Subject: Re: net, ipv6: out of bounds access in secret_stable

From: Hannes Frederic Sowa <[email-masked]>
Date: Mon, 21 Dec 2015 09:58:31 +0100

On 19.12.2015 21:50, Cong Wang wrote:
> On Fri, Dec 18, 2015 at 5:13 PM, Sasha Levin <[email-masked]> wrote:
>> Hi Hannes,
>>
>> I've hit the following out of bounds access while fuzzing on the latest -next kernel.
>>
>> This code was added in 3d1bec9932 ("ipv6: introduce secret_stable to ipv6_devconf").
>>
>> [  459.553655] BUG: KASAN: stack-out-of-bounds in strlen+0x58/0x90 at addr ffff8802ab0efb0e
>> [  459.554953] Read of size 1 by task trinity-c91/22576
>> [  459.555805] page:ffffea000aac3bc0 count:0 mapcount:0 mapping:          (null) index:0x0
>> [  459.556899] flags: 0x26fffff80000000()
>> [  459.557521] page dumped because: kasan: bad access detected
>> [  459.558320] CPU: 7 PID: 22576 Comm: trinity-c91 Not tainted 4.4.0-rc5-next-20151218-sasha-00021-gaba8d84-dirty #2750
>> [  459.559809]  0000000000000000 00000000549d0aa3 ffff8802ab0ef860 ffffffffa1042384
>> [  459.561036]  0000000041b58ab3 ffffffffac667cdb ffffffffa10422d9 ffff8802ab0ef848
>> [  459.562245]  ffffffff9f6a417e 00000000549d0aa3 ffff8802ab0efb0e ffff8802ab0efb0e
>> [  459.563429] Call Trace:
>> [  459.563831] dump_stack (lib/dump_stack.c:52)
>> [  459.564623] ? _atomic_dec_and_lock (lib/dump_stack.c:27)
>> [  459.565628] ? __dump_page (mm/debug.c:126)
>> [  459.566538] kasan_report_error (include/linux/kasan.h:28 mm/kasan/report.c:170 mm/kasan/report.c:237)
>> [  459.570997] __asan_report_load1_noabort (mm/kasan/report.c:277)
>> [  459.572119] ? check_preemption_disabled (lib/smp_processor_id.c:39)
>> [  459.573731] ? strlen (lib/string.c:481 (discriminator 1))
>> [  459.574646] strlen (lib/string.c:481 (discriminator 1))
>> [  459.575485] proc_dostring (kernel/sysctl.c:1825 kernel/sysctl.c:1906)
>> [  459.576445] ? alloc_debug_processing (mm/slub.c:1054)
>> [  459.577523] addrconf_sysctl_stable_secret (net/ipv6/addrconf.c:5395)
> 
> Looks like we don't initialize the array on stack for write case.
> At least other callers always initialize the data for both read
> and write.
> 
> Please try the attached patch.

Your patch is right. I am surprised you need to initialize the buffer
passed down to proc_dostring so that strlen can correctly operate on it,
but f4aacea2f5d1a5 ("sysctl: allow for strict write position handling")
explains why. Can you submit formally?

Acked-by: Hannes Frederic Sowa <[email-masked]>

Thanks,
Hannes