[PATCH 3.2 152/152] KVM: x86: SYSENTER emulation is broken

Ben Hutchings
2015-02-17T01:46:53Z

From: Nadav Amit <namit@cs.technion.ac.il>

commit f3747379accba8e95d70cec0eae0582c8c182050 upstream.

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>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
arch/x86/kvm/emulate.c | 27 ++++++++-------------------
1 file changed, 8 insertions(+), 19 deletions(-)

--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2083,7 +2083,7 @@ static int em_sysenter(struct x86_emulat
* 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);

@@ -2096,23 +2096,13 @@ static int em_sysenter(struct x86_emulat
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;
- }
+ if ((msr_data & 0xfffc) == 0x0)
+ return emulate_gp(ctxt, 0);

ctxt->eflags &= ~(EFLG_VM | EFLG_IF | EFLG_RF);
- 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;
}
@@ -2121,10 +2111,11 @@ static int em_sysenter(struct x86_emulat
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);
- ctxt->regs[VCPU_REGS_RSP] = msr_data;
+ ctxt->regs[VCPU_REGS_RSP] = (efer & EFER_LMA) ? msr_data :
+ (u32)msr_data;

return X86EMUL_CONTINUE;
}