[Xen-devel] [PATCH 1/4] hvm/fep: Allow testing of instructions crossing the -1 -> 0 virtual boundary

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The Force Emulation Prefix is named to follow its PV counterpart for cpuid or
rdtsc, but isn't really an instruction prefix.  It behaves as a break-out into
Xen, with the purpose of emulating the next instruction in the current state.

It is important to be able to test legal situations which occur in real
hardware, including instruction which cross certain boundaries, and
instructions starting at 0.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/hvm/hvm.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 787f055..596a903 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -3981,15 +3981,8 @@ void hvm_ud_intercept(struct cpu_user_regs *regs)
         unsigned long addr;
         char sig[5]; /* ud2; .ascii "xen" */
 
-        /*
-         * Note that in the call below we pass 1 more than the signature
-         * size, to guard against the overall code sequence wrapping between
-         * "prefix" and actual instruction. There's necessarily at least one
-         * actual instruction byte required, so this won't cause failure on
-         * legitimate uses.
-         */
         if ( hvm_virtual_to_linear_addr(x86_seg_cs, cs, regs->eip,
-                                        sizeof(sig) + 1, hvm_access_insn_fetch,
+                                        sizeof(sig), hvm_access_insn_fetch,
                                         (hvm_long_mode_enabled(cur) &&
                                          cs->attr.fields.l) ? 64 :
                                         cs->attr.fields.db ? 32 : 16, &addr) &&
@@ -3999,6 +3992,11 @@ void hvm_ud_intercept(struct cpu_user_regs *regs)
         {
             regs->eip += sizeof(sig);
             regs->eflags &= ~X86_EFLAGS_RF;
+
+            /* Zero the upper 32 bits of %rip if not in long mode. */
+            if ( !(hvm_long_mode_enabled(cur) && cs->attr.fields.l) )
+                regs->eip = regs->_eip;
+
             add_taint(TAINT_HVM_FEP);
         }
     }
-- 
2.1.4


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