linux-kernel - Re: [lkp-robot] [printk] 7f7c60e066: BUG:KASAN:slab-out-of-bounds

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Date:   Mon, 30 Oct 2017 19:47:04 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Tobin C. Harding" <me@...in.cc>
Cc:     kernel test robot <xiaolong.ye@...el.com>,
        kernel-hardening@...ts.openwall.com,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tycho Andersen <tycho@...ker.com>,
        "Roberts, William C" <william.c.roberts@...el.com>,
        Tejun Heo <tj@...nel.org>,
        Jordan Glover <Golden_Miller83@...tonmail.ch>,
        Greg KH <gregkh@...uxfoundation.org>,
        Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>,
        Ian Campbell <ijc@...lion.org.uk>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <wilal.deacon@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Chris Fries <cfries@...gle.com>,
        Dave Weinstein <olorin@...gle.com>,
        Daniel Micay <danielmicay@...il.com>,
        Djalal Harouni <tixxdz@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, LKP <lkp@...org>
Subject: Re: [lkp-robot] [printk] 7f7c60e066: BUG:KASAN:slab-out-of-bounds

On Mon, Oct 30, 2017 at 7:39 PM, Tobin C. Harding <me@...in.cc> wrote:
> On Mon, Oct 30, 2017 at 05:14:59PM -0700, Kees Cook wrote:
>> On Wed, Oct 25, 2017 at 9:22 AM, kernel test robot
>> <xiaolong.ye@...el.com> wrote:
>
> thanks for looking at this, I was at a loss as to what (if any) action I
> needed to take.

I have finally learned after several instances that if 0day produces a
report, it's real. :) Even the crazy stuff I haven't been able to
reproduce on my own has come back to bite me later, so now I'm very
attentive to 0day report. ;)

>> > FYI, we noticed the following commit (built with gcc-4.9):
>> >
>> > commit: 7f7c60e0663645e757e520245606fde9c6e326bb ("printk: hash addresses printed with %p")
>> > url: https://github.com/0day-ci/linux/commits/Tobin-C-Harding/printk-hash-addresses-printed-with-p/20171024-231922
>>
>> It's not clear to me which of the various versions this test ran
>> against, but it seems like the printf self-tests got very confused by
>> the results:
>>
>> > [   40.275423] test_printf: kvasprintf(..., "%p %p", ...) returned '3cf9adbe eff717bf', expected '0000000001234567 fffffffffedcba98'
>> > [   40.296739] test_printf: vsnprintf(buf, 256, "|%-*p|%*p|", ...) returned 19, expected 39
>> > [   40.322776] test_printf: vsnprintf(buf, 16, "|%-*p|%*p|", ...) returned 19, expected 39
>> > [   40.334834] test_printf: vsnprintf(buf, 0, "|%-*p|%*p|", ...) returned 19, expected 39
>>
>> I assume v10 will fix the width issues, but probably not the value tests...
>
> Oh, so I need to update lib/test_printf.c to cover hashed %p.

Yeah, looks like it.

>> And it claims a use-after-free, too:
>>
>> > [   39.757461] The buggy address belongs to the object at 22cb34bb
>> > [   39.757461]  which belongs to the cache kmalloc-32 of size 32
>> > [   39.757461] The buggy address is located 0 bytes inside of
>> > [   39.757461]  32-byte region [22cb34bb, 24ac3a60)
>>
>> Which becomes rather unreadable, since the address got hashed. :P
>
> So I think we need to patch mm/kasan/report to use %pK instead of %p.

Yeah, looks like it.

> I don't know what I should be doing about
>
> [   39.757461] BUG: KASAN: slab-out-of-bounds in __test+0xee/0x13f

Yeah, I have no idea where this error came from. It does seem
triggered by the printf self-test code, though, so if a KASAN bulid
doesn't show it, maybe a full SLAB DEBUG build will?

-Kees

-- 
Kees Cook
Pixel Security