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

Andy Lutomirski
2015-03-25T00:20:34Z

[add Borislav]

I swear it would actually be an improvement if we just randomized the
function names. fpu_817, fpu_717, etc. At least no one would think
they understand them...

On Tue, Mar 24, 2015 at 5:18 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> 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
>