Re: memcpy is leaking secret data through ZMM vector registers

[Florian Weimer <fweimer@redhat.com>]

* Mikulas Patocka:

> On Fri, 19 Apr 2024, Zack Weinberg wrote:
>
>> On Fri, Apr 19, 2024, at 4:15 PM, Mikulas Patocka wrote:
>> > On Fri, 19 Apr 2024, Zack Weinberg wrote:
>> >> ... the copy
>> >> of round_keys in the vector registers *won't* get erased -- the exact
>> >> problem being discussed in this thread.
>> >
>> > On the SYSV ABI, all the vector registers are volatile, so you can erase 
>> > them in explicit_bzero.
>> >
>> > On Windows 64-bit ABI, it is more problematic, because some of the vector 
>> > registers must be preserved.
>> 
>> Oh, huh. Yes, that would work.
>
> I've just realized that this wouldn't work - if the function 
> explicit_bzero is lazily resolved, the dynamic linker would spill the 
> vector registers to the stack prior to calling explicit_bzero.

No, the dynamic linker makes a tail call to explicit_bzero.  There's no
register restore on the return path, all that happens before the tail
call.

Thanks,
Florian