linux-kernel - Re: [PATCH] objtool,x86: Verify poke_int3

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Date:   Mon, 27 Jul 2020 15:11:13 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     peterz@...radead.org
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org, mbenes@...e.cz
Subject: Re: [PATCH] objtool,x86: Verify poke_int3_handler() is self contained


* peterz@...radead.org <peterz@...radead.org> wrote:

> On Mon, Jul 27, 2020 at 01:21:44PM +0200, Ingo Molnar wrote:
> > 
> > * peterz@...radead.org <peterz@...radead.org> wrote:
> > 
> > > 
> > > Abuse the SMAP rules to ensure poke_int3_handler() doesn't call out to
> > > anything.
> > > 
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > > ---
> > >  tools/objtool/check.c |    8 ++++++++
> > >  1 file changed, 8 insertions(+)
> > > 
> > > --- a/tools/objtool/check.c
> > > +++ b/tools/objtool/check.c
> > > @@ -551,6 +551,14 @@ static const char *uaccess_safe_builtin[
> > >  	"__memcpy_mcsafe",
> > >  	"mcsafe_handle_tail",
> > >  	"ftrace_likely_update", /* CONFIG_TRACE_BRANCH_PROFILING */
> > > +	/*
> > > +	 * Abuse alert!
> > > +	 *
> > > +	 * poke_int3_handler() is not in fact related to uaccess at all, we
> > > +	 * abuse the uaccess rules to ensure poke_int3_handler() is self
> > > +	 * contained and doesn't CALL out to other code.
> > > +	 */
> > > +	"poke_int3_handler",
> > 
> > So ->uaccess_safe makes sure that we don't CALL into non-uaccess-safe 
> > functions, but it still allows CALLs into *other* uaccess-safe 
> > functions, right?
> > 
> > So unless I missed something in the logic, the comment should say 
> > something like "doesn't CALL out to other non-uaccess safe code" or 
> > so? Which is, arguably, like 99% of all functions - but still, a whole 
> > bunch are allowed, such as low level instrumentation and other utility 
> > functions.
> 
> Right, so poke_int3_handler() is also noinstr and by that isn't allowed
> to call out into !noinstr code. The intersection should be small.
> 
> But yeah, perhaps this is a bad idea and I should add another annotation
> for this,.. dunno.

'nocall' ? :-)

	Ingo