Re: KVM vs AMD: Re: [PATCH v3 48/59] x86/retbleed: Add SKL return thunk

Nathan Chancellor
2022-11-04T15:33:22Z

On Fri, Nov 04, 2022 at 01:44:46PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 03, 2022 at 10:53:54PM +0000, Andrew Cooper wrote:
> > On 21/10/2022 16:21, Nathan Chancellor wrote:
> > > On Fri, Oct 21, 2022 at 11:53:09AM +0200, Peter Zijlstra wrote:
> > >> On Thu, Oct 20, 2022 at 04:10:28PM -0700, Nathan Chancellor wrote:
> > >>> This commit is now in -next as commit 5d8213864ade ("x86/retbleed: Add
> > >>> SKL return thunk"). I just bisected an immediate reboot on my AMD test
> > >>> system when starting a virtual machine with QEMU + KVM to it (see the
> > >>> bisect log below). My Intel test systems do not show this.
> > >>> Unfortunately, I do not have much more information, as there are no logs
> > >>> in journalctl, which makes sense as the reboot occurs immediately after
> > >>> I hit the enter key for the QEMU command.
> > >>>
> > >>> If there is any further information I can provide or patches I can test
> > >>> for further debugging, I am more than happy to do so.
> > >> Moo :-(
> > >>
> > >> you happen to have a .config for me?
> > > Sure thing, sorry I did not provide it in the first place! Attached. It
> > > has been run through localmodconfig for the particular machine but I
> > > assume the core pieces should still be present.
> >
> > Following up from some debugging on IRC.
> >
> > The problem is that FILL_RETURN_BUFFER now has a per-cpu variable
> > access, and AMD SVM has a fun optimisation where the VMRUN instruction
> > doesn't swap, amongst other things, %gs.
> >
> > per-cpu variables only become safe following
> > vmload(__sme_page_pa(sd->save_area)); in svm_vcpu_enter_exit().
> >
> > Given that retbleed=force ought to work on non-skylake hardware, the
> > appropriate fix is to move the VMLOAD/VMSAVE's down into asm and put
> > them adjacent to VMRUN.
> >
> > This also addresses an undocumented dependency where its only the memory
> > clobber in vmload() which stops the compiler moving
> > svm_vcpu_enter_exit()'s calculation of sd into an unsafe position.
>
> So, aside from wasting the entire morning on resuscitating my AMD
> Interlagos, I ended up with the below patch which seems to work.
>
> Not being a virt person, I'm sure I've messed up something, please
> advise.

I too am not a virt person but this survives spawning a guest on the
host and in the guest, which is the extent of the testing I do with KVM
on a regular basis.

Tested-by: Nathan Chancellor <nathan@kernel.org>

Thanks again for looking into it and Andrew for the assists along the
way!

> ---
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 58f0077d9357..f7ee1eedacfe 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3929,11 +3929,8 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu)
> * the state doesn't need to be copied between vmcb01 and
> * vmcb02 when switching vmcbs for nested virtualization.
> */
> - vmload(svm->vmcb01.pa);
> - __svm_vcpu_run(vmcb_pa, (unsigned long *)&vcpu->arch.regs);
> - vmsave(svm->vmcb01.pa);
> -
> - vmload(__sme_page_pa(sd->save_area));
> + __svm_vcpu_run(vmcb_pa, (unsigned long *)&vcpu->arch.regs,
> + svm->vmcb01.pa, __sme_page_pa(sd->save_area));
> }
>
> guest_state_exit_irqoff();
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 6a7686bf6900..2a038def7ac7 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -684,6 +684,7 @@ void sev_es_unmap_ghcb(struct vcpu_svm *svm);
> /* vmenter.S */
>
> void __svm_sev_es_vcpu_run(unsigned long vmcb_pa);
> -void __svm_vcpu_run(unsigned long vmcb_pa, unsigned long *regs);
> +void __svm_vcpu_run(unsigned long vmcb_pa, unsigned long *regs,
> + unsigned long guest_vmcb_pa, unsigned long host_vmcb_pa);
>
> #endif
> diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S
> index 09eacf19d718..50f200f7b773 100644
> --- a/arch/x86/kvm/svm/vmenter.S
> +++ b/arch/x86/kvm/svm/vmenter.S
> @@ -32,8 +32,10 @@
>
> /**
> * __svm_vcpu_run - Run a vCPU via a transition to SVM guest mode
> - * @vmcb_pa: unsigned long
> - * @regs: unsigned long * (to guest registers)
> + * @vmcb_pa: unsigned long
> + * @regs: unsigned long * (to guest registers)
> + * @guest_vmcb_pa: unsigned long
> + * @host_vmcb_pa: unsigned long
> */
> SYM_FUNC_START(__svm_vcpu_run)
> push %_ASM_BP
> @@ -51,9 +53,18 @@ SYM_FUNC_START(__svm_vcpu_run)
> /* Save @regs. */
> push %_ASM_ARG2
>
> + /* Save host_vmcb_pa */
> + push %_ASM_ARG4
> +
> + /* Save guest_vmcb_pa */
> + push %_ASM_ARG3
> +
> /* Save @vmcb. */
> push %_ASM_ARG1
>
> + /* Save guest_vmcb_pa */
> + push %_ASM_ARG3
> +
> /* Move @regs to RAX. */
> mov %_ASM_ARG2, %_ASM_AX
>
> @@ -75,15 +86,29 @@ SYM_FUNC_START(__svm_vcpu_run)
> mov VCPU_R15(%_ASM_AX), %r15
> #endif
>
> + /* POP and VMLOAD @guest_vmcb01_pa */
> + pop %_ASM_AX
> +1: vmload %_ASM_AX
> +2:
> /* "POP" @vmcb to RAX. */
> pop %_ASM_AX
>
> /* Enter guest mode */
> sti
>
> -1: vmrun %_ASM_AX
> +3: vmrun %_ASM_AX
> +4:
> + cli
>
> -2: cli
> + /* POP and VMSAVE @guest_vmcb01_pa */
> + pop %_ASM_AX
> +5: vmsave %_ASM_AX
> +6:
> + /* POP and VMLOAD @host_vmcb01_pa */
> + pop %_ASM_AX
> +7: vmload %_ASM_AX
> +8:
> + /* Now host %GS is live */
>
> #ifdef CONFIG_RETPOLINE
> /* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */
> @@ -160,11 +185,26 @@ SYM_FUNC_START(__svm_vcpu_run)
> pop %_ASM_BP
> RET
>
> -3: cmpb $0, kvm_rebooting
> +10: cmpb $0, kvm_rebooting
> jne 2b
> ud2
>
> - _ASM_EXTABLE(1b, 3b)
> +30: cmpb $0, kvm_rebooting
> + jne 4b
> + ud2
> +
> +50: cmpb $0, kvm_rebooting
> + jne 6b
> + ud2
> +
> +70: cmpb $0, kvm_rebooting
> + jne 8b
> + ud2
> +
> + _ASM_EXTABLE(1b, 10b)
> + _ASM_EXTABLE(3b, 30b)
> + _ASM_EXTABLE(5b, 50b)
> + _ASM_EXTABLE(7b, 70b)
>
> SYM_FUNC_END(__svm_vcpu_run)
>