Re: [PATCH] staging: dgnc: fix potential format string flaw

[Kees Cook]

On Wed, Sep 11, 2013 at 12:45 PM, Joe Perches <joe@xxxxxxxxxxx> wrote:
> On Wed, 2013-09-11 at 12:25 -0700, Kees Cook wrote:
>> On Wed, Sep 11, 2013 at 12:09 PM, Joe Perches <joe@xxxxxxxxxxx> wrote:
>> > On Wed, 2013-09-11 at 11:19 -0700, Kees Cook wrote:
>> >> On Wed, Sep 11, 2013 at 2:31 AM, Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:
>> >> > On Tue, Sep 10, 2013 at 10:19:17PM -0700, Kees Cook wrote:
>> >> >> In the former case, format characters will get processed by the
>> >> >> sprintf logic. In the latter, they are printed as-is. In this specific
>> >> >> case, if there was a way to inject strings like "ohai %n" into the
>> >> >> msgbuf string, the former would actually attempt to resolve the %n. In
>> >> >> the simple case, this could lead to Oopses, and in the unlucky case,
>> >> >> it could allow arbitrary memory writing and execution control.
>> >> >>
>> >> >> http://en.wikipedia.org/wiki/Uncontrolled_format_string
>> >> >
>> >> > The kernel ignores %n so hopefully it can't actually write to memory.
>> >>
>> >> I wish! This is not the case, though. See FORMAT_TYPE_NRCHARS in
>> >> lib/vsprintf.c's vsnprintf().
>> >>
>> >> $ git grep '%n' | wc -l
>> >> 111
>> >
>> > Umm.
>> >
>> > See: lib/vsprintf.c
>> >
>> > /**
>> >  * vsnprintf - Format a string and place it in a buffer
>> > [...]
>> >  * %n is ignored
>> >
>> > %n does work for vsscanf though.
>>
>> The comment is a lie:
>>
>>         int len = 0;
>>         printk("len:%d\n", len);
>>         printk("%s%n\n", "Ohai!", &len);
>>         printk("len:%d\n", len);
>>
>> [    0.025930] len:0
>> [    0.026003] Ohai!
>> [    0.026261] len:5
>>
>> The functionality between scanf and printf was, I think, merged in
>> 2009, if I'm reading the git blame correctly.
>
> Yeah.
>
> commit fef20d9c1380f04ba9492d6463148db07b413708
> Author: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> Date:   Fri Mar 6 17:21:50 2009 +0100
>
>     vsprintf: unify the format decoding layer for its 3 users
>
> Maybe it should be reignored...
>
> There are a few more in net/ though that may be pretty
> easy to change to use the seq_printf return value.

I would love to remove %n. It's not clear to me how to re-split it
from scanf, though. I'd need to study this code a bunch more. Dropping
%n from all its non-scanf uses would be a good first-step, though.

-Kees

>
>
> net/ipv4/fib_trie.c-                            seq_printf(seq,
> net/ipv4/fib_trie.c-                                     "%s\t%08X\t%08X\t%04X\t%d\t%u\t"
> net/ipv4/fib_trie.c:                                     "%d\t%08X\t%d\t%u\t%u%n",
> net/ipv4/fib_trie.c-                                     fi->fib_dev ? fi->fib_dev->name : "*",
> --
> net/ipv4/fib_trie.c-                            seq_printf(seq,
> net/ipv4/fib_trie.c-                                     "*\t%08X\t%08X\t%04X\t%d\t%u\t"
> net/ipv4/fib_trie.c:                                     "%d\t%08X\t%d\t%u\t%u%n",
> net/ipv4/fib_trie.c-                                     prefix, 0, flags, 0, 0, 0,
> --
> net/ipv4/ping.c-        seq_printf(f, "%5d: %08X:%04X %08X:%04X"
> net/ipv4/ping.c:                " %02X %08X:%08X %02X:%08lX %08X %5u %8d %lu %d %pK %d%n",
> net/ipv4/ping.c-                bucket, src, srcp, dest, destp, sp->sk_state,
> --
> net/ipv4/tcp_ipv4.c-    seq_printf(f, "%4d: %08X:%04X %08X:%04X"
> net/ipv4/tcp_ipv4.c:            " %02X %08X:%08X %02X:%08lX %08X %5u %8d %u %d %pK%n",
> net/ipv4/tcp_ipv4.c-            i,
> --
> net/ipv4/tcp_ipv4.c-    seq_printf(f, "%4d: %08X:%04X %08X:%04X %02X %08X:%08X %02X:%08lX "
> net/ipv4/tcp_ipv4.c:                    "%08X %5u %8d %lu %d %pK %lu %lu %u %u %d%n",
> net/ipv4/tcp_ipv4.c-            i, src, srcp, dest, destp, sk->sk_state,
> --
> net/ipv4/tcp_ipv4.c-    seq_printf(f, "%4d: %08X:%04X %08X:%04X"
> net/ipv4/tcp_ipv4.c:            " %02X %08X:%08X %02X:%08lX %08X %5d %8d %d %d %pK%n",
> net/ipv4/tcp_ipv4.c-            i, src, srcp, dest, destp, tw->tw_substate, 0, 0,
> --
> net/ipv4/udp.c- seq_printf(f, "%5d: %08X:%04X %08X:%04X"
> net/ipv4/udp.c:         " %02X %08X:%08X %02X:%08lX %08X %5u %8d %lu %d %pK %d%n",
> net/ipv4/udp.c-         bucket, src, srcp, dest, destp, sp->sk_state,
> --
> net/sctp/objcnt.c:      seq_printf(seq, "%s: %d%n", sctp_dbg_objcnt[i].label,
> net/sctp/objcnt.c-                              atomic_read(sctp_dbg_objcnt[i].counter), &len);
>
>
>



-- 
Kees Cook
Chrome OS Security
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