linux-kernel - Re: [syzbot] KASAN: stack-out-of-bounds Read in profile

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Message-ID: <20210603132958.3es4xagwdijmzpqh@treble>
Date:   Thu, 3 Jun 2021 08:29:58 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     syzbot <syzbot+84fe685c02cd112a2ac3@...kaller.appspotmail.com>,
        bp@...en8.de, hpa@...or.com, inglorion@...gle.com,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        syzkaller-bugs@...glegroups.com, tglx@...utronix.de,
        x86@...nel.org, Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [syzbot] KASAN: stack-out-of-bounds Read in profile_pc

On Wed, Jun 02, 2021 at 04:35:11PM -0700, Andi Kleen wrote:
> 
> > profile_pc() assumes the return address is either directly at regs->sp,
> > or one word adjacent to it due to saved flags, both of which are just
> > completely wrong.  This code has probably never worked with ORC, and
> > nobody noticed apparently.
> 
> I presume it used to work because the lock functions were really simple, but
> that's not true anymore.

Yeah, I figured as much.

> > We could just use ORC to unwind to the next frame.  Though, isn't
> > /proc/profile redundant, compared to all the more sophisticated options
> > nowadays?  Is there still a distinct use case for it or can we just
> > remove it?
> 
> It's still needed for some special cases. For example there is no other
> viable way to profile early boot without a VM
> 
> I would just drop the hack to unwind, at least for the early boot profile
> use case locking profiling is usually not needed.

Ok, I'll just get rid of the hack then.

-- 
Josh