Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

[Peter Zijlstra]

On Wed, Nov 28, 2018 at 10:05:54PM -0800, Andy Lutomirski wrote:

> >> +static void static_call_bp_handler(struct pt_regs *regs, void *_data)
> >> +{
> >> +    struct static_call_bp_data *data = _data;
> >> +
> >> +    /*
> >> +     * For inline static calls, push the return address on the stack so the
> >> +     * "called" function will return to the location immediately after the
> >> +     * call site.
> >> +     *
> >> +     * NOTE: This code will need to be revisited when kernel CET gets
> >> +     *       implemented.
> >> +     */
> >> +    if (data->ret) {
> >> +        regs->sp -= sizeof(long);
> >> +        *(unsigned long *)regs->sp = data->ret;
> >> +    }
> 
> You canât do this.  Depending on the alignment of the old RSP, which
> is not guaranteed, this overwrites regs->cs.  IRET goes boom.

I don't get it; can you spell that out?

The way I understand it is that we're at a location where a "E8 - Near
CALL" instruction should be, and thus RSP should be the regular kernel
stack, and the above simply does "PUSH ret", which is what that CALL
would've done too.