Re: [PATCH master/3.5.y] KVM: VMX: Fix ds/es corruption on i386 with preemption

[Chris Clayton <chris2553@googlemail.com>]

On 08/01/12 14:48, Avi Kivity wrote:
> Commit b2da15ac26a0c ("KVM: VMX: Optimize %ds, %es reload") broke i386
> in the following scenario:
>
>    vcpu_load
>    ...
>    vmx_save_host_state
>    vmx_vcpu_run
>    (ds.rpl, es.rpl cleared by hardware)
>
>    interrupt
>      push ds, es  # pushes bad ds, es
>      schedule
>        vmx_vcpu_put
>          vmx_load_host_state
>            reload ds, es (with __USER_DS)
>      pop ds, es  # of other thread's stack
>      iret
>    # other thread runs
>    interrupt
>      push ds, es
>      schedule  # back in vcpu thread
>      pop ds, es  # now with rpl=0
>      iret
>    ...
>    vcpu_put
>    resume_userspace
>    iret  # clears ds, es due to mismatched rpl
>
> (instead of resume_userspace, we might return with SYSEXIT and then
> take an exception; when the exception IRETs we end up with cleared
> ds, es)
>
> Fix by avoiding the optimization on i386 and reloading ds, es on the
> lightweight exit path.
>
> Reported-by: Chris Clayron <chris2553@googlemail.com>
> Signed-off-by: Avi Kivity <avi@redhat.com>

I've just had 15 successful runs of qemu-kvm-1.1.1 with kernel 3.5.0 
plus this patch, so:

Tested-by: Chris Clayton <chris2553@googlemail.com>

I assume this will be forwarded to stable once it has been applied to 
mainline.

> ---
>   arch/x86/kvm/vmx.c | 20 +++++++++++++-------
>   1 file changed, 13 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index c39b607..c00f03d 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -1488,13 +1488,6 @@ static void __vmx_load_host_state(struct vcpu_vmx *vmx)
>   		loadsegment(ds, vmx->host_state.ds_sel);
>   		loadsegment(es, vmx->host_state.es_sel);
>   	}
> -#else
> -	/*
> -	 * The sysexit path does not restore ds/es, so we must set them to
> -	 * a reasonable value ourselves.
> -	 */
> -	loadsegment(ds, __USER_DS);
> -	loadsegment(es, __USER_DS);
>   #endif
>   	reload_tss();
>   #ifdef CONFIG_X86_64
> @@ -6370,6 +6363,19 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>   #endif
>   	      );
>
> +#ifndef CONFIG_X86_64
> +	/*
> +	 * The sysexit path does not restore ds/es, so we must set them to
> +	 * a reasonable value ourselves.
> +	 *
> +	 * We can't defer this to vmx_load_host_state() since that function
> +	 * may be executed in interrupt context, which saves and restore segments
> +	 * around it, nullifying its effect.
> +	 */
> +	loadsegment(ds, __USER_DS);
> +	loadsegment(es, __USER_DS);
> +#endif
> +
>   	vcpu->arch.regs_avail = ~((1 << VCPU_REGS_RIP) | (1 << VCPU_REGS_RSP)
>   				  | (1 << VCPU_EXREG_RFLAGS)
>   				  | (1 << VCPU_EXREG_CPL)
>