KVM: x86: Always use 32-bit SMRAM save state for 32-bit kernels
Invoking the 64-bit variation on a 32-bit kenrel will crash the guest,
trigger a WARN, and/or lead to a buffer overrun in the host, e.g.
rsm_load_state_64() writes r8-r15 unconditionally, but enum kvm_reg and
thus x86_emulate_ctxt._regs only define r8-r15 for CONFIG_X86_64.
KVM allows userspace to report long mode support via CPUID, even though
the guest is all but guaranteed to crash if it actually tries to enable
long mode. But, a pure 32-bit guest that is ignorant of long mode will
happily plod along.
SMM complicates things as 64-bit CPUs use a different SMRAM save state
area. KVM handles this correctly for 64-bit kernels, e.g. uses the
legacy save state map if userspace has hid long mode from the guest,
but doesn't fare well when userspace reports long mode support on a
32-bit host kernel (32-bit KVM doesn't support 64-bit guests).
Since the alternative is to crash the guest, e.g. by not loading state
or explicitly requesting shutdown, unconditionally use the legacy SMRAM
save state map for 32-bit KVM. If a guest has managed to get far enough
to handle SMIs when running under a weird/buggy userspace hypervisor,
then don't deliberately crash the guest since there are no downsides
(from KVM's perspective) to allow it to continue running.
Fixes: 660a5d517aaab ("KVM: x86: save/load state on SMM switch")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index f328482..d0d5dd4 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2331,12 +2331,16 @@ static int em_lseg(struct x86_emulate_ctxt *ctxt)
static int emulator_has_longmode(struct x86_emulate_ctxt *ctxt)
{
+#ifdef CONFIG_X86_64
u32 eax, ebx, ecx, edx;
eax = 0x80000001;
ecx = 0;
ctxt->ops->get_cpuid(ctxt, &eax, &ebx, &ecx, &edx, false);
return edx & bit(X86_FEATURE_LM);
+#else
+ return false;
+#endif
}
static void rsm_set_desc_flags(struct desc_struct *desc, u32 flags)
@@ -2372,6 +2376,7 @@ static int rsm_load_seg_32(struct x86_emulate_ctxt *ctxt, const char *smstate,
return X86EMUL_CONTINUE;
}
+#ifdef CONFIG_X86_64
static int rsm_load_seg_64(struct x86_emulate_ctxt *ctxt, const char *smstate,
int n)
{
@@ -2391,6 +2396,7 @@ static int rsm_load_seg_64(struct x86_emulate_ctxt *ctxt, const char *smstate,
ctxt->ops->set_segment(ctxt, selector, &desc, base3, n);
return X86EMUL_CONTINUE;
}
+#endif
static int rsm_enter_protected_mode(struct x86_emulate_ctxt *ctxt,
u64 cr0, u64 cr3, u64 cr4)
@@ -2492,6 +2498,7 @@ static int rsm_load_state_32(struct x86_emulate_ctxt *ctxt,
return rsm_enter_protected_mode(ctxt, cr0, cr3, cr4);
}
+#ifdef CONFIG_X86_64
static int rsm_load_state_64(struct x86_emulate_ctxt *ctxt,
const char *smstate)
{
@@ -2554,6 +2561,7 @@ static int rsm_load_state_64(struct x86_emulate_ctxt *ctxt,
return X86EMUL_CONTINUE;
}
+#endif
static int em_rsm(struct x86_emulate_ctxt *ctxt)
{
@@ -2621,9 +2629,11 @@ static int em_rsm(struct x86_emulate_ctxt *ctxt)
if (ctxt->ops->pre_leave_smm(ctxt, buf))
return X86EMUL_UNHANDLEABLE;
+#ifdef CONFIG_X86_64
if (emulator_has_longmode(ctxt))
ret = rsm_load_state_64(ctxt, buf);
else
+#endif
ret = rsm_load_state_32(ctxt, buf);
if (ret != X86EMUL_CONTINUE) {