On Thu, Nov 29, 2018 at 10:42:10AM +0100, Peter Zijlstra 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?
I don't quite follow that either. Maybe Andy is referring to x86-32, for which regs->sp isn't actually saved: see kernel_stack_pointer(). This code is 64-bit only so that's not a concern. > 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. Right. -- Josh