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

Andy Lutomirski
2018-11-29T13:37:45Z

> On Nov 29, 2018, at 1:42 AM, Peter Zijlstra <peterz@infradead.org> 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 can’t 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 isn’t IST anymore, so the int3 instruction conditionally subtracts 8 from RSP and then pushes SS, etc. So my email was obviously wrong wrt “cs”, but you’re still potentially overwriting the int3 IRET frame.