[Xen-changelog] [xen master] x86emul: special case far branch validation outside of long mode

[patchbot@xxxxxxx]

commit 46c2337ce61816d85a895b57d2792d88a55d9179
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Apr 25 14:18:55 2016 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Apr 25 14:18:55 2016 +0200

    x86emul: special case far branch validation outside of long mode
    
    In that case (with the new value being held in, or now in one case cast
    to, a 32-bit variable) there's no need to go through the long mode part
    of the checks.
    
    Primarily this was meant to hopefully address Coverity ID 1355278, but
    since the change produces smaller code as well I think we should use it
    even if it doesn't help that (benign) warning.
    
    Also it's more in line with jmp_rel() for commit_far_branch() to do the
    _regs.eip update, so adjust that at once.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Release-acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>
---
 xen/arch/x86/x86_emulate/x86_emulate.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c 
b/xen/arch/x86/x86_emulate/x86_emulate.c
index 10a2959..621332b 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -652,13 +652,20 @@ do {                                                      
              \
     _regs.eip = ip;                                                     \
 } while (0)
 
-#define validate_far_branch(cs, ip)                                     \
-    generate_exception_if(in_longmode(ctxt, ops) && (cs)->attr.fields.l \
-                          ? !is_canonical_address(ip)                   \
-                          : (ip) > (cs)->limit, EXC_GP, 0)
+#define validate_far_branch(cs, ip) ({                                  \
+    if ( sizeof(ip) <= 4 ) {                                            \
+        ASSERT(in_longmode(ctxt, ops) <= 0);                            \
+        generate_exception_if((ip) > (cs)->limit, EXC_GP, 0);           \
+    } else                                                              \
+        generate_exception_if(in_longmode(ctxt, ops) &&                 \
+                              (cs)->attr.fields.l                       \
+                              ? !is_canonical_address(ip)               \
+                              : (ip) > (cs)->limit, EXC_GP, 0);         \
+})
 
 #define commit_far_branch(cs, ip) ({                                    \
     validate_far_branch(cs, ip);                                        \
+    _regs.eip = (ip);                                                   \
     ops->write_segment(x86_seg_cs, cs, ctxt);                           \
 })
 
@@ -2802,7 +2809,6 @@ x86_emulate(
              (rc = load_seg(x86_seg_cs, src.val, 1, &cs, ctxt, ops)) ||
              (rc = commit_far_branch(&cs, dst.val)) )
             goto done;
-        _regs.eip = dst.val;
         break;
     }
 
@@ -2845,9 +2851,8 @@ x86_emulate(
         eflags &= 0x257fd5;
         _regs.eflags &= mask;
         _regs.eflags |= (eflags & ~mask) | 0x02;
-        _regs.eip = eip;
         if ( (rc = load_seg(x86_seg_cs, sel, 1, &cs, ctxt, ops)) ||
-             (rc = commit_far_branch(&cs, eip)) )
+             (rc = commit_far_branch(&cs, (uint32_t)eip)) )
             goto done;
         break;
     }
@@ -3480,7 +3485,6 @@ x86_emulate(
         if ( (rc = load_seg(x86_seg_cs, sel, 0, &cs, ctxt, ops)) ||
              (rc = commit_far_branch(&cs, eip)) )
             goto done;
-        _regs.eip = eip;
         break;
     }
 
@@ -3782,11 +3786,11 @@ x86_emulate(
                                       &_regs.eip, op_bytes, ctxt)) ||
                      (rc = ops->write_segment(x86_seg_cs, &cs, ctxt)) )
                     goto done;
+                _regs.eip = src.val;
             }
             else if ( (rc = load_seg(x86_seg_cs, sel, 0, &cs, ctxt, ops)) ||
                       (rc = commit_far_branch(&cs, src.val)) )
                 goto done;
-            _regs.eip = src.val;
 
             dst.type = OP_NONE;
             break;
--
generated by git-patchbot for /home/xen/git/xen.git#master

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