File xsa444-1.patch of Package xen.30996

From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: x86/svm: Fix asymmetry with AMD DR MASK context switching

The handling of MSR_DR{0..3}_MASK is asymmetric between PV and HVM guests.

HVM guests context switch in based on the guest view of DBEXT, whereas PV
guest switch in base on the host capability.  Both guest types leave the
context dirty for the next vCPU.

This leads to the following issue:

 * PV or HVM vCPU has debugging active (%dr7 + mask)
 * Switch out deactivates %dr7 but leaves other state stale in hardware
 * HVM vCPU with debugging activate but can't see DBEXT is switched in
 * Switch in loads %dr7 but leaves the mask MSRs alone

Now, the HVM vCPU is operating in the context of the prior vCPU's mask MSR,
and furthermore in a case where it genuinely expects there to be no masking
MSRs.

As a stopgap, adjust the HVM path to switch in/out the masks based on host
capabilities rather than guest visibility (i.e. like the PV path).  Adjustment
of the of the intercepts still needs to be dependent on the guest visibility
of DBEXT.

This is part of XSA-444 / CVE-2023-34327

Fixes: c097f54912d3 ("x86/SVM: support data breakpoint extension registers")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index a019d196e071..ba4069f9100a 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -185,6 +185,10 @@ static void svm_save_dr(struct vcpu *v)
     v->arch.hvm.flag_dr_dirty = 0;
     vmcb_set_dr_intercepts(vmcb, ~0u);
 
+    /*
+     * The guest can only have changed the mask MSRs if we previous dropped
+     * intercepts.  Re-read them from hardware.
+     */
     if ( v->domain->arch.cpuid->extd.dbext )
     {
         svm_intercept_msr(v, MSR_AMD64_DR0_ADDRESS_MASK, MSR_INTERCEPT_RW);
@@ -216,17 +220,25 @@ static void __restore_debug_registers(struct vmcb_struct *vmcb, struct vcpu *v)
 
     ASSERT(v == current);
 
-    if ( v->domain->arch.cpuid->extd.dbext )
+    /*
+     * Both the PV and HVM paths leave stale DR_MASK values in hardware on
+     * context-switch-out.  If we're activating %dr7 for the guest, we must
+     * sync the DR_MASKs too, whether or not the guest can see them.
+     */
+    if ( boot_cpu_has(X86_FEATURE_DBEXT) )
     {
-        svm_intercept_msr(v, MSR_AMD64_DR0_ADDRESS_MASK, MSR_INTERCEPT_NONE);
-        svm_intercept_msr(v, MSR_AMD64_DR1_ADDRESS_MASK, MSR_INTERCEPT_NONE);
-        svm_intercept_msr(v, MSR_AMD64_DR2_ADDRESS_MASK, MSR_INTERCEPT_NONE);
-        svm_intercept_msr(v, MSR_AMD64_DR3_ADDRESS_MASK, MSR_INTERCEPT_NONE);
-
         wrmsrl(MSR_AMD64_DR0_ADDRESS_MASK, v->arch.msrs->dr_mask[0]);
         wrmsrl(MSR_AMD64_DR1_ADDRESS_MASK, v->arch.msrs->dr_mask[1]);
         wrmsrl(MSR_AMD64_DR2_ADDRESS_MASK, v->arch.msrs->dr_mask[2]);
         wrmsrl(MSR_AMD64_DR3_ADDRESS_MASK, v->arch.msrs->dr_mask[3]);
+
+        if ( v->domain->arch.cpuid->extd.dbext )
+        {
+            svm_intercept_msr(v, MSR_AMD64_DR0_ADDRESS_MASK, MSR_INTERCEPT_NONE);
+            svm_intercept_msr(v, MSR_AMD64_DR1_ADDRESS_MASK, MSR_INTERCEPT_NONE);
+            svm_intercept_msr(v, MSR_AMD64_DR2_ADDRESS_MASK, MSR_INTERCEPT_NONE);
+            svm_intercept_msr(v, MSR_AMD64_DR3_ADDRESS_MASK, MSR_INTERCEPT_NONE);
+        }
     }
 
     write_debugreg(0, v->arch.dr[0]);
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index f7992ff230b5..a142a63dd869 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2314,6 +2314,11 @@ void activate_debugregs(const struct vcpu *curr)
     if ( curr->arch.dr7 & DR7_ACTIVE_MASK )
         write_debugreg(7, curr->arch.dr7);
 
+    /*
+     * Both the PV and HVM paths leave stale DR_MASK values in hardware on
+     * context-switch-out.  If we're activating %dr7 for the guest, we must
+     * sync the DR_MASKs too, whether or not the guest can see them.
+     */
     if ( boot_cpu_has(X86_FEATURE_DBEXT) )
     {
         wrmsrl(MSR_AMD64_DR0_ADDRESS_MASK, curr->arch.msrs->dr_mask[0]);