[PATCH 2/5] KVM: SVM: Guard against DEACTIVATE when performing WBINVD/DF_FLUSH

[Lendacky, Thomas]

From: Tom Lendacky <thomas.lenda...@amd.com>

The SEV firmware DEACTIVATE command disassociates an SEV guest from an
ASID, clears the WBINVD indicator on all threads and indicates that the
SEV firmware DF_FLUSH command must be issued before the ASID can be
re-used. The SEV firmware DF_FLUSH command will return an error if a
WBINVD has not been performed on every thread before it has been invoked.
A window exists between the WBINVD and the invocation of the DF_FLUSH
command where an SEV firmware DEACTIVATE command could be invoked on
another thread, clearing the WBINVD indicator. This will cause the
subsequent SEV firmware DF_FLUSH command to fail which, in turn, results
in the SEV firmware ACTIVATE command failing for the reclaimed ASID.
This results in the SEV guest failing to start.

Use a mutex to close the WBINVD/DF_FLUSH window by obtaining the mutex
before the DEACTIVATE and releasing it after the DF_FLUSH. This ensures
that any DEACTIVATE cannot run before a DF_FLUSH has completed.

Fixes: 59414c989220 ("KVM: SVM: Add support for KVM_SEV_LAUNCH_START command")
Tested-by: David Rientjes <rient...@google.com>
Signed-off-by: Tom Lendacky <thomas.lenda...@amd.com>
---
 arch/x86/kvm/svm.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index d371007ab109..1d217680cf83 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -418,6 +418,7 @@ enum {
 
 #define VMCB_AVIC_APIC_BAR_MASK                0xFFFFFFFFFF000ULL
 
+static DEFINE_MUTEX(sev_deactivate_lock);
 static DEFINE_MUTEX(sev_bitmap_lock);
 static unsigned int max_sev_asid;
 static unsigned int min_sev_asid;
@@ -1756,10 +1757,20 @@ static void sev_unbind_asid(struct kvm *kvm, unsigned 
int handle)
 
        /* deactivate handle */
        data->handle = handle;
+
+       /*
+        * Guard against a parallel DEACTIVATE command before the DF_FLUSH
+        * command has completed.
+        */
+       mutex_lock(&sev_deactivate_lock);
+
        sev_guest_deactivate(data, NULL);
 
        wbinvd_on_all_cpus();
        sev_guest_df_flush(NULL);
+
+       mutex_unlock(&sev_deactivate_lock);
+
        kfree(data);
 
        decommission = kzalloc(sizeof(*decommission), GFP_KERNEL);
@@ -6318,9 +6329,18 @@ static int sev_bind_asid(struct kvm *kvm, unsigned int 
handle, int *error)
        int asid = sev_get_asid(kvm);
        int ret;
 
+       /*
+        * Guard against a DEACTIVATE command before the DF_FLUSH command
+        * has completed.
+        */
+       mutex_lock(&sev_deactivate_lock);
+
        wbinvd_on_all_cpus();
 
        ret = sev_guest_df_flush(error);
+
+       mutex_unlock(&sev_deactivate_lock);
+
        if (ret)
                return ret;
 
-- 
2.17.1