[Xen-devel] [PATCH v3] x86/emul: Fix the handling of unimplemented Grp7 instructions

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Grp7 is abnormally complicated to decode, even by x86's standards, with
{s,l}msw being the problematic cases.

Previously, any value which fell through the first switch statement (looking
for instructions with entirely implicit operands) would be interpreted by the
second switch statement (handling instructions with memory operands).

Unimplemented instructions would then hit the #UD case for having a non-memory
operand, rather than taking the cannot_emulate path.

Consolidate the two switch statements into a single one, using ranges to cover
the instructions with memory operands.

This also fixes the emulation of xend, which would hit the #UD path when it
should complete with no side effects.

Reported-by: Petre Pircalabu <ppircalabu@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Petre Pircalabu <ppircalabu@xxxxxxxxxxxxxxx>

v3:
 * Use a single switch statement.

v2:
 * Use break rather than goto complete_insn for implicit instructions.
 * Note that we actually fix the behaviour of xend.

RFC as I've only done light testing so far.
---
 xen/arch/x86/x86_emulate/x86_emulate.c | 49 +++++++++++++++++++++-------------
 1 file changed, 31 insertions(+), 18 deletions(-)

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c 
b/xen/arch/x86/x86_emulate/x86_emulate.c
index 2201852..43e0e00 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -4987,9 +4987,12 @@ x86_emulate(
         }
         break;
 
-    case X86EMUL_OPC(0x0f, 0x01): /* Grp7 */ {
+    case X86EMUL_OPC(0x0f, 0x01): /* Grp7 */
+    {
         unsigned long base, limit, cr0, cr0w;
 
+        seg = (modrm_reg & 1) ? x86_seg_idtr : x86_seg_gdtr;
+
         switch( modrm )
         {
         case 0xca: /* clac */
@@ -5000,7 +5003,7 @@ x86_emulate(
             _regs.eflags &= ~X86_EFLAGS_AC;
             if ( modrm == 0xcb )
                 _regs.eflags |= X86_EFLAGS_AC;
-            goto complete_insn;
+            break;
 
 #ifdef __XEN__
         case 0xd1: /* xsetbv */
@@ -5012,7 +5015,7 @@ x86_emulate(
                                   handle_xsetbv(_regs.ecx,
                                                 _regs.eax | (_regs.rdx << 32)),
                                   EXC_GP, 0);
-            goto complete_insn;
+            break;
 #endif
 
         case 0xd4: /* vmfunc */
@@ -5020,7 +5023,7 @@ x86_emulate(
             fail_if(!ops->vmfunc);
             if ( (rc = ops->vmfunc(ctxt)) != X86EMUL_OKAY )
                 goto done;
-            goto complete_insn;
+            break;
 
         case 0xd5: /* xend */
             generate_exception_if(vex.pfx, EXC_UD);
@@ -5034,7 +5037,7 @@ x86_emulate(
                                   EXC_UD);
             /* Neither HLE nor RTM can be active when we get here. */
             _regs.eflags |= X86_EFLAGS_ZF;
-            goto complete_insn;
+            break;
 
         case 0xdf: /* invlpga */
             generate_exception_if(!in_protmode(ctxt, ops), EXC_UD);
@@ -5043,7 +5046,7 @@ x86_emulate(
             if ( (rc = ops->invlpg(x86_seg_none, truncate_ea(_regs.r(ax)),
                                    ctxt)) )
                 goto done;
-            goto complete_insn;
+            break;
 
         case 0xf9: /* rdtscp */
             fail_if(ops->read_msr == NULL);
@@ -5091,16 +5094,16 @@ x86_emulate(
                 base += sizeof(zero);
                 limit -= sizeof(zero);
             }
-            goto complete_insn;
-        }
+            break;
         }
 
-        seg = (modrm_reg & 1) ? x86_seg_idtr : x86_seg_gdtr;
+#define _GRP7(mod, reg) \
+            (((mod) << 6) | ((reg) << 3)) ... (((mod) << 6) | ((reg) << 3) | 7)
+#define GRP7_MEM(reg) _GRP7(0, reg): case _GRP7(1, reg): case _GRP7(2, reg)
+#define GRP7_ALL(reg) GRP7_MEM(reg): case _GRP7(3, reg)
 
-        switch ( modrm_reg & 7 )
-        {
-        case 0: /* sgdt */
-        case 1: /* sidt */
+        case GRP7_MEM(0): /* sgdt */
+        case GRP7_MEM(1): /* sidt */
             generate_exception_if(ea.type != OP_MEM, EXC_UD);
             generate_exception_if(umip_active(ctxt, ops), EXC_GP, 0);
             fail_if(!ops->read_segment || !ops->write);
@@ -5119,8 +5122,9 @@ x86_emulate(
                                   op_bytes, ctxt)) != X86EMUL_OKAY )
                 goto done;
             break;
-        case 2: /* lgdt */
-        case 3: /* lidt */
+
+        case GRP7_MEM(2): /* lgdt */
+        case GRP7_MEM(3): /* lidt */
             generate_exception_if(!mode_ring0(), EXC_GP, 0);
             generate_exception_if(ea.type != OP_MEM, EXC_UD);
             fail_if(ops->write_segment == NULL);
@@ -5138,7 +5142,8 @@ x86_emulate(
             if ( (rc = ops->write_segment(seg, &sreg, ctxt)) )
                 goto done;
             break;
-        case 4: /* smsw */
+
+        case GRP7_ALL(4): /* smsw */
             generate_exception_if(umip_active(ctxt, ops), EXC_GP, 0);
             if ( ea.type == OP_MEM )
             {
@@ -5153,7 +5158,8 @@ x86_emulate(
             if ( (rc = ops->read_cr(0, &dst.val, ctxt)) )
                 goto done;
             break;
-        case 6: /* lmsw */
+
+        case GRP7_ALL(6): /* lmsw */
             fail_if(ops->read_cr == NULL);
             fail_if(ops->write_cr == NULL);
             generate_exception_if(!mode_ring0(), EXC_GP, 0);
@@ -5169,16 +5175,23 @@ x86_emulate(
             if ( (rc = ops->write_cr(0, cr0, ctxt)) )
                 goto done;
             break;
-        case 7: /* invlpg */
+
+        case GRP7_MEM(7): /* invlpg */
             generate_exception_if(!mode_ring0(), EXC_GP, 0);
             generate_exception_if(ea.type != OP_MEM, EXC_UD);
             fail_if(ops->invlpg == NULL);
             if ( (rc = ops->invlpg(ea.mem.seg, ea.mem.off, ctxt)) )
                 goto done;
             break;
+
         default:
             goto cannot_emulate;
         }
+
+#undef GRP7_ALL
+#undef GRP7_MEM
+#undef _GRP7
+
         break;
     }
 
-- 
2.1.4


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