Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast path with retpolines on

[Andy Lutomirski]

On Thu, Jan 25, 2018 at 1:39 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> On Thu, Jan 25, 2018 at 1:31 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> On Thu, Jan 25, 2018 at 1:20 PM, Linus Torvalds
>> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>> On Thu, Jan 25, 2018 at 1:08 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>>>
>>>> With retpoline, the retpoline in the trampoline sucks.  I don't need
>>>> perf for that -- I've benchmarked it both ways.  It sucks.  I'll fix
>>>> it, but it'll be kind of complicated.
>>>
>>> Ahh, I'd forgotten about that (and obviously didn't see it in the profiles).
>>>
>>> But yeah, that is fixable even if it does require a page per CPU. Or
>>> did you have some clever scheme in mind?
>>
>> Nothing clever.  I was going to see if I could get actual
>> binutils-generated relocations to work in the trampoline.  We already
>> have code to parse ELF relocations and turn them into a simple table,
>> and it shouldn't be *that* hard to run a separate pass on the entry
>> trampoline.
>>
>> Another potentially useful if rather minor optimization would be to
>> rejigger the SYSCALL_DEFINE macros a bit.  Currently we treat all
>> syscalls like this:
>>
>> long func(long arg0, long arg1, long arg2, long arg3, long arg4, long arg5);
>>
>> I wonder if we'd be better off doing:
>>
>> long func(const struct pt_regs *regs);
>>
>> and autogenerating:
>>
>> static long SyS_read(const struct pt_regs *regs)
>> {
>>    return sys_reg(regs->di, ...);
>> }
>
> If you're rejiggering, can we also put in a mechanism for detecting
> which registers to clear so that userspace can't inject useful values
> into speculation paths?
>
> https://patchwork.kernel.org/patch/10153753/

My SYSCALL_DEFINE rejigger suggestion up-thread does this for free as
a side effect.

That being said, I think this would be more accurately characterized
as "so that userspace has a somewhat harder time injecting useful
values into speculation paths".