linux-kernel - Re: [PATCH 09/14] x86/ibt: Implement IBT+

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Message-ID: <CAADnVQ+26b2t9LSwsSZuhL3CppvJvaucqU3UvO=ioLNdC67cyQ@mail.gmail.com>
Date: Mon, 30 Sep 2024 10:00:44 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	alyssa.milburn@...el.com, scott.d.constable@...el.com, 
	Joao Moreira <joao@...rdrivepizza.com>, Andrew Cooper <andrew.cooper3@...rix.com>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, "Jose E. Marchesi" <jose.marchesi@...cle.com>, 
	"H.J. Lu" <hjl.tools@...il.com>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Sami Tolvanen <samitolvanen@...gle.com>, Nathan Chancellor <nathan@...nel.org>, ojeda@...nel.org, 
	Kees Cook <kees@...nel.org>
Subject: Re: [PATCH 09/14] x86/ibt: Implement IBT+

On Mon, Sep 30, 2024 at 1:23 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Sun, Sep 29, 2024 at 10:38:58AM -0700, Alexei Starovoitov wrote:
> > On Fri, Sep 27, 2024 at 12:50 PM Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > --- a/arch/x86/net/bpf_jit_comp.c
> > > +++ b/arch/x86/net/bpf_jit_comp.c
> > > @@ -555,6 +555,8 @@ static int emit_patch(u8 **pprog, void *
> > >
> > >  static int emit_call(u8 **pprog, void *func, void *ip)
> > >  {
> > > +       if (is_endbr(func))
> > > +               func += ENDBR_INSN_SIZE;
> > >         return emit_patch(pprog, func, ip, 0xE8);
> > >  }
> > >
> > > @@ -562,11 +564,13 @@ static int emit_rsb_call(u8 **pprog, voi
> > >  {
> > >         OPTIMIZER_HIDE_VAR(func);
> > >         ip += x86_call_depth_emit_accounting(pprog, func, ip);
> > > -       return emit_patch(pprog, func, ip, 0xE8);
> > > +       return emit_call(pprog, func, ip);
> > >  }
> > >
> > >  static int emit_jump(u8 **pprog, void *func, void *ip)
> > >  {
> > > +       if (is_endbr(func))
> > > +               func += ENDBR_INSN_SIZE;
> > >         return emit_patch(pprog, func, ip, 0xE9);
> > >  }
> >
> > Makes sense, but it feels like it's fixing the existing bug
> > that we somehow didn't notice earlier?
>
> Before all this func()+0 was a valid call address -- as it's been
> forever.
>
> While it is true that with the introduction of ENDBR some compilers will
> do direct calls to func()+4 to avoid the ENDBR (less instructions, more
> faster etc..) this was very much an optional thing.
>
> Notably, up until this point we would use a 4 byte NOP to seal(*)
> functions, specifically so that anybody doing direct calls to func()+0
> would continue to work.
>
> These patches however change all that by sealing with a 4 byte UD1
> instruction, which makes direct calls to func()+0 fatal. As such, we
> must guarantee all direct calls are to func()+4. So what used to be an
> optimization is now a strict requirement.
>
> Indirect calls still go to func()+0 (or func()-16 for FineIBT) and will
> go *bang* if !ENDBR or UD1 (depending on the hardware having CET/IBT
> support).
>
> (*) with sealing we mean the explicit action of disallowing indirect
> calls to a given function.

I see. Thanks for explaining. I would copy paste the above details
into the commit log.