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

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Message-ID: <a826082f-d80e-4aa3-982e-df7c723e2d2b@amd.com>
Date: Fri, 13 Sep 2024 09:56:52 +0530
From: "Nikunj A. Dadhania" <nikunj@....com>
To: Tom Lendacky <thomas.lendacky@....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

Hi Tom,

On 9/13/2024 3:24 AM, Tom Lendacky wrote:
> On 7/31/24 10:08, Nikunj A Dadhania wrote:
>> @@ -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?

We can move the check inside the lock, and get_* will try to prepare
the message and after grabbing the lock if the the VMPCK is empty we
would fail. Something like below:

diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
index 8a2d0d751685..537f59358090 100644
--- a/drivers/virt/coco/sev-guest/sev-guest.c
+++ b/drivers/virt/coco/sev-guest/sev-guest.c
@@ -347,6 +347,12 @@ static int snp_send_guest_request(struct snp_guest_dev *snp_dev, struct snp_gues
 
 	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");
+		return -ENOTTY;
+        }
+
 	/* Get message sequence and verify that its a non-zero */
 	seqno = snp_get_msg_seqno(snp_dev);
 	if (!seqno)
@@ -594,12 +600,6 @@ static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long
 	if (!input.msg_version)
 		return -EINVAL;
 
-	/* Check if the VMPCK is not empty */
-	if (is_vmpck_empty(snp_dev)) {
-		dev_err_ratelimited(snp_dev->dev, "VMPCK is disabled\n");
-		return -ENOTTY;
-	}
-
 	switch (ioctl) {
 	case SNP_GET_REPORT:
 		ret = get_report(snp_dev, &input);
@@ -869,12 +869,6 @@ static int sev_report_new(struct tsm_report *report, void *data)
 	if (!buf)
 		return -ENOMEM;
 
-	/* Check if the VMPCK is not empty */
-	if (is_vmpck_empty(snp_dev)) {
-		dev_err_ratelimited(snp_dev->dev, "VMPCK is disabled\n");
-		return -ENOTTY;
-	}
-
 	cert_table = buf + report_size;
 	struct snp_ext_report_req ext_req = {
 		.data = { .vmpl = desc->privlevel },


> 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?

The above code will fail early.

> 
> Thanks,
> Tom
> 

Regards
Nikunj