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

Dave Hansen
2015-03-25T00:12:12Z

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.