On 06/15/2018 01:33 PM, Jason A. Donenfeld wrote:
> On Fri, Jun 15, 2018 at 8:32 PM Andy Lutomirski <l...@kernel.org> wrote:
>> quite in the form you imagined. The idea that we've tossed around is
>> to restore FPU state on return to user mode. Roughly, we'd introduce
>> a new thread flag TIF_FPU_UNLOADED (name TBD).
>> prepare_exit_to_usermode() would notice this flag, copy the fpstate to
>> fpregs, and clear the flag. (Or maybe exit_to_usermode_loop() -- No
>> one has quite thought it through, but I think it should be outside the
>> loop.) We'd update all the FPU accessors to understand the flag.
> Yes! This is exactly what I was thinking. Then those calls to begin()
> and end() could be placed as close to the actual FPU usage as
> possible.
Andy, what was the specific concern about PKRU? That we might do:
kernel_fpu_begin(); <- Saves the first time
something()
kernel_fpu_end(); <- Does not XRSTOR
copy_from_user(); <- Sees old PKRU, does the wrong thing
prepare_exit_to_usermode(); <- Does the XRSTOR
// only now does PKRU have the right value
SYSRET/IRET
?
Does that *matter* unless something() modified PKRU? We could just make
the rule that nobody is supposed to mess with it and that it's not
covered by kernel_fpu_begin/end() semantics. We could even
theoretically enforce that in a debug environment if we watch its value.
