is_vmpck_empty() in
> preparation for moving to sev.c.
>
> Signed-off-by: Nikunj A Dadhania <nikunj@....com>
> ---
>  drivers/virt/coco/sev-guest/sev-guest.c | 85 ++++++++++++-------------
>  1 file changed, 42 insertions(+), 43 deletions(-)
>
> diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
> index 5801dd52ffdf..4dd094c73e2f 100644
> --- a/drivers/virt/coco/sev-guest/sev-guest.c
> +++ b/drivers/virt/coco/sev-guest/sev-guest.c
> @@ -50,8 +50,7 @@ struct snp_guest_dev {
>
>         struct snp_secrets_page_layout *layout;
>         struct snp_req_data input;
> -       u32 *os_area_msg_seqno;
> -       u8 *vmpck;
> +       unsigned int vmpck_id;
>  };
>
>  static u32 vmpck_id;
> @@ -61,14 +60,22 @@ MODULE_PARM_DESC(vmpck_id, "The VMPCK ID to use when communicating with the PSP.
>  /* Mutex to serialize the shared buffer access and command handling. */
>  static DEFINE_MUTEX(snp_cmd_mutex);
>
> -static bool is_vmpck_empty(struct snp_guest_dev *snp_dev)
> +static inline u8 *snp_get_vmpck(struct snp_guest_dev *snp_dev)
>  {
> -       char zero_key[VMPCK_KEY_LEN] = {0};
> +       return snp_dev->layout->vmpck0 + snp_dev->vmpck_id * VMPCK_KEY_LEN;
> +}
>
> -       if (snp_dev->vmpck)
> -               return !memcmp(snp_dev->vmpck, zero_key, VMPCK_KEY_LEN);
> +static inline u32 *snp_get_os_area_msg_seqno(struct snp_guest_dev *snp_dev)
> +{
> +       return &snp_dev->layout->os_area.msg_seqno_0 + snp_dev->vmpck_id;
> +}
>
> -       return true;
> +static bool snp_is_vmpck_empty(struct snp_guest_dev *snp_dev)
> +{
> +       char zero_key[VMPCK_KEY_LEN] = {0};
> +       u8 *key = snp_get_vmpck(snp_dev);
> +
> +       return !memcmp(key, zero_key, VMPCK_KEY_LEN);
>  }
>
>  /*
> @@ -90,20 +97,22 @@ static bool is_vmpck_empty(struct snp_guest_dev *snp_dev)
>   */
>  static void snp_disable_vmpck(struct snp_guest_dev *snp_dev)
>  {
> +       u8 *key = snp_get_vmpck(snp_dev);
> +
>         dev_alert(snp_dev->dev, "Disabling vmpck_id %d to prevent IV reuse.\n",
> -                 vmpck_id);
> -       memzero_explicit(snp_dev->vmpck, VMPCK_KEY_LEN);
> -       snp_dev->vmpck = NULL;
> +                 snp_dev->vmpck_id);
> +       memzero_explicit(key, VMPCK_KEY_LEN);
>  }

We disable the VMPCK because we believe the guest to be under attack,
but this only clears a single key. Shouldn't we clear all VMPCK keys
in the secrets page for good measure? If at VMPCK > 0, most likely the
0..VMPCK-1 keys have been zeroed by whatever was prior in the boot
stack, but still better to be safe.

>
>  static inline u64 __snp_get_msg_seqno(struct snp_guest_dev *snp_dev)
>  {
> +       u32 *os_area_msg_seqno = snp_get_os_area_msg_seqno(snp_dev);
>         u64 count;
>
>         lockdep_assert_held(&snp_dev->cmd_mutex);
>
>         /* Read the current message sequence counter from secrets pages */
> -       count = *snp_dev->os_area_msg_seqno;
> +       count = *os_area_msg_seqno;
>
>         return count + 1;
>  }
> @@ -131,11 +140,13 @@ static u64 snp_get_msg_seqno(struct snp_guest_dev *snp_dev)
>
>  static void snp_inc_msg_seqno(struct snp_guest_dev *snp_dev)
>  {
> +       u32 *os_area_msg_seqno = snp_get_os_area_msg_seqno(snp_dev);
> +
>         /*
>          * The counter is also incremented by the PSP, so increment it by 2
>          * and save in secrets page.
>          */
> -       *snp_dev->os_area_msg_seqno += 2;
> +       *os_area_msg_seqno += 2;
>  }
>
>  static inline struct snp_guest_dev *to_snp_dev(struct file *file)
> @@ -145,15 +156,22 @@ static inline struct snp_guest_dev *to_snp_dev(struct file *file)
>         return container_of(dev, struct snp_guest_dev, misc);
>  }
>
> -static struct aesgcm_ctx *snp_init_crypto(u8 *key, size_t keylen)
> +static struct aesgcm_ctx *snp_init_crypto(struct snp_guest_dev *snp_dev)
>  {
>         struct aesgcm_ctx *ctx;
> +       u8 *key;
> +
> +       if (snp_is_vmpck_empty(snp_dev)) {
> +               pr_err("SNP: vmpck id %d is null\n", snp_dev->vmpck_id);
> +               return NULL;
> +       }
>
>         ctx = kzalloc(sizeof(*ctx), GFP_KERNEL_ACCOUNT);
>         if (!ctx)
>                 return NULL;
>
> -       if (aesgcm_expandkey(ctx, key, keylen, AUTHTAG_LEN)) {
> +       key = snp_get_vmpck(snp_dev);
> +       if (aesgcm_expandkey(ctx, key, VMPCK_KEY_LEN, AUTHTAG_LEN)) {
>                 pr_err("SNP: crypto init failed\n");
>                 kfree(ctx);
>                 return NULL;
> @@ -586,7 +604,7 @@ static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long
>         mutex_lock(&snp_dev->cmd_mutex);
>
>         /* Check if the VMPCK is not empty */
> -       if (is_vmpck_empty(snp_dev)) {
> +       if (snp_is_vmpck_empty(snp_dev)) {
>                 dev_err_ratelimited(snp_dev->dev, "VMPCK is disabled\n");
>                 mutex_unlock(&snp_dev->cmd_mutex);
>                 return -ENOTTY;
> @@ -656,32 +674,14 @@ static const struct file_operations snp_guest_fops = {
>         .unlocked_ioctl = snp_guest_ioctl,
>  };
>
> -static u8 *get_vmpck(int id, struct snp_secrets_page_layout *layout, u32 **seqno)
> +bool snp_assign_vmpck(struct snp_guest_dev *dev, int vmpck_id)
>  {
> -       u8 *key = NULL;
> +       if (WARN_ON(vmpck_id > 3))
> +               return false;

The vmpck_id is an int for some reason, so < 0 is also a problem. Can
we not use unsigned int?

>
> -       switch (id) {
> -       case 0:
> -               *seqno = &layout->os_area.msg_seqno_0;
> -               key = layout->vmpck0;
> -               break;
> -       case 1:
> -               *seqno = &layout->os_area.msg_seqno_1;
> -               key = layout->vmpck1;
> -               break;
> -       case 2:
> -               *seqno = &layout->os_area.msg_seqno_2;
> -               key = layout->vmpck2;
> -               break;
> -       case 3:
> -               *seqno = &layout->os_area.msg_seqno_3;
> -               key = layout->vmpck3;
> -               break;
> -       default:
> -               break;
> -       }
> +       dev->vmpck_id = vmpck_id;
>
> -       return key;
> +       return true;
>  }
>
>  static int __init sev_guest_probe(struct platform_device *pdev)
> @@ -713,14 +713,14 @@ static int __init sev_guest_probe(struct platform_device *pdev)
>                 goto e_unmap;
>
>         ret = -EINVAL;
> -       snp_dev->vmpck = get_vmpck(vmpck_id, layout, &snp_dev->os_area_msg_seqno);
> -       if (!snp_dev->vmpck) {
> +       snp_dev->layout = layout;
> +       if (!snp_assign_vmpck(snp_dev, vmpck_id)) {
>                 dev_err(dev, "invalid vmpck id %d\n", vmpck_id);
>                 goto e_unmap;
>         }
>
>         /* Verify that VMPCK is not zero. */
> -       if (is_vmpck_empty(snp_dev)) {
> +       if (snp_is_vmpck_empty(snp_dev)) {
>                 dev_err(dev, "vmpck id %d is null\n", vmpck_id);
>                 goto e_unmap;
>         }
> @@ -728,7 +728,6 @@ static int __init sev_guest_probe(struct platform_device *pdev)
>         mutex_init(&snp_dev->cmd_mutex);
>         platform_set_drvdata(pdev, snp_dev);
>         snp_dev->dev = dev;
> -       snp_dev->layout = layout;
>
>         /* Allocate the shared page used for the request and response message. */
>         snp_dev->request = alloc_shared_pages(dev, sizeof(struct snp_guest_msg));
> @@ -744,7 +743,7 @@ static int __init sev_guest_probe(struct platform_device *pdev)
>                 goto e_free_response;
>
>         ret = -EIO;
> -       snp_dev->ctx = snp_init_crypto(snp_dev->vmpck, VMPCK_KEY_LEN);
> +       snp_dev->ctx = snp_init_crypto(snp_dev);
>         if (!snp_dev->ctx)
>                 goto e_free_cert_data;
>
> --
> 2.34.1
>


-- 
-Dionna Glaze, PhD (she/her)

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