[Xen-changelog] [xen staging] x86emul: vendor specific SYSENTER/SYSEXIT behavior in long mode

[patchbot@xxxxxxx]

commit 94d7992ba21c29d4ea1c6a94e83b8ab2040ad01b
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Mar 26 12:36:30 2020 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Mar 26 12:36:30 2020 +0100

    x86emul: vendor specific SYSENTER/SYSEXIT behavior in long mode
    
    Intel CPUs permit both insns there while AMD ones don't.
    
    While at it also
    - drop the ring 0 check from SYSENTER handling - neither Intel's nor
      AMD's insn pages have any indication of #GP(0) getting raised when
      executed from ring 0, and trying it out in practice also confirms
      the check shouldn't be there,
    - move SYSENTER segment register writing until after the (in principle
      able to fail) MSR reads.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/x86_emulate/x86_emulate.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c 
b/xen/arch/x86/x86_emulate/x86_emulate.c
index 1ecdf5315a..c3efaff633 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -6587,7 +6587,7 @@ x86_emulate(
 
     case X86EMUL_OPC(0x0f, 0x34): /* sysenter */
         vcpu_must_have(sep);
-        generate_exception_if(mode_ring0(), EXC_GP, 0);
+        generate_exception_if(amd_like(ctxt) && ctxt->lma, EXC_UD);
         generate_exception_if(!in_protmode(ctxt, ops), EXC_GP, 0);
 
         fail_if(ops->read_msr == NULL);
@@ -6610,11 +6610,6 @@ x86_emulate(
         sreg.limit = ~0u;  /* 4GB limit */
         sreg.attr = 0xc93; /* G+DB+P+S+Data */
 
-        fail_if(ops->write_segment == NULL);
-        if ( (rc = ops->write_segment(x86_seg_cs, &cs, ctxt)) != 0 ||
-             (rc = ops->write_segment(x86_seg_ss, &sreg, ctxt)) != 0 )
-            goto done;
-
         if ( (rc = ops->read_msr(MSR_IA32_SYSENTER_EIP,
                                  &msr_val, ctxt)) != X86EMUL_OKAY )
             goto done;
@@ -6625,11 +6620,19 @@ x86_emulate(
             goto done;
         _regs.r(sp) = ctxt->lma ? msr_val : (uint32_t)msr_val;
 
+        fail_if(!ops->write_segment);
+        if ( (rc = ops->write_segment(x86_seg_cs, &cs,
+                                      ctxt)) != X86EMUL_OKAY ||
+             (rc = ops->write_segment(x86_seg_ss, &sreg,
+                                      ctxt)) != X86EMUL_OKAY )
+            goto done;
+
         singlestep = _regs.eflags & X86_EFLAGS_TF;
         break;
 
     case X86EMUL_OPC(0x0f, 0x35): /* sysexit */
         vcpu_must_have(sep);
+        generate_exception_if(amd_like(ctxt) && ctxt->lma, EXC_UD);
         generate_exception_if(!mode_ring0(), EXC_GP, 0);
         generate_exception_if(!in_protmode(ctxt, ops), EXC_GP, 0);
 
--
generated by git-patchbot for /home/xen/git/xen.git#staging

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