linux-kernel - Re: [patch V4 01/32] mm/slab: Fix broken stack trace storage

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Message-ID: <alpine.DEB.2.21.1904152318020.1806@nanos.tec.linutronix.de>
Date:   Mon, 15 Apr 2019 23:20:42 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [patch V4 01/32] mm/slab: Fix broken stack trace storage

On Mon, 15 Apr 2019, Josh Poimboeuf wrote:
> On Mon, Apr 15, 2019 at 06:07:44PM +0200, Thomas Gleixner wrote:
> > > 
> > > Looks like stack_trace.nr_entries isn't initialized?  (though this code
> > > gets eventually replaced by a later patch)
> > 
> > struct initializer initialized the non mentioned fields to 0, if I'm not
> > totally mistaken.
> 
> Hm, it seems you are correct.  And I thought I knew C.

:)

> > > Who actually reads this stack trace?  I couldn't find a consumer.
> > 
> > It's stored directly in the memory pointed to by @addr and that's the freed
> > cache memory. If that is used later (UAF) then the stack trace can be
> > printed to see where it was freed.
> 
> Right... but who reads it?

Indeed. I didn't check but I know that I saw that info printed at least a
decade ago. Looks like that debug magic in slab.c has seen major changes
since then.

Thanks,

	tglx