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

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Thu, Nov 29, 2018 at 03:38:53PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 29, 2018 at 05:37:39AM -0800, Andy Lutomirski wrote:
> > 
> > 
> > > 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.
> 
> ARGH!..
> 
> can't we 'fix' that again? The alternative is moving that IRET-frame and
> fixing everything up, which is going to be fragile, ugly and such
> things more.
> 
> Commit d8ba61ba58c8 ("x86/entry/64: Don't use IST entry for #BP stack")
> doesn't list any strong reasons for why it should NOT be an IST.

This seems to work...

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index ce25d84023c0..184523447d35 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -876,7 +876,7 @@ apicinterrupt IRQ_WORK_VECTOR			irq_work_interrupt		smp_irq_work_interrupt
  * @paranoid == 2 is special: the stub will never switch stacks.  This is for
  * #DF: if the thread stack is somehow unusable, we'll still get a useful OOPS.
  */
-.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1
+.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1 create_gap=0
 ENTRY(\sym)
 	UNWIND_HINT_IRET_REGS offset=\has_error_code*8
 
@@ -891,6 +891,12 @@ ENTRY(\sym)
 	pushq	$-1				/* ORIG_RAX: no syscall to restart */
 	.endif
 
+	.if \create_gap == 1
+	.rept 6
+	pushq 5*8(%rsp)
+	.endr
+	.endif
+
 	.if \paranoid == 1
 	testb	$3, CS-ORIG_RAX(%rsp)		/* If coming from userspace, switch stacks */
 	jnz	.Lfrom_usermode_switch_stack_\@
@@ -1126,7 +1132,7 @@ apicinterrupt3 HYPERV_STIMER0_VECTOR \
 #endif /* CONFIG_HYPERV */
 
 idtentry debug			do_debug		has_error_code=0	paranoid=1 shift_ist=DEBUG_STACK
-idtentry int3			do_int3			has_error_code=0
+idtentry int3			do_int3			has_error_code=0	create_gap=1
 idtentry stack_segment		do_stack_segment	has_error_code=1
 
 #ifdef CONFIG_XEN_PV