Message-ID: <74f17321-42ed-417c-ad24-8bc4e7207117@amd.com>
Date: Tue, 16 Apr 2024 11:27:24 +0530
From: "Nikunj A. Dadhania" <nikunj@....com>
To: Borislav Petkov <bp@...en8.de>
Cc: linux-kernel@...r.kernel.org, thomas.lendacky@....com, x86@...nel.org,
 kvm@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
 dave.hansen@...ux.intel.com, pgonda@...gle.com, seanjc@...gle.com,
 pbonzini@...hat.com
Subject: Re: [PATCH v8 04/16] virt: sev-guest: Add vmpck_id to snp_guest_dev
 struct

On 4/9/2024 3:53 PM, Borislav Petkov wrote:
> On Thu, Feb 15, 2024 at 05:01:16PM +0530, Nikunj A Dadhania 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. Added define for maximum supported VMPCK.
> 
> Do not talk about *what* the patch is doing in the commit message - that
> should be obvious from the diff itself. Rather, concentrate on the *why*
> it needs to be done.
> 
> Imagine one fine day you're doing git archeology, you find the place in
> the code about which you want to find out why it was changed the way it
> is now.
> 
> You do git annotate <filename> ... find the line, see the commit id and
> you do:
> 
> git show <commit id>
> 
> You read the commit message and there's just gibberish and nothing's
> explaining *why* that change was done. And you start scratching your
> head, trying to figure out why...
> 
> I'm sure you're getting the idea.

Sure, will reword the commit message and send the patch.

> 
>> 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>
>> Reviewed-by: Tom Lendacky <thomas.lendacky@....com>
>> Tested-by: Peter Gonda <pgonda@...gle.com>
>> ---
>>  arch/x86/include/asm/sev.h              |  1 +
>>  drivers/virt/coco/sev-guest/sev-guest.c | 95 ++++++++++++-------------
>>  2 files changed, 48 insertions(+), 48 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
>> index 0c0b11af9f89..e4f52a606487 100644
>> --- a/arch/x86/include/asm/sev.h
>> +++ b/arch/x86/include/asm/sev.h
>> @@ -135,6 +135,7 @@ struct secrets_os_area {
>>  } __packed;
>>  
>>  #define VMPCK_KEY_LEN		32
>> +#define VMPCK_MAX_NUM		4
>>  
>>  /* See the SNP spec version 0.9 for secrets page format */
>>  struct snp_secrets_page_layout {
>> diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
>> index 596cec03f9eb..646eb215f3c7 100644
>> --- a/drivers/virt/coco/sev-guest/sev-guest.c
>> +++ b/drivers/virt/coco/sev-guest/sev-guest.c
>> @@ -55,8 +55,7 @@ struct snp_guest_dev {
>>  		struct snp_derived_key_req derived_key;
>>  		struct snp_ext_report_req ext_report;
>>  	} req;
>> -	u32 *os_area_msg_seqno;
>> -	u8 *vmpck;
>> +	unsigned int vmpck_id;
>>  };
>>  
>>  static u32 vmpck_id;
>> @@ -66,14 +65,22 @@ 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)
> 
> static functions don't need a prefix like "snp_".

Sure

> 
>>  {
>> -	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)
> 
> Ditto.

Sure

> 
>> +{
>> +	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);
>>  }
>>  
>>  /*
>> @@ -95,20 +102,22 @@ static bool is_vmpck_empty(struct snp_guest_dev *snp_dev)
>>   */
>>  static void snp_disable_vmpck(struct snp_guest_dev *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;
>> +	u8 *key = snp_get_vmpck(snp_dev);
> 
> Check whether is_vmpck_empty before you disable?
> 
>> +
>> +	dev_alert(snp_dev->dev, "Disabling vmpck_id %u to prevent IV reuse.\n",
>> +		  snp_dev->vmpck_id);
>> +	memzero_explicit(key, VMPCK_KEY_LEN);
>>  }
>>  
>>  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_cmd_mutex);
>>  
>>  	/* Read the current message sequence counter from secrets pages */
>> -	count = *snp_dev->os_area_msg_seqno;
>> +	count = *os_area_msg_seqno;
> 
> Why does that snp_get_os_area_msg_seqno() returns a pointer when you
> deref it here again?
> 
> A function which returns a sequence number should return that number
> - not a pointer to it.
> 
> Which then makes that u32 *os_area_msg_seqno redundant and you can use
> the function directly.
> 
> IOW:
> 
> 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;

This patch removes setting of layour page in snp_dev structure.

static inline u32 snp_get_os_area_msg_seqno(struct snp_guest_dev *snp_dev)
{
        if (!platform_data)
                return NULL;

        return *(&platform_data->layout->os_area.msg_seqno_0 + vmpck_id);
}

> }
> 

> Simple.
> 
>>  
>>  	return count + 1;
>>  }
>> @@ -136,11 +145,13 @@ 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;
> 
> Yah, you have a getter but not a setter. You're setting it through the
> pointer. 

I had a getter for getting the os_area_msg_seqno pointer, probably not a good function name.

> Do you see the imbalance in the APIs?

The msg_seqno should only be incremented by 2 (always), that was the reason to avoid a setter.

> 
>>  }
>>  
>>  static inline struct snp_guest_dev *to_snp_dev(struct file *file)
>> @@ -150,15 +161,22 @@ 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("VM communication key VMPCK%u is null\n", vmpck_id);
> 
> 		      "Empty/invalid VMPCK%u communication key"
> 
> or so.
> 
> In a pre-patch, fix all your user-visible strings to say "VMPCK"
> - capitalized as it is an abbreviation.

Sure, will do.

> 
>> +		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("Crypto context initialization failed\n");
>>  		kfree(ctx);
>>  		return NULL;
>> @@ -590,7 +608,7 @@ static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long
>>  	mutex_lock(&snp_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_cmd_mutex);
>>  		return -ENOTTY;
>> @@ -667,32 +685,14 @@ 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)
>> +static bool snp_assign_vmpck(struct snp_guest_dev *dev, unsigned int vmpck_id)
>>  {
>> -	u8 *key = NULL;
>> +	if (WARN_ON((vmpck_id + 1) > VMPCK_MAX_NUM))
>> +		return false;
> 
> So this will warn *and*, at the call site too. Let's tone that down.

Sure.

> 
>>  
>> -	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;
>> -	}
> 
> Your commit message could explain why this is not needed, all of
> a sudden.

This was replaced by two independent APIs returning pointers to VMPCK and seqno pointer.

static inline u8 *snp_get_vmpck(unsigned int vmpck_id)
{
        if (!platform_data)
                return NULL;

        return platform_data->layout->vmpck0 + vmpck_id * VMPCK_KEY_LEN;
}

static inline u32 *snp_get_os_area_msg_seqno(unsigned int vmpck_id)
{
        if (!platform_data)
                return NULL;

        return &platform_data->layout->os_area.msg_seqno_0 + vmpck_id;
}

I will add more details.

> 
>> +	dev->vmpck_id = vmpck_id;
>>  
>> -	return key;
>> +	return true;
>>  }
>>  
>>  struct snp_msg_report_resp_hdr {
>> @@ -728,7 +728,7 @@ static int sev_report_new(struct tsm_report *report, void *data)
>>  	guard(mutex)(&snp_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");
>>  		return -ENOTTY;
>>  	}
>> @@ -848,21 +848,20 @@ 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) {
>> -		dev_err(dev, "invalid vmpck id %d\n", vmpck_id);
>> +	snp_dev->layout = layout;
>> +	if (!snp_assign_vmpck(snp_dev, vmpck_id)) {
>> +		dev_err(dev, "invalid vmpck id %u\n", vmpck_id);
>>  		goto e_unmap;
>>  	}
>>  
>>  	/* Verify that VMPCK is not zero. */
>> -	if (is_vmpck_empty(snp_dev)) {
>> -		dev_err(dev, "vmpck id %d is null\n", vmpck_id);
>> +	if (snp_is_vmpck_empty(snp_dev)) {
>> +		dev_err(dev, "vmpck id %u is null\n", vmpck_id);
> 
> s!null!Invalid/Empty!

Okay

> 
>>  		goto e_unmap;
>>  	}
>>  
>>  	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));
>> @@ -878,7 +877,7 @@ 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;
>>  
>> @@ -903,7 +902,7 @@ static int __init sev_guest_probe(struct platform_device *pdev)
>>  	if (ret)
>>  		goto e_free_ctx;
>>  
>> -	dev_info(dev, "Initialized SEV guest driver (using vmpck_id %d)\n", vmpck_id);
>> +	dev_info(dev, "Initialized SEV guest driver (using vmpck_id %u)\n", vmpck_id);
> 
> Yet another spelling: "vmpck_id". Unify all those in a pre-patch pls
> because it looks stupid.

Sure
> 
> Thx.
> 

Thanks for the review.

Regards
Nikunj

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