linux-kernel - Re: [PATCH 1/2] x86/dumpstack: on oops do not rewind stack for kthread

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20161021055630.GA3102@twins.programming.kicks-ass.net>
Date:   Fri, 21 Oct 2016 07:56:30 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Roman Pen <roman.penyaev@...fitbricks.com>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86/dumpstack: on oops do not rewind stack for
 kthread

On Thu, Oct 20, 2016 at 04:07:28PM -0700, Andy Lutomirski wrote:
> On Wed, Sep 21, 2016 at 8:43 AM, Roman Pen
> <roman.penyaev@...fitbricks.com> wrote:
> > kthread uses stack and keeps completion structure on it to be woken up
> > on vfork_done completion.
> >
> > In commit 2deb4be28 Andy Lutomirski rewinds the stack unconditionally
> > and further completion of task->vfork_done for any kthread leads to stack
> > corruption (or infinite spin on attempt to spin lock on garbage memory).
> 
> This is sort of okay, but it will blow up pretty badly if a kthread
> overflows its stack.  Would it make more sense to change
> rewind_stack_do_exit() to leave a big enough gap at the top of the
> stack to avoid clobbering the completion?

We need to preserve the entire struct kthread on the stack, kthread just
abuses that pointer to stash an on-stack kthread descriptor. See
kthread():

  current->vfork_done = &self.exited;

Its a horrible horrible thing kthread does. I suppose there might have
been some intent by keeping that exited completion last in the
structure, but *shudder*.

But yes, leaving enough stack to not clobber that might keep this horror
show working.

ISTR talk about alternative schemes for this a long time ago, but I
cannot recall :-(