Re: [PATCH 01/17] x86, fpu: wrap get_xsave_addr() to make it safer

Andy Lutomirski
2015-03-25T00:19:02Z

On Tue, Mar 24, 2015 at 5:12 PM, Dave Hansen <dave.hansen@intel.com> wrote:
> On 03/24/2015 04:52 PM, Andy Lutomirski wrote:
>> On Tue, Mar 24, 2015 at 4:42 PM, Dave Hansen <dave.hansen@intel.com> wrote:
>>> On 03/24/2015 03:28 PM, Andy Lutomirski wrote:
>>>> Your function appears to be getting it for write (I assume that's what
>>>> the unlazy_fpu is for), so I'd rather have it called
>>>> tsk_get_xsave_field_for_write or something like that.
>>>
>>> It should be entirely read-only.
>>>
>>> For MPX (the only user of get_xsave_addr() iirc), we are only worried
>>> about getting the status codes (and addresses) out of the bndstatus
>>> register and making sure that the kernel-recorded bounds directory
>>> address matches the bndcfgu (configuration) register.
>>>
>>> We don't ever write to the registers.
>>
>> So why are you unlazying it?
>
> Oleg actually suggested it.
>
>> IIUC, the xstae for current can be in one of three logical states:
>>
>> 1. Live in CPU regs. The in-memory copy is garbage and the state is
>> in CPU regs.
>> 2. Lazy. The in-memory copy and the CPU regs match. Writing to
>> either copy is illegal.
>> 3. In memory only. Writing to the in-memory copy is safe.
>>
>> IIUC, you want to read the xstate, do you're okay with #2 or #3. This
>> would be tsk_get_xsave_field_for_read in my terminology.
>>
>> If you want to write the xstate, you'd need to be in state #3, which
>> would be tsk_get_xsave_field_for_write.
>>
>> IIUC, unlazy_fpu just moves from from state 2 to 3.
>
> I won't completely claim to understand what's going on with the FPU
> code, but I think your analysis is a bit off.
>
> unlazy_fpu() does __save_init_fpu() which (among other things) calls
> xsave to dump the CPU registers to memory. That doesn't make any sense
> to do if "The in-memory copy and the CPU regs match."
>
> IOW, unlazy_fpu() is called when the in-memory copy is garbage and takes
> us to a state where we can look at the in-memory copy.

I think that __save_init_fpu (called by unlazy_fpu) does that, but
__thread_fpu_end calls __thread_clear_has_fpu, which, in turn, zaps
fpu_owner_task, which will force an unnecessary xrstor. Or maybe not
if we have further bugs.

Holy crap these functions are poorly named. Also, what, if anything,
guarantees that fpu_owner_task is set on entry to userspace? Do we
even need it to be set? Oleg, help?

--Andy

--
Andy Lutomirski
AMA Capital Management, LLC