Re: [PATCH v2 01/12] xen/trace: Don't over-read trace objects

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/09/2021 08:01, Jan Beulich wrote:
> On 21.09.2021 19:51, Andrew Cooper wrote:
>> On 21/09/2021 07:53, Jan Beulich wrote:
>>> On 20.09.2021 19:25, Andrew Cooper wrote:
>>>> --- a/xen/arch/x86/hvm/hvm.c
>>>> +++ b/xen/arch/x86/hvm/hvm.c
>>>> @@ -5063,8 +5063,9 @@ long do_hvm_op(unsigned long op, 
>>>> XEN_GUEST_HANDLE_PARAM(void) arg)
>>>>          if ( copy_from_guest(&tr, arg, 1 ) )
>>>>              return -EFAULT;
>>>>  
>>>> -        if ( tr.extra_bytes > sizeof(tr.extra)
>>>> -             || (tr.event & ~((1u<<TRC_SUBCLS_SHIFT)-1)) )
>>>> +        if ( tr.extra_bytes % sizeof(uint32_t) ||
>>>> +             tr.extra_bytes > sizeof(tr.extra) ||
>>>> +             tr.event >> TRC_SUBCLS_SHIFT )
>>>>              return -EINVAL;
>>> Despite this being a function that supposedly no-one is to really
>>> use, you're breaking the interface here when really there would be a
>>> way to be backwards compatible: Instead of failing, pad the data to
>>> a 32-bit boundary. As the interface struct is large enough, this
>>> would look to be as simple as a memset() plus aligning extra_bytes
>>> upwards. Otherwise the deliberate breaking of potential existing
>>> callers wants making explicit in the respective paragraph of the
>>> description.
>> It is discussed, along with a justification for why an ABI change is fine.
> What you say is "This has no business being a hypercall in the first
> place", yet to me that's not a justification to break an interface.

No, but "cannot be used outside of custom debugging" means there are no
users in practice, and therefore it really doesn't matter.

> It is part of the ABI, so disallowing what was previously allowed
> may break people's (questionable, yes) code.
>
>> But I could do
>>
>> tr.extra_bytes = ROUNDUP(tr.extra_bytes, sizeof(uint32_t));
>>
>> if you'd really prefer.
> I would, indeed, and as said ideally alongside clearing the excess
> bytes in the buffer.

Why?  The entire structure is copied out of guest memory, with a fixed size.

It's not Xen's fault/problem if the VM didn't initialise it correctly,
and an explicit ROUNDUP() here maintains the current behaviour.

>>>> --- a/xen/common/sched/rt.c
>>>> +++ b/xen/common/sched/rt.c
>>>> @@ -968,18 +968,20 @@ burn_budget(const struct scheduler *ops, struct 
>>>> rt_unit *svc, s_time_t now)
>>>>      /* TRACE */
>>>>      {
>>>>          struct __packed {
>>>> -            unsigned unit:16, dom:16;
>>>> +            uint16_t unit, dom;
>>>>              uint64_t cur_budget;
>>>> -            int delta;
>>>> -            unsigned priority_level;
>>>> -            bool has_extratime;
>>>> -        } d;
>>>> -        d.dom = svc->unit->domain->domain_id;
>>>> -        d.unit = svc->unit->unit_id;
>>>> -        d.cur_budget = (uint64_t) svc->cur_budget;
>>>> -        d.delta = delta;
>>>> -        d.priority_level = svc->priority_level;
>>>> -        d.has_extratime = svc->flags & RTDS_extratime;
>>>> +            uint32_t delta;
>>> The original field was plain int, and aiui for a valid reason. I
>>> don't see why you couldn't use int32_t here.
>> delta can't be negative, because there is a check earlier in the function.
> Oh, yes, didn't spot that.
>
>> What is a problem is the 63=>32 bit truncation, and uint32_t here is
>> half as bad as int32_t.
> Agreed. Whether the truncation is an issue in practice is questionable,
> as I wouldn't expect budget to be consumed in multiple-second individual
> steps. But I didn't check whether this scheduler might allow a vCPU to
> run for this long all in one go.

I expect it's marginal too.  Honestly, its not a bug I care to fix right
about now.  I could leave a /* TODO: truncation? */ in place so whomever
encounters weird behaviour from this trace record has a bit more help of
where to look?

~Andrew