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

[Andy Lutomirski]

> On Nov 29, 2018, at 1:42 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> 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.
> 

int3 isnât IST anymore, so the int3 instruction conditionally subtracts 8 from RSP and then pushes SS, etc. So my email was obviously wrong wrt âcsâ, but youâre still potentially overwriting the int3 IRET frame.