linux-kernel - Re: KASAN: slab-out-of-bounds Read in strcmp

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Date:   Mon, 4 Dec 2017 19:49:10 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     james.l.morris@...cle.com
Cc:     bot+015afdb01dbf2abb6a6bfdd5430b72e5503fca6d@...kaller.appspotmail.com,
        linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov,
        syzkaller-bugs@...glegroups.com, danielj@...lanox.com,
        dledford@...hat.com, eparis@...isplace.org, junil0814.lee@....com,
        kyeongdon.kim@....com, linux-kernel@...r.kernel.org,
        mka@...omium.org, paul@...l-moore.com, sds@...ho.nsa.gov,
        serge@...lyn.com, penguin-kernel@...ove.SAKURA.ne.jp
Subject: Re: KASAN: slab-out-of-bounds Read in strcmp

Tetsuo Handa wrote:
> James Morris wrote:
> > On Sun, 3 Dec 2017, Tetsuo Handa wrote:
> > 
> > > Tetsuo Handa wrote:
> > > > which will allow strcmp() to trigger out of bound read when "size" is
> > > > larger than strlen(initial_sid_to_string[i]).
> > > 
> > > Oops. "smaller" than.
> > > 
> > > > 
> > > > Thus, I guess the simplest fix is to use strncmp() instead of strcmp().
> > > 
> > > Can somebody test below patch? (My CentOS 7 environment does not support
> > > enabling SELinux in linux.git . Userspace tool is too old to support?)
> > 
> > You mean enabling KASAN?  Yep, you need gcc 4.9.2 or better.  Recent 
> > Fedora has it.
> 
> I was not able to find "SELinux:  Initializing." line for some reason, and
> it turned out that I just forgot to run "make install". ;-)
> 
> I tested using debug printk() and init for built-in initramfs shown below.
> It is strange that KASAN does not trigger upon strcmp()ing
> initial_sid_to_string[1]. But anyway, my patch fixes this problem.

Sorry for noise. It was just initial_sid_to_string[10] which compared the second byte.
Thus, nothing is strange.