Re: [Xen-devel] [PATCH] x86/vm_event: correctly gather gs_shadow value

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/05/2019 15:58, Tamas K Lengyel wrote:
> On Wed, May 1, 2019 at 7:45 AM Tamas K Lengyel <tamas@xxxxxxxxxxxxx> wrote:
>> On Wed, May 1, 2019 at 1:50 AM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
>> wrote:
>>> On 01/05/2019 05:22, Tamas K Lengyel wrote:
>>>> Currently the gs_shadow value is only cached when the vCPU is being 
>>>> scheduled
>>>> out by Xen. Reporting this (usually) stale value through vm_event is 
>>>> incorrect,
>>>> since it doesn't represent the actual state of the vCPU at the time the 
>>>> event
>>>> was recorded. This prevents vm_event subscribers from correctly finding 
>>>> kernel
>>>> structures in the guest when it is trapped while in ring3.
>>>>
>>>> Signed-off-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
>>>> Cc: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
>>>> Cc: Jan Beulich <jbeulich@xxxxxxxx>
>>>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
>>>> Cc: Roger Pau Monne <roger.pau@xxxxxxxxxx>
>>>> ---
>>>>  xen/arch/x86/vm_event.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/xen/arch/x86/vm_event.c b/xen/arch/x86/vm_event.c
>>>> index 51c3493b1d..4464940da7 100644
>>>> --- a/xen/arch/x86/vm_event.c
>>>> +++ b/xen/arch/x86/vm_event.c
>>> Actually, come to think of it, the same is true for the SYSENTER
>>> details, which by default are read/write to the guest while it is
>>> scheduled.  As a result, the details reported here will from the last
>>> vcpu context switch, and possibly stale.
>> I'll look into it.
> The sysenter values look fine to me because vmx_save_vmcs_ctxt calls
> vmx_vmcs_save, which refreshes the values from the actual VMCS. It's
> only shadow_gs that is not refreshed.

So, you are correct that we call into vmx_vmcs_save() which causes the
SYSENTER details to be correct.

However, the same path also calls vmx_save_cpu_state() which saves
shadow_gs, and therefore ought to DTRT for the previous use in vm_event.

The problem is that vmx_{save,load}_cpu_state() only function correctly
in remote context, which is why the stale shadow_gs persists.

Contrary to what I said earlier, I now think that a better fix would be
to make the above functions safe to use use in current context (at least
for the save side - the restore side can probably just ASSERT() atm, as
it doesn't seem to have an equivalent use).

That way, the vm_event code doesn't need to contain any
context-sensitive code, which is a better overall, IMO.

~Andrew

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