Re: [RFC PATCH 1/1] xen: Use a global mapping for runstate

[Bertrand Marquis <Bertrand.Marquis@arm.com>]

Hi Julien,

> On 28 May 2020, at 19:54, Julien Grall <julien@xen.org> wrote:
> 
> Hi Bertrand,
> 
> Thank you for the patch.
> 
> On 28/05/2020 16:25, Bertrand Marquis wrote:
>> At the moment on Arm, a Linux guest running with KTPI enabled will
>> cause the following error when a context switch happens in user mode:
>> (XEN) p2m.c:1890: d1v0: Failed to walk page-table va 0xffffff837ebe0cd0
>> This patch is modifying runstate handling to map the area given by the
>> guest inside Xen during the hypercall.
>> This is removing the guest virtual to physical conversion during context
>> switches which removes the bug
> 
> It would be good to spell out that a virtual address is not stable. So relying on it is wrong.
> 
>> and improve performance by preventing to
>> walk page tables during context switches.
> 
> With Secret free hypervisor in mind, I would like to suggest to map/unmap the runstate during context switch.
> 
> The cost should be minimal when there is a direct map (i.e on Arm64 and x86) and still provide better performance on Arm32.

Even with a minimal cost this is still adding some non real-time behaviour to the context switch.
But definitely from the security point of view as we have to map a page from the guest, we could have accessible in Xen some data that should not be there.
There is a trade here where:
- xen can protect by map/unmapping
- a guest which wants to secure his data should either not use it or make sure there is nothing else in the page

That sounds like a thread local storage kind of problematic where we want data from xen to be accessible fast from the guest and easy to be modified from xen.

> 
> The change should be minimal compare to the current approach but this could be taken care separately if you don't have time.

I could add that to the serie in a separate patch so that it can be discussed and test separately ?

> 
>> --
>> In the current status, this patch is only working on Arm and needs to
>> be fixed on X86 (see #error on domain.c for missing get_page_from_gva).
>> Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
>> ---
>>  xen/arch/arm/domain.c   | 32 +++++++++-------
>>  xen/arch/x86/domain.c   | 51 ++++++++++++++-----------
>>  xen/common/domain.c     | 84 ++++++++++++++++++++++++++++++++++-------
>>  xen/include/xen/sched.h | 11 ++++--
>>  4 files changed, 124 insertions(+), 54 deletions(-)
>> diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
>> index 31169326b2..799b0e0103 100644
>> --- a/xen/arch/arm/domain.c
>> +++ b/xen/arch/arm/domain.c
>> @@ -278,33 +278,37 @@ static void ctxt_switch_to(struct vcpu *n)
>>  /* Update per-VCPU guest runstate shared memory area (if registered). */
>>  static void update_runstate_area(struct vcpu *v)
>>  {
>> -    void __user *guest_handle = NULL;
>> -    struct vcpu_runstate_info runstate;
>> +    struct vcpu_runstate_info *runstate;
>>  -    if ( guest_handle_is_null(runstate_guest(v)) )
>> +    /* XXX why do we accept not to block here */
>> +    if ( !spin_trylock(&v->runstate_guest_lock) )
>>          return;
>>  -    memcpy(&runstate, &v->runstate, sizeof(runstate));
>> +    runstate = runstate_guest(v);
>> +
>> +    if (runstate == NULL)
>> +    {
>> +        spin_unlock(&v->runstate_guest_lock);
>> +        return;
>> +    }
>>        if ( VM_ASSIST(v->domain, runstate_update_flag) )
>>      {
>> -        guest_handle = &v->runstate_guest.p->state_entry_time + 1;
>> -        guest_handle--;
>> -        runstate.state_entry_time |= XEN_RUNSTATE_UPDATE;
>> -        __raw_copy_to_guest(guest_handle,
>> -                            (void *)(&runstate.state_entry_time + 1) - 1, 1);
>> +        runstate->state_entry_time |= XEN_RUNSTATE_UPDATE;
>>          smp_wmb();
> 
> Because you set v->runstate.state_entry_time below, the placement of the barrier seems a bit odd.
> 
> I would suggest to update v->runstate.state_entry_time first and then update runstate->state_entry_time.

We do want the guest to know when we modify the runstate so we need to make sure the XEN_RUNSTATE_UPDATE is actually set in a visible way before we do the memcpy.
That’s why the barrier is after.

> 
>> +        v->runstate.state_entry_time |= XEN_RUNSTATE_UPDATE;
>>      }
>>  -    __copy_to_guest(runstate_guest(v), &runstate, 1);
>> +    memcpy(runstate, &v->runstate, sizeof(v->runstate));
>>  -    if ( guest_handle )
>> +    if ( VM_ASSIST(v->domain, runstate_update_flag) )
>>      {
>> -        runstate.state_entry_time &= ~XEN_RUNSTATE_UPDATE;
>> +        runstate->state_entry_time &= ~XEN_RUNSTATE_UPDATE;
>>          smp_wmb();
> 
> You want to update runstate->state_entry_time after the barrier not before.
Agree

> 
>>  static void _update_runstate_area(struct vcpu *v)
>>  {
>> +    /* XXX: this should be removed */
>>      if ( !update_runstate_area(v) && is_pv_vcpu(v) &&
>>           !(v->arch.flags & TF_kernel_mode) )
>>          v->arch.pv.need_update_runstate_area = 1;
>> diff --git a/xen/common/domain.c b/xen/common/domain.c
>> index 7cc9526139..acc6f90ba3 100644
>> --- a/xen/common/domain.c
>> +++ b/xen/common/domain.c
>> @@ -161,6 +161,7 @@ struct vcpu *vcpu_create(struct domain *d, unsigned int vcpu_id)
>>      v->dirty_cpu = VCPU_CPU_CLEAN;
>>        spin_lock_init(&v->virq_lock);
>> +    spin_lock_init(&v->runstate_guest_lock);
>>        tasklet_init(&v->continue_hypercall_tasklet, NULL, NULL);
>>  @@ -691,6 +692,66 @@ int rcu_lock_live_remote_domain_by_id(domid_t dom, struct domain **d)
>>      return 0;
>>  }
>>  +static void  unmap_runstate_area(struct vcpu *v, unsigned int lock)
> 
> NIT: There is an extra space after void
> 
> Also, AFAICT, the lock is only taking two values. Please switch to a bool.

Agree

> 
>> +{
>> +    mfn_t mfn;
>> +
>> +    if ( ! runstate_guest(v) )
> 
> NIT: We don't usually put a space after !.
> 
> But shouldn't this be checked within the lock?

Agree

> 
> 
>> +        return;
>> +
>> +    if (lock)
> 
> NIT: if ( ... )
> 

Ack

>> +        spin_lock(&v->runstate_guest_lock);
>> +
>> +    mfn = domain_page_map_to_mfn(runstate_guest(v));
>> +
>> +    unmap_domain_page_global((void *)
>> +                            ((unsigned long)v->runstate_guest &
>> +                             PAGE_MASK));
>> +
>> +    put_page_and_type(mfn_to_page(mfn));
>> +    runstate_guest(v) = NULL;
>> +
>> +    if (lock)
> 
> Ditto.

Ack

> 
>> +        spin_unlock(&v->runstate_guest_lock);
>> +}
>> +
>> +static int map_runstate_area(struct vcpu *v,
>> +                    struct vcpu_register_runstate_memory_area *area)
>> +{
>> +    unsigned long offset = area->addr.p & ~PAGE_MASK;
>> +    void *mapping;
>> +    struct page_info *page;
>> +    size_t size = sizeof(struct vcpu_runstate_info);
>> +
>> +    ASSERT(runstate_guest(v) == NULL);
>> +
>> +    /* do not allow an area crossing 2 pages */
>> +    if ( offset > (PAGE_SIZE - size) )
>> +        return -EINVAL;
> 
> This is a change in behavior for the guest. If we are going forward with this, this will want a separate patch with its own explanation why this is done.

Ack I need to add support for an area crossing pages

> 
>> +
>> +#ifdef CONFIG_ARM
>> +    page = get_page_from_gva(v, area->addr.p, GV2M_WRITE);
> 
> A guest is allowed to setup the runstate for a different vCPU than the current one. This will lead to get_page_from_gva() to fail as the function cannot yet work with a vCPU other than current.

If the area is mapped per cpu but isn’t the aim of this to have a way to check other cpus status ?

> 
> AFAICT, there is no restriction on when the runstate hypercall can be called. So this can even be called before the vCPU is brought up.

I understand the remark but it still feels very weird to allow an invalid address in an hypercall.
Wouldn’t we have a lot of potential issues accepting an address that we cannot check ?


> 
> I was going to suggest to use the current vCPU for translating the address. However, it would be reasonable for an OS to use the same virtual address for all the vCPUs assuming the page-tables are different per vCPU.
> 
> Recent Linux are using a per-cpu area, so the virtual address should be different for each vCPU. But I don't know how the other OSes works. Roger should be able to help for FreeBSD at least.
> 
> I have CCed Paul for the Windows drivers.
> 
> If we decide to introduce some restriction then they should be explained in the commit message and also documented in the public header (we have been pretty bad at documenting change in the past!).

Agree

Cheers
Bertrand