KVM: x86: SYSENTER emulation is broken

SYSENTER emulation is broken in several ways:
1. It misses the case of 16-bit code segments completely (CVE-2015-0239).
2. MSR_IA32_SYSENTER_CS is checked in 64-bit mode incorrectly (bits 0 and 1 can
   still be set without causing #GP).
3. MSR_IA32_SYSENTER_EIP and MSR_IA32_SYSENTER_ESP are not masked in
   legacy-mode.
4. There is some unneeded code.

Fix it.

Cc: stable@vger.linux.org
Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 8 insertions, 19 deletions

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 817c6ca2c4fb..de12c1d379f1 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2348,7 +2348,7 @@ static int em_sysenter(struct x86_emulate_ctxt *ctxt)
          * Not recognized on AMD in compat mode (but is recognized in legacy
          * mode).
          */
-        if ((ctxt->mode == X86EMUL_MODE_PROT32) && (efer & EFER_LMA)
+        if ((ctxt->mode != X86EMUL_MODE_PROT64) && (efer & EFER_LMA)
             && !vendor_intel(ctxt))
                 return emulate_ud(ctxt);
 
@@ -2359,25 +2359,13 @@ static int em_sysenter(struct x86_emulate_ctxt *ctxt)
         setup_syscalls_segments(ctxt, &cs, &ss);
 
         ops->get_msr(ctxt, MSR_IA32_SYSENTER_CS, &msr_data);
-        switch (ctxt->mode) {
-        case X86EMUL_MODE_PROT32:
-                if ((msr_data & 0xfffc) == 0x0)
-                        return emulate_gp(ctxt, 0);
-                break;
-        case X86EMUL_MODE_PROT64:
-                if (msr_data == 0x0)
-                        return emulate_gp(ctxt, 0);
-                break;
-        default:
-                break;
-        }
+        if ((msr_data & 0xfffc) == 0x0)
+                return emulate_gp(ctxt, 0);
 
         ctxt->eflags &= ~(EFLG_VM | EFLG_IF);
-        cs_sel = (u16)msr_data;
-        cs_sel &= ~SELECTOR_RPL_MASK;
+        cs_sel = (u16)msr_data & ~SELECTOR_RPL_MASK;
         ss_sel = cs_sel + 8;
-        ss_sel &= ~SELECTOR_RPL_MASK;
-        if (ctxt->mode == X86EMUL_MODE_PROT64 || (efer & EFER_LMA)) {
+        if (efer & EFER_LMA) {
                 cs.d = 0;
                 cs.l = 1;
         }
@@ -2386,10 +2374,11 @@ static int em_sysenter(struct x86_emulate_ctxt *ctxt)
         ops->set_segment(ctxt, ss_sel, &ss, 0, VCPU_SREG_SS);
 
         ops->get_msr(ctxt, MSR_IA32_SYSENTER_EIP, &msr_data);
-        ctxt->_eip = msr_data;
+        ctxt->_eip = (efer & EFER_LMA) ? msr_data : (u32)msr_data;
 
         ops->get_msr(ctxt, MSR_IA32_SYSENTER_ESP, &msr_data);
-        *reg_write(ctxt, VCPU_REGS_RSP) = msr_data;
+        *reg_write(ctxt, VCPU_REGS_RSP) = (efer & EFER_LMA) ? msr_data :
+                                                              (u32)msr_data;
 
         return X86EMUL_CONTINUE;
 }