Re: [PATCH 1/3] x86/entry: Clear extra registers beyond syscall arguments for 64bit kernels

[Andy Lutomirski]

On Mon, Feb 5, 2018 at 4:26 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>> [...]
>>
>> But as the commit message says, the system call argument registers are
>> also likely to be aggressively clobbered unless used, since the low
>> registers are preferred for code generation (smaller code, and many of
>> them are special anyway in various ways and have forced uses for
>> shifts, function arguments, or just are special in general like %rax).
>>
>> So the actual argument registers tend to not be an issue anyway.
>

>
> So I submit that we should probably extend the register clearing/sanitization to
> R10 and R11 as well, because while they are technically caller-saved and freely
> clobberable, in practice they don't get clobbered all that often and there might
> be various code paths into complex system calls where these R10/R11 values survive
> just fine and can be used in Spectre gadgets.


Maybe R11, but we have to be careful, since R11 is used as scratch
space in a bunch of the asm.  Clearing R10 is mostly useless in the
syscall path because we'll just unconditionally reload it in
do_syscall_64().  If we manage to change the way syscall wrappers
work, then we can think about clearing R10 and maybe even more regs.