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

[Andy Lutomirski]

On Mon, Nov 26, 2018 at 9:10 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
> On Mon, Nov 26, 2018 at 05:02:17PM +0100, Peter Zijlstra wrote:
> > On Mon, Nov 26, 2018 at 07:55:00AM -0600, Josh Poimboeuf wrote:
> > > diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
> > > index 8026d176f25c..d3869295b88d 100644
> > > --- a/arch/x86/kernel/static_call.c
> > > +++ b/arch/x86/kernel/static_call.c
> > > @@ -9,13 +9,21 @@
> > >
> > >  void static_call_bp_handler(void);
> > >  void *bp_handler_dest;
> > > +void *bp_handler_continue;
> > >
> > >  asm(".pushsection .text, \"ax\"                                            \n"
> > >      ".globl static_call_bp_handler                                 \n"
> > >      ".type static_call_bp_handler, @function                               \n"
> > >      "static_call_bp_handler:                                               \n"
> > > -    "ANNOTATE_RETPOLINE_SAFE                                               \n"
> > > +#ifdef CONFIG_HAVE_STATIC_CALL_INLINE
> > > +    ANNOTATE_RETPOLINE_SAFE
> > > +    "call *bp_handler_dest                                         \n"
> > > +    ANNOTATE_RETPOLINE_SAFE
> > > +    "jmp *bp_handler_continue                                              \n"
> > > +#else /* !CONFIG_HAVE_STATIC_CALL_INLINE */
> > > +    ANNOTATE_RETPOLINE_SAFE
> > >      "jmp *bp_handler_dest                                          \n"
> > > +#endif
> > >      ".popsection                                                   \n");
> > >
> > >  void arch_static_call_transform(void *site, void *tramp, void *func)
> > > @@ -25,7 +33,10 @@ void arch_static_call_transform(void *site, void *tramp, void *func)
> > >     unsigned char insn_opcode;
> > >     unsigned char opcodes[CALL_INSN_SIZE];
> > >
> > > -   insn = (unsigned long)tramp;
> > > +   if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE))
> > > +           insn = (unsigned long)site;
> > > +   else
> > > +           insn = (unsigned long)tramp;
> > >
> > >     mutex_lock(&text_mutex);
> > >
> > > @@ -41,8 +52,10 @@ void arch_static_call_transform(void *site, void *tramp, void *func)
> > >     opcodes[0] = insn_opcode;
> > >     memcpy(&opcodes[1], &dest_relative, CALL_INSN_SIZE - 1);
> > >
> > > -   /* Set up the variable for the breakpoint handler: */
> > > +   /* Set up the variables for the breakpoint handler: */
> > >     bp_handler_dest = func;
> > > +   if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE))
> > > +           bp_handler_continue = (void *)(insn + CALL_INSN_SIZE);
> > >
> > >     /* Patch the call site: */
> > >     text_poke_bp((void *)insn, opcodes, CALL_INSN_SIZE,
> >
> > OK, so this is where that static_call_bp_handler comes from; you need
> > that CALL to frob the stack.
> >
> > But I still think it is broken; consider:
> >
> >       CPU0                            CPU1
> >
> >       bp_handler = ponies;
> >
> >       text_poke_bp(, &static_call_bp_handler)
> >         text_poke(&int3);
> >         on_each_cpu(sync)
> >                                       <IPI>
> >                                         ...
> >                                       </IPI>
> >
> >         text_poke(/* all but first bytes */)
> >         on_each_cpu(sync)
> >                                       <IPI>
> >                                         ...
> >                                       </IPI>
> >
> >                                       <int3>
> >                                         pt_regs->ip = &static_call_bp_handler
> >                                       </int3>
> >
> >                                       // VCPU takes a nap...
> >         text_poke(/* first byte */)
> >         on_each_cpu(sync)
> >                                       <IPI>
> >                                         ...
> >                                       </IPI>
> >
> >                                       // VCPU sleeps more
> >       bp_handler = unicorn;
> >
> >                                       CALL unicorn
> >
> > *whoops*
> >
> > Now, granted, that is all rather 'unlikely', but that never stopped
> > Murphy.
>
> Good find, thanks Peter.
>
> As we discussed on IRC, we'll need to fix this from within the int3
> exception handler by faking the call: putting a fake return address on
> the stack (pointing to right after the call) and setting regs->ip to the
> called function.

Can you add a comment that it will need updating when kernel CET is added?