[PATCH v2 5/6] x86/hvm: Forced Emulation Prefix for debug builds of Xen

[Andrew Cooper <andrew.cooper3@citrix.com>]

Analysis of XSAs 105 and 106 show that is possible to force a race condition
which causes any arbitrary instruction to be emulated.

To aid testing, explicitly introduce the Forced Emulation Prefix for debug
builds alone.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Keir Fraser <keir@xen.org>
CC: Jan Beulich <JBeulich@suse.com>
CC: Boris Ostrovsky <boris.ostrovsky@oracle.com>
CC: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
CC: Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>
CC: Jun Nakajima <jun.nakajima@intel.com>
CC: Eddie Dong <eddie.dong@intel.com>
CC: Kevin Tian <kevin.tian@intel.com>

---
v2: (all suggested by Jan)
 * Use hvm_fetch_from_guest_virt_nofault() in preference to copy_from_guest()
 * Vastly reduce use of #ifndef NDEBUG
---
 docs/misc/xen-command-line.markdown |   11 +++++++++++
 xen/arch/x86/hvm/hvm.c              |    5 +++++
 xen/arch/x86/hvm/svm/svm.c          |   13 +++++++++++++
 xen/arch/x86/hvm/vmx/vmx.c          |   13 +++++++++++++
 xen/include/asm-x86/hvm/hvm.h       |    7 +++++++
 5 files changed, 49 insertions(+)

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index af93e17..389701a 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -682,6 +682,17 @@ Bit 11 - MSR operation logging
 
 Recognized in debug builds of the hypervisor only.
 
+### hvm\_fep
+> `= <boolean>`
+
+> Default: `false`
+
+Allow use of the Forced Emulation Prefix in HVM guests, to allow emulation of
+arbitrary instructions.
+
+This option is intended for development purposes, and is only available in
+debug builds of the hypervisor.
+
 ### hvm\_port80
 > `= <boolean>`
 
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 5c7e0a4..34f28d0 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -86,6 +86,11 @@ unsigned long __attribute__ ((__section__ (".bss.page_aligned")))
 static bool_t __initdata opt_hap_enabled = 1;
 boolean_param("hap", opt_hap_enabled);
 
+#ifndef opt_hvm_fep
+bool_t opt_hvm_fep;
+boolean_param("hvm_fep", opt_hvm_fep);
+#endif
+
 static int cpu_callback(
     struct notifier_block *nfb, unsigned long action, void *hcpu)
 {
diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index b6beefc..cda968b 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -2118,6 +2118,19 @@ static void svm_vmexit_ud_intercept(struct cpu_user_regs *regs)
     struct hvm_emulate_ctxt ctxt;
     int rc;
 
+    if ( opt_hvm_fep )
+    {
+        char sig[5]; /* ud2; .ascii "xen" */
+
+        if ( (hvm_fetch_from_guest_virt_nofault(
+                  sig, regs->eip, sizeof(sig), 0) == HVMCOPY_okay) &&
+             (memcmp(sig, "\xf\xbxen", sizeof(sig)) == 0) )
+        {
+            regs->eip += sizeof(sig);
+            regs->eflags &= ~X86_EFLAGS_RF;
+        }
+    }
+
     hvm_emulate_prepare(&ctxt, regs);
 
     rc = hvm_emulate_one(&ctxt);
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index addaa81..7f02ba2 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -2499,6 +2499,19 @@ static void vmx_vmexit_ud_intercept(struct cpu_user_regs *regs)
     struct hvm_emulate_ctxt ctxt;
     int rc;
 
+    if ( opt_hvm_fep )
+    {
+        char sig[5]; /* ud2; .ascii "xen" */
+
+        if ( (hvm_fetch_from_guest_virt_nofault(
+                  sig, regs->eip, sizeof(sig), 0) == HVMCOPY_okay) &&
+             (memcmp(sig, "\xf\xbxen", sizeof(sig)) == 0) )
+        {
+            regs->eip += sizeof(sig);
+            regs->eflags &= ~X86_EFLAGS_RF;
+        }
+    }
+
     hvm_emulate_prepare(&ctxt, regs);
 
     rc = hvm_emulate_one(&ctxt);
diff --git a/xen/include/asm-x86/hvm/hvm.h b/xen/include/asm-x86/hvm/hvm.h
index 3e66276..c0fbc8b 100644
--- a/xen/include/asm-x86/hvm/hvm.h
+++ b/xen/include/asm-x86/hvm/hvm.h
@@ -517,6 +517,13 @@ bool_t nhvm_vmcx_hap_enabled(struct vcpu *v);
 /* interrupt */
 enum hvm_intblk nhvm_interrupt_blocked(struct vcpu *v);
 
+#ifndef NDEBUG
+/* Permit use of the Forced Emulation Prefix in HVM guests */
+extern bool_t opt_hvm_fep;
+#else
+#define opt_hvm_fep 0
+#endif
+
 #endif /* __ASM_X86_HVM_HVM_H__ */
 
 /*
-- 
1.7.10.4