[PATCH V4] xen: Introduce VM_EVENT_FLAG_SET_REGISTERS

[PATCH V4] xen: Introduce VM_EVENT_FLAG_SET_REGISTERS

[Razvan Cojocaru]

A previous version of this patch dealing with support for skipping
the current instruction when a vm_event response requested it
computed the instruction length in the hypervisor, adding non-trivial
code dependencies. This patch allows a userspace vm_event client to
simply request that the guest's EIP is set to an arbitary value,
computed by the introspection application. The registers that can
now be set are EAX-EDX, ESP, EBP, ESI, EDI, R8-R15, EFLAGS, and EIP.
CR0, CR3 and CR4 are not set, as at the time of vm_event_resume()
we can't call hvm_set_cr{0,3,4}() and simply setting
v->arch.hvm_vcpu.guest_cr[{0,3,4}] is unlikely to have the desired
effect. The rest of the vm_event registers are not set because
they're not being filled by hvm_event_fill_regs(), but only by
p2m_vm_event_fill_regs(). Currently x86-only.
The VCPU needs to be paused for this flag to take effect.

Signed-off-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>

---
Changes since V3:
 - Added Reviewed-by and Acked-by tags.
 - Updated the header comment and patch description to explicitly
   state that support is only being added for x86 registers.
---
 xen/arch/x86/vm_event.c        |   24 ++++++++++++++++++++++++
 xen/common/vm_event.c          |    3 +++
 xen/include/asm-arm/vm_event.h |    6 ++++++
 xen/include/asm-x86/vm_event.h |    2 ++
 xen/include/public/vm_event.h  |    7 +++++++
 5 files changed, 42 insertions(+)

diff --git a/xen/arch/x86/vm_event.c b/xen/arch/x86/vm_event.c
index c38d37b..9677ecc 100644
--- a/xen/arch/x86/vm_event.c
+++ b/xen/arch/x86/vm_event.c
@@ -97,6 +97,30 @@ void vm_event_register_write_resume(struct vcpu *v, vm_event_response_t *rsp)
     }
 }
 
+void vm_event_set_registers(struct vcpu *v, vm_event_response_t *rsp)
+{
+    v->arch.user_regs.eax = rsp->data.regs.x86.rax;
+    v->arch.user_regs.ebx = rsp->data.regs.x86.rbx;
+    v->arch.user_regs.ecx = rsp->data.regs.x86.rcx;
+    v->arch.user_regs.edx = rsp->data.regs.x86.rdx;
+    v->arch.user_regs.esp = rsp->data.regs.x86.rsp;
+    v->arch.user_regs.ebp = rsp->data.regs.x86.rbp;
+    v->arch.user_regs.esi = rsp->data.regs.x86.rsi;
+    v->arch.user_regs.edi = rsp->data.regs.x86.rdi;
+
+    v->arch.user_regs.r8 = rsp->data.regs.x86.r8;
+    v->arch.user_regs.r9 = rsp->data.regs.x86.r9;
+    v->arch.user_regs.r10 = rsp->data.regs.x86.r10;
+    v->arch.user_regs.r11 = rsp->data.regs.x86.r11;
+    v->arch.user_regs.r12 = rsp->data.regs.x86.r12;
+    v->arch.user_regs.r13 = rsp->data.regs.x86.r13;
+    v->arch.user_regs.r14 = rsp->data.regs.x86.r14;
+    v->arch.user_regs.r15 = rsp->data.regs.x86.r15;
+
+    v->arch.user_regs.eflags = rsp->data.regs.x86.rflags;
+    v->arch.user_regs.eip = rsp->data.regs.x86.rip;
+}
+
 /*
  * Local variables:
  * mode: C
diff --git a/xen/common/vm_event.c b/xen/common/vm_event.c
index ef84b0f..e1f9580 100644
--- a/xen/common/vm_event.c
+++ b/xen/common/vm_event.c
@@ -417,6 +417,9 @@ void vm_event_resume(struct domain *d, struct vm_event_domain *ved)
 
         if ( rsp.flags & VM_EVENT_FLAG_VCPU_PAUSED )
         {
+            if ( rsp.flags & VM_EVENT_FLAG_SET_REGISTERS )
+                vm_event_set_registers(v, &rsp);
+
             if ( rsp.flags & VM_EVENT_FLAG_TOGGLE_SINGLESTEP )
                 vm_event_toggle_singlestep(d, v);
 
diff --git a/xen/include/asm-arm/vm_event.h b/xen/include/asm-arm/vm_event.h
index 976fdf1..4d0fbf7 100644
--- a/xen/include/asm-arm/vm_event.h
+++ b/xen/include/asm-arm/vm_event.h
@@ -47,4 +47,10 @@ void vm_event_register_write_resume(struct vcpu *v, vm_event_response_t *rsp)
     /* Not supported on ARM. */
 }
 
+static inline
+void vm_event_set_registers(struct vcpu *v, vm_event_response_t *rsp)
+{
+    /* Not supported on ARM. */
+}
+
 #endif /* __ASM_ARM_VM_EVENT_H__ */
diff --git a/xen/include/asm-x86/vm_event.h b/xen/include/asm-x86/vm_event.h
index 2ff2cab..5aff834 100644
--- a/xen/include/asm-x86/vm_event.h
+++ b/xen/include/asm-x86/vm_event.h
@@ -42,4 +42,6 @@ void vm_event_toggle_singlestep(struct domain *d, struct vcpu *v);
 
 void vm_event_register_write_resume(struct vcpu *v, vm_event_response_t *rsp);
 
+void vm_event_set_registers(struct vcpu *v, vm_event_response_t *rsp);
+
 #endif /* __ASM_X86_VM_EVENT_H__ */
diff --git a/xen/include/public/vm_event.h b/xen/include/public/vm_event.h
index ff2f217..9270d52 100644
--- a/xen/include/public/vm_event.h
+++ b/xen/include/public/vm_event.h
@@ -89,6 +89,13 @@
  * by the altp2m_idx response field if possible.
  */
 #define VM_EVENT_FLAG_ALTERNATE_P2M      (1 << 7)
+/*
+ * Set the vCPU registers to the values in the  vm_event response.
+ * At the moment x86-only, applies to EAX-EDX, ESP, EBP, ESI, EDI, R8-R15,
+ * EFLAGS, and EIP.
+ * Requires the vCPU to be paused already (synchronous events only).
+ */
+#define VM_EVENT_FLAG_SET_REGISTERS      (1 << 8)
 
 /*
  * Reasons for the vm event request
-- 
1.7.9.5