KVM: x86: allow RSM from 64-bit mode

The SDM says that exiting system management mode from 64-bit mode
is invalid, but that would be too good to be true.  But actually,
most of the code is already there to support exiting from compat
mode (EFER.LME=1, EFER.LMA=0).  Getting all the way from 64-bit
mode to real mode only requires clearing CS.L and CR4.PCIDE.

Cc: stable@vger.kernel.org
Fixes: 660a5d517aaab9187f93854425c4c63f4a09195c
Tested-by: Laszlo Ersek <lersek@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -2484,16 +2484,36 @@ static int em_rsm(struct x86_emulate_ctxt *ctxt)
 
 	/*
 	 * Get back to real mode, to prepare a safe state in which to load
-	 * CR0/CR3/CR4/EFER.
-	 *
-	 * CR4.PCIDE must be zero, because it is a 64-bit mode only feature.
+	 * CR0/CR3/CR4/EFER.  It's all a bit more complicated if the vCPU
+	 * supports long mode.
 	 */
+	cr4 = ctxt->ops->get_cr(ctxt, 4);
+	if (emulator_has_longmode(ctxt)) {
+		struct desc_struct cs_desc;
+
+		/* Zero CR4.PCIDE before CR0.PG.  */
+		if (cr4 & X86_CR4_PCIDE) {
+			ctxt->ops->set_cr(ctxt, 4, cr4 & ~X86_CR4_PCIDE);
+			cr4 &= ~X86_CR4_PCIDE;
+		}
+
+		/* A 32-bit code segment is required to clear EFER.LMA.  */
+		memset(&cs_desc, 0, sizeof(cs_desc));
+		cs_desc.type = 0xb;
+		cs_desc.s = cs_desc.g = cs_desc.p = 1;
+		ctxt->ops->set_segment(ctxt, 0, &cs_desc, 0, VCPU_SREG_CS);
+	}
+
+	/* For the 64-bit case, this will clear EFER.LMA.  */
 	cr0 = ctxt->ops->get_cr(ctxt, 0);
 	if (cr0 & X86_CR0_PE)
 		ctxt->ops->set_cr(ctxt, 0, cr0 & ~(X86_CR0_PG | X86_CR0_PE));
-	cr4 = ctxt->ops->get_cr(ctxt, 4);
+
+	/* Now clear CR4.PAE (which must be done before clearing EFER.LME).  */
 	if (cr4 & X86_CR4_PAE)
 		ctxt->ops->set_cr(ctxt, 4, cr4 & ~X86_CR4_PAE);
+
+	/* And finally go back to 32-bit mode.  */
 	efer = 0;
 	ctxt->ops->set_msr(ctxt, MSR_EFER, efer);
 
@@ -4454,7 +4474,7 @@ static const struct opcode twobyte_table[256] = {
 	F(DstMem | SrcReg | Src2CL | ModRM, em_shld), N, N,
 	/* 0xA8 - 0xAF */
 	I(Stack | Src2GS, em_push_sreg), I(Stack | Src2GS, em_pop_sreg),
-	II(No64 | EmulateOnUD | ImplicitOps, em_rsm, rsm),
+	II(EmulateOnUD | ImplicitOps, em_rsm, rsm),
 	F(DstMem | SrcReg | ModRM | BitOp | Lock | PageTable, em_bts),
 	F(DstMem | SrcReg | Src2ImmByte | ModRM, em_shrd),
 	F(DstMem | SrcReg | Src2CL | ModRM, em_shrd),