linux-kernel - Re: [PATCH 04/29] x86/livepatch: Validate __fentry_

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Message-ID: <20220223092327.0c73872d@gandalf.local.home>
Date:   Wed, 23 Feb 2022 09:23:27 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, Miroslav Benes <mbenes@...e.cz>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [PATCH 04/29] x86/livepatch: Validate __fentry__ location

On Wed, 23 Feb 2022 07:41:39 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -1578,7 +1578,23 @@ unsigned long ftrace_location_range(unsi
> >   */
> >  unsigned long ftrace_location(unsigned long ip)
> >  {
> > -	return ftrace_location_range(ip, ip);
> > +	struct dyn_ftrace *rec;
> > +	unsigned long offset;
> > +	unsigned long size;
> > +
> > +	rec = lookup_rec(ip, ip);
> > +	if (!rec) {
> > +		if (!kallsyms_lookup_size_offset(ip, &size, &offset))
> > +			goto out;
> > +
> > +		rec = lookup_rec(ip - offset, (ip - offset) + size);
> > +	}
> > +  
> 
> Please create a new function for this. Perhaps find_ftrace_location().
> 
> ftrace_location() is used to see if the address given is a ftrace
> nop or not. This change will make it always return true.

Now we could do:

	return ip <= (rec->ip + MCOUNT_INSN_SIZE) ? rec->ip : 0;

Since we would want rec->ip if the pointer is before the ftrace
instruction. But we would need to audit all use cases and make sure this is
not called from any hot paths (in a callback).

This will affect kprobes and BPF as they both use ftrace_location() as well.

-- Steve