linux-kernel - Re: [PATCH v11 09/20] virt: sev-guest: Reduce the scope of SNP command mutex

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Message-ID: <30a5505f-8c9c-6f13-6f90-8d5b6826acb5@amd.com>
Date: Thu, 12 Sep 2024 16:54:10 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Nikunj A Dadhania <nikunj@....com>, linux-kernel@...r.kernel.org,
 bp@...en8.de, x86@...nel.org, kvm@...r.kernel.org
Cc: mingo@...hat.com, tglx@...utronix.de, dave.hansen@...ux.intel.com,
 pgonda@...gle.com, seanjc@...gle.com, pbonzini@...hat.com
Subject: Re: [PATCH v11 09/20] virt: sev-guest: Reduce the scope of SNP
 command mutex

On 7/31/24 10:08, Nikunj A Dadhania wrote:
> The SNP command mutex is used to serialize access to the shared buffer,
> command handling, and message sequence number.
> 
> All shared buffer, command handling, and message sequence updates are done
> within snp_send_guest_request(), so moving the mutex to this function is
> appropriate and maintains the critical section.
> 
> Since the mutex is now taken at a later point in time, remove the lockdep
> checks that occur before taking the mutex.
> 
> Signed-off-by: Nikunj A Dadhania <nikunj@....com>
> ---
>  drivers/virt/coco/sev-guest/sev-guest.c | 17 ++---------------
>  1 file changed, 2 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
> index 92734a2345a6..42f7126f1718 100644
> --- a/drivers/virt/coco/sev-guest/sev-guest.c
> +++ b/drivers/virt/coco/sev-guest/sev-guest.c
> @@ -345,6 +345,8 @@ static int snp_send_guest_request(struct snp_guest_dev *snp_dev, struct snp_gues
>  	u64 seqno;
>  	int rc;
>  
> +	guard(mutex)(&snp_cmd_mutex);
> +
>  	/* Get message sequence and verify that its a non-zero */
>  	seqno = snp_get_msg_seqno(snp_dev);
>  	if (!seqno)
> @@ -419,8 +421,6 @@ static int get_report(struct snp_guest_dev *snp_dev, struct snp_guest_request_io
>  	struct snp_report_resp *report_resp;
>  	int rc, resp_len;
>  
> -	lockdep_assert_held(&snp_cmd_mutex);
> -
>  	if (!arg->req_data || !arg->resp_data)
>  		return -EINVAL;
>  
> @@ -458,8 +458,6 @@ 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;
>  
> @@ -501,8 +499,6 @@ static int get_ext_report(struct snp_guest_dev *snp_dev, struct snp_guest_reques
>  	int ret, npages = 0, resp_len;
>  	sockptr_t certs_address;
>  
> -	lockdep_assert_held(&snp_cmd_mutex);
> -
>  	if (sockptr_is_null(io->req_data) || sockptr_is_null(io->resp_data))
>  		return -EINVAL;
>  
> @@ -590,12 +586,9 @@ static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long
>  	if (!input.msg_version)
>  		return -EINVAL;
>  
> -	mutex_lock(&snp_cmd_mutex);
> -
>  	/* Check if the VMPCK is not empty */
>  	if (is_vmpck_empty(snp_dev)) {

Are we ok with this being outside of the lock now?

I believe is_vmpck_empty() can get a false and then be waiting on the
mutex while snp_disable_vmpck() is called. Suddenly the code thinks the
VMPCK is valid when it isn't anymore. Not sure if that matters, as the
guest request will fail anyway?

Thanks,
Tom

>  		dev_err_ratelimited(snp_dev->dev, "VMPCK is disabled\n");
> -		mutex_unlock(&snp_cmd_mutex);
>  		return -ENOTTY;
>  	}
>  
> @@ -620,8 +613,6 @@ static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long
>  		break;
>  	}
>  
> -	mutex_unlock(&snp_cmd_mutex);
> -
>  	if (input.exitinfo2 && copy_to_user(argp, &input, sizeof(input)))
>  		return -EFAULT;
>  
> @@ -736,8 +727,6 @@ static int sev_svsm_report_new(struct tsm_report *report, void *data)
>  	man_len = SZ_4K;
>  	certs_len = SEV_FW_BLOB_MAX_SIZE;
>  
> -	guard(mutex)(&snp_cmd_mutex);
> -
>  	if (guid_is_null(&desc->service_guid)) {
>  		call_id = SVSM_ATTEST_CALL(SVSM_ATTEST_SERVICES);
>  	} else {
> @@ -872,8 +861,6 @@ 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 (is_vmpck_empty(snp_dev)) {
>  		dev_err_ratelimited(snp_dev->dev, "VMPCK is disabled\n");