[Xen-devel] [PATCH v5 1/4] nvmx: implement support for MSR bitmaps

[Roger Pau Monne <roger.pau@citrix.com>]

Current implementation of nested VMX has a half baked handling of MSR
bitmaps for the L1 VMM: it maps the L1 VMM provided MSR bitmap, but
doesn't actually load it into the nested vmcs, and thus the nested
guest vmcs ends up using the same MSR bitmap as the L1 VMM.

This is wrong as there's no assurance that the set of features enabled
for the L1 vmcs are the same that L1 itself is going to use in the
nested vmcs, and thus can lead to misconfigurations.

For example L1 vmcs can use x2APIC virtualization and virtual
interrupt delivery, and thus some x2APIC MSRs won't be trapped so that
they can be handled directly by the hardware using virtualization
extensions. On the other hand, the nested vmcs created by L1 VMM might
not use any of such features, so using a MSR bitmap that doesn't trap
accesses to the x2APIC MSRs will be leaking them to the underlying
hardware.

Fix this by crafting a merged MSR bitmap between the one used by L1
and the nested guest.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Changes since v4:
 - Add static to vcpu_relinquish_resources.

Changes since v3:
 - Free the merged MSR bitmap page in nvmx_purge_vvmcs.

Changes since v2:
 - Pass shadow_ctrl into update_msrbitmap, and check there if
   CPU_BASED_ACTIVATE_MSR_BITMAP is set.
 - Do not enable MSR bitmap unless it's enabled in both L1 and L2.
 - Rename L1 guest to L2 in nestedvmx struct comment.

Changes since v1:
 - Split the x2APIC MSR fix into a separate patch.
 - Move setting MSR_BITMAP vmcs field into load_vvmcs_host_state for
   virtual vmexit.
 - Allocate memory with MEMF_no_owner.
 - Use tabs to align comment of the nestedvmx struct field.
---
 xen/arch/x86/hvm/vmx/vvmx.c        | 73 ++++++++++++++++++++++++++++--
 xen/include/asm-x86/hvm/vmx/vvmx.h |  3 +-
 2 files changed, 71 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/hvm/vmx/vvmx.c b/xen/arch/x86/hvm/vmx/vvmx.c
index 47eee1e5b9..3337260d4b 100644
--- a/xen/arch/x86/hvm/vmx/vvmx.c
+++ b/xen/arch/x86/hvm/vmx/vvmx.c
@@ -128,6 +128,16 @@ int nvmx_vcpu_initialise(struct vcpu *v)
         unmap_domain_page(vw);
     }
 
+    if ( cpu_has_vmx_msr_bitmap )
+    {
+        nvmx->msr_merged = alloc_domheap_page(d, MEMF_no_owner);
+        if ( !nvmx->msr_merged )
+        {
+            gdprintk(XENLOG_ERR, "nest: allocation for MSR bitmap failed\n");
+            return -ENOMEM;
+        }
+    }
+
     nvmx->ept.enabled = 0;
     nvmx->guest_vpid = 0;
     nvmx->vmxon_region_pa = INVALID_PADDR;
@@ -183,13 +193,27 @@ void nvmx_vcpu_destroy(struct vcpu *v)
         v->arch.hvm.vmx.vmwrite_bitmap = NULL;
     }
 }
- 
+
+static void vcpu_relinquish_resources(struct vcpu *v)
+{
+    struct nestedvmx *nvmx = &vcpu_2_nvmx(v);
+
+    if ( nvmx->msr_merged )
+    {
+        free_domheap_page(nvmx->msr_merged);
+        nvmx->msr_merged = NULL;
+    }
+}
+
 void nvmx_domain_relinquish_resources(struct domain *d)
 {
     struct vcpu *v;
 
     for_each_vcpu ( d, v )
+    {
         nvmx_purge_vvmcs(v);
+        vcpu_relinquish_resources(v);
+    }
 }
 
 int nvmx_vcpu_reset(struct vcpu *v)
@@ -548,6 +572,35 @@ unsigned long *_shadow_io_bitmap(struct vcpu *v)
     return nestedhvm_vcpu_iomap_get(port80, portED);
 }
 
+static void update_msrbitmap(struct vcpu *v, uint32_t shadow_ctrl)
+{
+    struct nestedvmx *nvmx = &vcpu_2_nvmx(v);
+    struct vmx_msr_bitmap *msr_bitmap;
+
+    if ( !(shadow_ctrl & CPU_BASED_ACTIVATE_MSR_BITMAP) ||
+         !nvmx->msrbitmap )
+       return;
+
+    msr_bitmap = __map_domain_page(nvmx->msr_merged);
+
+    bitmap_or(msr_bitmap->read_low, nvmx->msrbitmap->read_low,
+              v->arch.hvm.vmx.msr_bitmap->read_low,
+              sizeof(msr_bitmap->read_low) * 8);
+    bitmap_or(msr_bitmap->read_high, nvmx->msrbitmap->read_high,
+              v->arch.hvm.vmx.msr_bitmap->read_high,
+              sizeof(msr_bitmap->read_high) * 8);
+    bitmap_or(msr_bitmap->write_low, nvmx->msrbitmap->write_low,
+              v->arch.hvm.vmx.msr_bitmap->write_low,
+              sizeof(msr_bitmap->write_low) * 8);
+    bitmap_or(msr_bitmap->write_high, nvmx->msrbitmap->write_high,
+              v->arch.hvm.vmx.msr_bitmap->write_high,
+              sizeof(msr_bitmap->write_high) * 8);
+
+    unmap_domain_page(msr_bitmap);
+
+    __vmwrite(MSR_BITMAP, page_to_maddr(nvmx->msr_merged));
+}
+
 void nvmx_update_exec_control(struct vcpu *v, u32 host_cntrl)
 {
     u32 pio_cntrl = (CPU_BASED_ACTIVATE_IO_BITMAP
@@ -558,10 +611,17 @@ void nvmx_update_exec_control(struct vcpu *v, u32 host_cntrl)
     shadow_cntrl = __n2_exec_control(v);
     pio_cntrl &= shadow_cntrl;
     /* Enforce the removed features */
-    shadow_cntrl &= ~(CPU_BASED_ACTIVATE_MSR_BITMAP
-                      | CPU_BASED_ACTIVATE_IO_BITMAP
+    shadow_cntrl &= ~(CPU_BASED_ACTIVATE_IO_BITMAP
                       | CPU_BASED_UNCOND_IO_EXITING);
-    shadow_cntrl |= host_cntrl;
+    /*
+     * Do NOT enforce the MSR bitmap currently used by L1, as certain hardware
+     * virtualization features require specific MSR bitmap settings, but
+     * without the guest also using these same features the bitmap could be
+     * leaking through unwanted MSR accesses.
+     */
+    shadow_cntrl |= host_cntrl & ~CPU_BASED_ACTIVATE_MSR_BITMAP;
+    if ( !(shadow_cntrl & host_cntrl & CPU_BASED_ACTIVATE_MSR_BITMAP) )
+      shadow_cntrl &= ~CPU_BASED_ACTIVATE_MSR_BITMAP;
     if ( pio_cntrl == CPU_BASED_UNCOND_IO_EXITING ) {
         /* L1 VMM intercepts all I/O instructions */
         shadow_cntrl |= CPU_BASED_UNCOND_IO_EXITING;
@@ -584,6 +644,8 @@ void nvmx_update_exec_control(struct vcpu *v, u32 host_cntrl)
         __vmwrite(IO_BITMAP_B, virt_to_maddr(bitmap) + PAGE_SIZE);
     }
 
+    update_msrbitmap(v, shadow_cntrl);
+
     /* TODO: change L0 intr window to MTF or NMI window */
     __vmwrite(CPU_BASED_VM_EXEC_CONTROL, shadow_cntrl);
 }
@@ -1278,6 +1340,9 @@ static void load_vvmcs_host_state(struct vcpu *v)
     hvm_set_tsc_offset(v, v->arch.hvm.cache_tsc_offset, 0);
 
     set_vvmcs(v, VM_ENTRY_INTR_INFO, 0);
+
+    if ( v->arch.hvm.vmx.exec_control & CPU_BASED_ACTIVATE_MSR_BITMAP )
+        __vmwrite(MSR_BITMAP, virt_to_maddr(v->arch.hvm.vmx.msr_bitmap));
 }
 
 static void sync_exception_state(struct vcpu *v)
diff --git a/xen/include/asm-x86/hvm/vmx/vvmx.h b/xen/include/asm-x86/hvm/vmx/vvmx.h
index 6b9c4ae0b2..c41f089939 100644
--- a/xen/include/asm-x86/hvm/vmx/vvmx.h
+++ b/xen/include/asm-x86/hvm/vmx/vvmx.h
@@ -37,7 +37,8 @@ struct nestedvmx {
      */
     paddr_t    vmxon_region_pa;
     void       *iobitmap[2];		/* map (va) of L1 guest I/O bitmap */
-    void       *msrbitmap;		/* map (va) of L1 guest MSR bitmap */
+    struct vmx_msr_bitmap *msrbitmap;	/* map (va) of L1 guest MSR bitmap */
+    struct page_info *msr_merged;	/* merged L1 and L2 MSR bitmap */
     /* deferred nested interrupt */
     struct {
         unsigned long intr_info;
-- 
2.25.0


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