[Xen-devel] [PATCH v2 1/4] x86/vmx: introduce vmwrite_safe()

[Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>]

Any fail during the original __vmwrite() leads to BUG() which can be
easily exploited from a guest in the nested vmx mode.

The new function returns error code depending on the outcome:

          VMsucceed: 0
        VMfailValid: VM Instruction Error Number
      VMfailInvalid: a new VMX_INSN_FAIL_INVALID

A new macro GAS_VMX_OP is introduced in order to improve the
readability of asm.  Existing ASM_FLAG_OUT macro is reused and copied
into asm_defns.h

Signed-off-by: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
---
 xen/arch/x86/hvm/vmx/vvmx.c        |  3 ++-
 xen/include/asm-x86/asm_defns.h    |  6 ++++++
 xen/include/asm-x86/hvm/vmx/vmcs.h |  1 +
 xen/include/asm-x86/hvm/vmx/vmx.h  | 29 +++++++++++++++++++++++++++++
 4 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/vmx/vvmx.c b/xen/arch/x86/hvm/vmx/vvmx.c
index 9caebe5..105a3c0 100644
--- a/xen/arch/x86/hvm/vmx/vvmx.c
+++ b/xen/arch/x86/hvm/vmx/vvmx.c
@@ -483,7 +483,8 @@ static void vmfail_invalid(struct cpu_user_regs *regs)
 
 static void vmfail(struct cpu_user_regs *regs, enum vmx_insn_errno errno)
 {
-    if ( vcpu_nestedhvm(current).nv_vvmcxaddr != INVALID_PADDR )
+    if ( vcpu_nestedhvm(current).nv_vvmcxaddr != INVALID_PADDR &&
+         errno != VMX_INSN_FAIL_INVALID )
         vmfail_valid(regs, errno);
     else
         vmfail_invalid(regs);
diff --git a/xen/include/asm-x86/asm_defns.h b/xen/include/asm-x86/asm_defns.h
index f1c6fa1..220ae2e 100644
--- a/xen/include/asm-x86/asm_defns.h
+++ b/xen/include/asm-x86/asm_defns.h
@@ -413,4 +413,10 @@ static always_inline void stac(void)
 #define REX64_PREFIX "rex64/"
 #endif
 
+#ifdef __GCC_ASM_FLAG_OUTPUTS__
+# define ASM_FLAG_OUT(yes, no) yes
+#else
+# define ASM_FLAG_OUT(yes, no) no
+#endif
+
 #endif /* __X86_ASM_DEFNS_H__ */
diff --git a/xen/include/asm-x86/hvm/vmx/vmcs.h 
b/xen/include/asm-x86/hvm/vmx/vmcs.h
index d71de04..8d43efd 100644
--- a/xen/include/asm-x86/hvm/vmx/vmcs.h
+++ b/xen/include/asm-x86/hvm/vmx/vmcs.h
@@ -526,6 +526,7 @@ enum vmx_insn_errno
     VMX_INSN_VMPTRLD_INVALID_PHYADDR       = 9,
     VMX_INSN_UNSUPPORTED_VMCS_COMPONENT    = 12,
     VMX_INSN_VMXON_IN_VMX_ROOT             = 15,
+    VMX_INSN_FAIL_INVALID                  = ~0,
 };
 
 void vmx_disable_intercept_for_msr(struct vcpu *v, u32 msr, int type);
diff --git a/xen/include/asm-x86/hvm/vmx/vmx.h 
b/xen/include/asm-x86/hvm/vmx/vmx.h
index 4bf4d50..d40d6a5 100644
--- a/xen/include/asm-x86/hvm/vmx/vmx.h
+++ b/xen/include/asm-x86/hvm/vmx/vmx.h
@@ -306,6 +306,12 @@ extern uint8_t posted_intr_vector;
 #define INVVPID_ALL_CONTEXT                     2
 #define INVVPID_SINGLE_CONTEXT_RETAINING_GLOBAL 3
 
+#ifdef HAVE_GAS_VMX
+# define GAS_VMX_OP(yes, no) yes
+#else
+# define GAS_VMX_OP(yes, no) no
+#endif
+
 static always_inline void __vmptrld(u64 addr)
 {
     asm volatile (
@@ -423,6 +429,29 @@ static inline bool_t __vmread_safe(unsigned long field, 
unsigned long *value)
     return okay;
 }
 
+static always_inline unsigned long vmwrite_safe(unsigned long field,
+                                                unsigned long value)
+{
+    unsigned long ret = 0;
+    bool fail_invalid, fail_valid;
+
+    asm volatile ( GAS_VMX_OP("vmwrite %[value], %[field]\n\t",
+                              VMWRITE_OPCODE MODRM_EAX_ECX)
+                   ASM_FLAG_OUT(, "setc %[invalid]\n\t")
+                   ASM_FLAG_OUT(, "setz %[valid]\n\t")
+                   : ASM_FLAG_OUT("=@ccc", [invalid] "=rm") (fail_invalid),
+                     ASM_FLAG_OUT("=@ccz", [valid] "=rm") (fail_valid)
+                   : [field] GAS_VMX_OP("r", "a") (field),
+                     [value] GAS_VMX_OP("rm", "c") (value));
+
+    if ( unlikely(fail_invalid) )
+        ret = VMX_INSN_FAIL_INVALID;
+    else if ( unlikely(fail_valid) )
+        __vmread(VM_INSTRUCTION_ERROR, &ret);
+
+    return ret;
+}
+
 static always_inline void __invept(unsigned long type, u64 eptp, u64 gpa)
 {
     struct {
-- 
2.9.3


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