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SubjectRe: [Part2 PATCH v6 13/38] crypto: ccp: Add Secure Encrypted Virtualization (SEV) command support
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On 10/26/2017 12:44 PM, Borislav Petkov wrote:
> On Thu, Oct 26, 2017 at 11:56:57AM -0500, Brijesh Singh wrote:
>> The variable is used as ref counter.
>
> ... and it can't be converted to a boolean because...?
>

SHUTDOWN command unconditionally transitions a platform to uninitialized
state. The command does not care how many processes are actively using
the PSP. We don't want to shutdown the firmware while other process is
still using it.

e.g consider three processes (A, B, C)

Process A:
----------
sev_platform_init()
sev_do_cmd(..)
...
...
sev_do_cmd(..)
...
sev_platform_shutdown()

Process B:
-----------
sev_platform_init()
sev_do_cmd(...)
sev_platform_shutdown()

Process C:
----------
sev_platform_init()
sev_do_cmd(...)
sev_do_cmd(...)
sev_do_cmd(...)
sev_platform_shutdown()

As per the SEV spec section 5.1.2 (platform state machine), several
commands require that platform should be initialized before issuing the
actual command. As you can see Process B may finish quickly and SHUTDOWN
from process B will simply uninitialize the firmware and cause
unexpected result to process A and C.


>> In your previous reply you comments on global semaphore (fw_init_mutex) and
>> in response I tried to highlight why we need the global semaphore. Did I
>> misunderstood your comment ?
>
> Yes, what happens if you get preempted while holding the mutex? Will the other
> process be able to do anything?
>

If other process tries to issue the sev_platform_init/shutdown() then
they have to wait.

The sev_platform_init() and sev_platform_shutdown() uses the same global
mutex. See the original code below.

+static int __sev_platform_init(struct sev_data_init *data, int *error)
+{
+ int rc = 0;
+
+ mutex_lock(&fw_init_mutex);
+
+ if (!fw_init_count) {
+ rc = sev_do_cmd(SEV_CMD_INIT, data, error);
+ if (rc)
+ goto unlock;
+ }
+
+ fw_init_count++;
+
+unlock:
+ mutex_unlock(&fw_init_mutex);
+ return rc;
+
+}
+
+int sev_platform_shutdown(int *error)
+{
+ int rc = 0;
+
+ mutex_lock(&fw_init_mutex);
+
+ if (!fw_init_count)
+ goto unlock;
+
+ if (fw_init_count == 1) {
+ rc = sev_do_cmd(SEV_CMD_SHUTDOWN, 0, error);
+ if (rc)
+ goto unlock;
+ }
+
+ fw_init_count--;
+
+unlock:
+ mutex_unlock(&fw_init_mutex);
+ return rc;
+}

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