Re: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of dynamic user state

[Andy Lutomirski]

On 6/16/21 9:27 AM, Dave Hansen wrote:
> On 5/23/21 12:32 PM, Chang S. Bae wrote:
>> @@ -571,6 +612,8 @@ static inline void switch_fpu_finish(struct fpu *new_fpu)
>>  
>>  	set_thread_flag(TIF_NEED_FPU_LOAD);
>>  
>> +	xfd_switch(old_fpu, new_fpu);
> 
> This seems fragile.
> 
> XSAVE* will decline to write out state for feature i when XFD[i]=1 and
> will instead write out the init state.  That means that, at this point,
> we switch XFD and yet leave state for feature i in place.
> 
> That means this *MUST* come before any copy_fpregs_to_fpstate() occurs.
> 
> Could we please add some FPU_WARN_ON() checks to ensure that no XSAVE*
> is ever performed with XINUSE=1 *and* XFD armed?
> 

Wait, really?  I somehow thought that XSAVE* would write out the actual
state even if XFD=1.

This seems like it's asking for some kind of bug.