Re: [PATCH] KVM: x86: nVMX: allow RSM to restore VMXE CR4 flag

[Liran Alon]

> On 26 Mar 2019, at 15:07, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
> 
> Commit 5bea5123cbf0 ("KVM: VMX: check nested state and CR4.VMXE against
> SMM") introduced a check to vmx_set_cr4() forbidding to set VMXE from SMM.
> The check is correct, however, there is a special case when RSM is called
> to leave SMM: rsm_enter_protected_mode() is called with HF_SMM_MASK still
> set and in case VMXE was set before entering SMM we're failing to return.
> 
> Resolve the issue by temporary dropping HF_SMM_MASK around set_cr4() calls
> when ops->set_cr() is called from RSM.
> 
> Reported-by: Jon Doron <arilou@xxxxxxxxx>
> Suggested-by: Liran Alon <liran.alon@xxxxxxxxxx>
> Fixes: 5bea5123cbf0 ("KVM: VMX: check nested state and CR4.VMXE against SMM")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

Patch looks good to me.
Reviewed-by: Liran Alon <liran.alon@xxxxxxxxxx>

> ---
> - Instread of putting the temporary HF_SMM_MASK drop to
>  rsm_enter_protected_mode() (as was suggested by Liran), move it to
>  emulator_set_cr() modifying its interface. emulate.c seems to be
>  vcpu-specifics-free at this moment, we may want to keep it this way.
> - It seems that Hyper-V+UEFI on KVM is still broken, I'm observing sporadic
>  hangs even with this patch. These hangs, however, seem to be unrelated to
>  rsm.

Feel free to share details on these hangs ;)

Great work,
-Liran

> ---
> arch/x86/include/asm/kvm_emulate.h |  3 ++-
> arch/x86/kvm/emulate.c             | 27 ++++++++++++++-------------
> arch/x86/kvm/x86.c                 | 12 +++++++++++-
> 3 files changed, 27 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
> index 93c4bf598fb0..6c33caa82fa5 100644
> --- a/arch/x86/include/asm/kvm_emulate.h
> +++ b/arch/x86/include/asm/kvm_emulate.h
> @@ -203,7 +203,8 @@ struct x86_emulate_ops {
> 	void (*set_gdt)(struct x86_emulate_ctxt *ctxt, struct desc_ptr *dt);
> 	void (*set_idt)(struct x86_emulate_ctxt *ctxt, struct desc_ptr *dt);
> 	ulong (*get_cr)(struct x86_emulate_ctxt *ctxt, int cr);
> -	int (*set_cr)(struct x86_emulate_ctxt *ctxt, int cr, ulong val);
> +	int (*set_cr)(struct x86_emulate_ctxt *ctxt, int cr, ulong val,
> +		      bool from_rsm);
> 	int (*cpl)(struct x86_emulate_ctxt *ctxt);
> 	int (*get_dr)(struct x86_emulate_ctxt *ctxt, int dr, ulong *dest);
> 	int (*set_dr)(struct x86_emulate_ctxt *ctxt, int dr, ulong value);
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index c338984c850d..a6204105d4d7 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -2413,7 +2413,7 @@ static int rsm_enter_protected_mode(struct x86_emulate_ctxt *ctxt,
> 		cr3 &= ~0xfff;
> 	}
> 
> -	bad = ctxt->ops->set_cr(ctxt, 3, cr3);
> +	bad = ctxt->ops->set_cr(ctxt, 3, cr3, true);
> 	if (bad)
> 		return X86EMUL_UNHANDLEABLE;
> 
> @@ -2422,20 +2422,20 @@ static int rsm_enter_protected_mode(struct x86_emulate_ctxt *ctxt,
> 	 * Then enable protected mode.	However, PCID cannot be enabled
> 	 * if EFER.LMA=0, so set it separately.
> 	 */
> -	bad = ctxt->ops->set_cr(ctxt, 4, cr4 & ~X86_CR4_PCIDE);
> +	bad = ctxt->ops->set_cr(ctxt, 4, cr4 & ~X86_CR4_PCIDE, true);
> 	if (bad)
> 		return X86EMUL_UNHANDLEABLE;
> 
> -	bad = ctxt->ops->set_cr(ctxt, 0, cr0);
> +	bad = ctxt->ops->set_cr(ctxt, 0, cr0, true);
> 	if (bad)
> 		return X86EMUL_UNHANDLEABLE;
> 
> 	if (cr4 & X86_CR4_PCIDE) {
> -		bad = ctxt->ops->set_cr(ctxt, 4, cr4);
> +		bad = ctxt->ops->set_cr(ctxt, 4, cr4, true);
> 		if (bad)
> 			return X86EMUL_UNHANDLEABLE;
> 		if (pcid) {
> -			bad = ctxt->ops->set_cr(ctxt, 3, cr3 | pcid);
> +			bad = ctxt->ops->set_cr(ctxt, 3, cr3 | pcid, true);
> 			if (bad)
> 				return X86EMUL_UNHANDLEABLE;
> 		}
> @@ -2581,7 +2581,7 @@ static int em_rsm(struct x86_emulate_ctxt *ctxt)
> 
> 		/* Zero CR4.PCIDE before CR0.PG.  */
> 		if (cr4 & X86_CR4_PCIDE) {
> -			ctxt->ops->set_cr(ctxt, 4, cr4 & ~X86_CR4_PCIDE);
> +			ctxt->ops->set_cr(ctxt, 4, cr4 & ~X86_CR4_PCIDE, true);
> 			cr4 &= ~X86_CR4_PCIDE;
> 		}
> 
> @@ -2595,11 +2595,12 @@ static int em_rsm(struct x86_emulate_ctxt *ctxt)
> 	/* 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));
> +		ctxt->ops->set_cr(ctxt, 0, cr0 & ~(X86_CR0_PG | X86_CR0_PE),
> +				  true);
> 
> 	/* 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);
> +		ctxt->ops->set_cr(ctxt, 4, cr4 & ~X86_CR4_PAE, true);
> 
> 	/* And finally go back to 32-bit mode.  */
> 	efer = 0;
> @@ -3131,7 +3132,7 @@ static int load_state_from_tss32(struct x86_emulate_ctxt *ctxt,
> 	int ret;
> 	u8 cpl;
> 
> -	if (ctxt->ops->set_cr(ctxt, 3, tss->cr3))
> +	if (ctxt->ops->set_cr(ctxt, 3, tss->cr3, false))
> 		return emulate_gp(ctxt, 0);
> 	ctxt->_eip = tss->eip;
> 	ctxt->eflags = tss->eflags | 2;
> @@ -3331,7 +3332,7 @@ static int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt,
> 		write_segment_descriptor(ctxt, tss_selector, &next_tss_desc);
> 	}
> 
> -	ops->set_cr(ctxt, 0,  ops->get_cr(ctxt, 0) | X86_CR0_TS);
> +	ops->set_cr(ctxt, 0,  ops->get_cr(ctxt, 0) | X86_CR0_TS, false);
> 	ops->set_segment(ctxt, tss_selector, &next_tss_desc, 0, VCPU_SREG_TR);
> 
> 	if (has_error_code) {
> @@ -3633,7 +3634,7 @@ static int em_movbe(struct x86_emulate_ctxt *ctxt)
> 
> static int em_cr_write(struct x86_emulate_ctxt *ctxt)
> {
> -	if (ctxt->ops->set_cr(ctxt, ctxt->modrm_reg, ctxt->src.val))
> +	if (ctxt->ops->set_cr(ctxt, ctxt->modrm_reg, ctxt->src.val, false))
> 		return emulate_gp(ctxt, 0);
> 
> 	/* Disable writeback. */
> @@ -3766,7 +3767,7 @@ static int em_clts(struct x86_emulate_ctxt *ctxt)
> 
> 	cr0 = ctxt->ops->get_cr(ctxt, 0);
> 	cr0 &= ~X86_CR0_TS;
> -	ctxt->ops->set_cr(ctxt, 0, cr0);
> +	ctxt->ops->set_cr(ctxt, 0, cr0, false);
> 	return X86EMUL_CONTINUE;
> }
> 
> @@ -3866,7 +3867,7 @@ static int em_smsw(struct x86_emulate_ctxt *ctxt)
> static int em_lmsw(struct x86_emulate_ctxt *ctxt)
> {
> 	ctxt->ops->set_cr(ctxt, 0, (ctxt->ops->get_cr(ctxt, 0) & ~0x0eul)
> -			  | (ctxt->src.val & 0x0f));
> +			  | (ctxt->src.val & 0x0f), false);
> 	ctxt->dst.type = OP_NONE;
> 	return X86EMUL_CONTINUE;
> }
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a419656521b6..f2745e3170b6 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5739,7 +5739,8 @@ static unsigned long emulator_get_cr(struct x86_emulate_ctxt *ctxt, int cr)
> 	return value;
> }
> 
> -static int emulator_set_cr(struct x86_emulate_ctxt *ctxt, int cr, ulong val)
> +static int emulator_set_cr(struct x86_emulate_ctxt *ctxt, int cr, ulong val,
> +			   bool from_rsm)
> {
> 	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
> 	int res = 0;
> @@ -5755,7 +5756,16 @@ static int emulator_set_cr(struct x86_emulate_ctxt *ctxt, int cr, ulong val)
> 		res = kvm_set_cr3(vcpu, val);
> 		break;
> 	case 4:
> +		/*
> +		 * set_cr4() may forbid to set certain flags (e.g. VMXE) from
> +		 * SMM but we're actually leaving it; temporary drop HF_SMM_MASK
> +		 * when setting CR4.
> +		 */
> +		if (from_rsm)
> +			vcpu->arch.hflags &= ~HF_SMM_MASK;
> 		res = kvm_set_cr4(vcpu, mk_cr_64(kvm_read_cr4(vcpu), val));
> +		if (from_rsm)
> +			vcpu->arch.hflags |= HF_SMM_MASK;
> 		break;
> 	case 8:
> 		res = kvm_set_cr8(vcpu, val);
> -- 
> 2.20.1
>