Re: [PATCH v7 06/16] virt: sev-guest: Move SNP Guest command mutex

[Tom Lendacky]

On 12/20/23 09:13, Nikunj A Dadhania wrote:
> SNP command mutex is used to serialize the shared buffer access, command
> handling and message sequence number races. Move the SNP guest command
> mutex out of the sev guest driver and provide accessors to sev-guest
> driver. Remove multiple lockdep check in sev-guest driver, next patch adds
> a single lockdep check in snp_send_guest_request().
>
> Signed-off-by: Nikunj A Dadhania <nikunj@xxxxxxx>
> ---
>   arch/x86/include/asm/sev-guest.h        |  3 +++
>   arch/x86/kernel/sev.c                   | 21 +++++++++++++++++++++
>   drivers/virt/coco/sev-guest/sev-guest.c | 23 +++++++----------------
>   3 files changed, 31 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/include/asm/sev-guest.h b/arch/x86/include/asm/sev-guest.h
> index 27cc15ad6131..2f3cceb88396 100644
> --- a/arch/x86/include/asm/sev-guest.h
> +++ b/arch/x86/include/asm/sev-guest.h
> @@ -81,4 +81,7 @@ struct snp_guest_req {
>   
>   int snp_issue_guest_request(struct snp_guest_req *req, struct snp_req_data *input,
>   			    struct snp_guest_request_ioctl *rio);
> +void snp_guest_cmd_lock(void);
> +void snp_guest_cmd_unlock(void);
> +
>   #endif /* __VIRT_SEVGUEST_H__ */
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 6aa0bdf8a7a0..191193924b22 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -941,6 +941,21 @@ static void snp_cleanup_vmsa(struct sev_es_save_area *vmsa)
>   		free_page((unsigned long)vmsa);
>   }
>   
> +/*  SNP Guest command mutex to serialize the shared buffer access and command handling. */
> +static struct mutex snp_guest_cmd_mutex;

You should probably use:

static DEFINE_MUTEX(snp_guest_cmd_mutex);

That way you can avoid the initialization in snp_init_platform_device().

With that:

Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

> +
> +void snp_guest_cmd_lock(void)
> +{
> +	mutex_lock(&snp_guest_cmd_mutex);
> +}
> +EXPORT_SYMBOL_GPL(snp_guest_cmd_lock);
> +
> +void snp_guest_cmd_unlock(void)
> +{
> +	mutex_unlock(&snp_guest_cmd_mutex);
> +}
> +EXPORT_SYMBOL_GPL(snp_guest_cmd_unlock);
> +
>   static int wakeup_cpu_via_vmgexit(u32 apic_id, unsigned long start_ip)
>   {
>   	struct sev_es_save_area *cur_vmsa, *vmsa;
> @@ -2240,6 +2255,12 @@ static int __init snp_init_platform_device(void)
>   		return -ENODEV;
>   	}
>   
> +	/*
> +	 * Initialize snp command mutex that is used to serialize the shared
> +	 * buffer access and use of the vmpck and message sequence number
> +	 */
> +	mutex_init(&snp_guest_cmd_mutex);
> +
>   	data.secrets_gpa = secrets_pa;
>   	if (platform_device_add_data(&sev_guest_device, &data, sizeof(data)))
>   		return -ENODEV;
> diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
> index 9c0ff69a16da..bd30a9ff82c1 100644
> --- a/drivers/virt/coco/sev-guest/sev-guest.c
> +++ b/drivers/virt/coco/sev-guest/sev-guest.c
> @@ -63,9 +63,6 @@ static u32 vmpck_id;
>   module_param(vmpck_id, uint, 0444);
>   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 inline u8 *snp_get_vmpck(struct snp_guest_dev *snp_dev)
>   {
>   	return snp_dev->layout->vmpck0 + snp_dev->vmpck_id * VMPCK_KEY_LEN;
> @@ -115,8 +112,6 @@ 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 = *os_area_msg_seqno;
>   
> @@ -409,8 +404,6 @@ static int get_report(struct snp_guest_dev *snp_dev, struct snp_guest_request_io
>   	struct snp_report_resp *resp;
>   	int rc, resp_len;
>   
> -	lockdep_assert_held(&snp_cmd_mutex);
> -
>   	if (!arg->req_data || !arg->resp_data)
>   		return -EINVAL;
>   
> @@ -457,8 +450,6 @@ static int get_derived_key(struct snp_guest_dev *snp_dev, struct snp_guest_reque
>   	/* Response data is 64 bytes and max authsize for GCM is 16 bytes. */
>   	u8 buf[64 + 16];
>   
> -	lockdep_assert_held(&snp_cmd_mutex);
> -
>   	if (!arg->req_data || !arg->resp_data)
>   		return -EINVAL;
>   
> @@ -507,8 +498,6 @@ static int get_ext_report(struct snp_guest_dev *snp_dev, struct snp_guest_reques
>   	sockptr_t certs_address;
>   	int ret, resp_len;
>   
> -	lockdep_assert_held(&snp_cmd_mutex);
> -
>   	if (sockptr_is_null(io->req_data) || sockptr_is_null(io->resp_data))
>   		return -EINVAL;
>   
> @@ -604,12 +593,12 @@ static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long
>   	if (!input.msg_version)
>   		return -EINVAL;
>   
> -	mutex_lock(&snp_cmd_mutex);
> +	snp_guest_cmd_lock();
>   
>   	/* Check if the VMPCK is not empty */
>   	if (snp_is_vmpck_empty(snp_dev)) {
>   		dev_err_ratelimited(snp_dev->dev, "VMPCK is disabled\n");
> -		mutex_unlock(&snp_cmd_mutex);
> +		snp_guest_cmd_unlock();
>   		return -ENOTTY;
>   	}
>   
> @@ -634,7 +623,7 @@ static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long
>   		break;
>   	}
>   
> -	mutex_unlock(&snp_cmd_mutex);
> +	snp_guest_cmd_unlock();
>   
>   	if (input.exitinfo2 && copy_to_user(argp, &input, sizeof(input)))
>   		return -EFAULT;
> @@ -724,14 +713,14 @@ static int sev_report_new(struct tsm_report *report, void *data)
>   	if (!buf)
>   		return -ENOMEM;
>   
> -	guard(mutex)(&snp_cmd_mutex);
> -
>   	/* Check if the VMPCK is not empty */
>   	if (snp_is_vmpck_empty(snp_dev)) {
>   		dev_err_ratelimited(snp_dev->dev, "VMPCK is disabled\n");
>   		return -ENOTTY;
>   	}
>   
> +	snp_guest_cmd_lock();
> +
>   	cert_table = buf + report_size;
>   	struct snp_ext_report_req ext_req = {
>   		.data = { .vmpl = desc->privlevel },
> @@ -752,6 +741,8 @@ static int sev_report_new(struct tsm_report *report, void *data)
>   	};
>   
>   	ret = get_ext_report(snp_dev, &input, &io);
> +	snp_guest_cmd_unlock();
> +
>   	if (ret)
>   		return ret;
>