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SubjectRe: [PATCH] staging: dgnc: fix potential format string flaw
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On Wed, Sep 11, 2013 at 12:22 PM, Dan Carpenter
<dan.carpenter@oracle.com> wrote:
> On Wed, Sep 11, 2013 at 11:19:11AM -0700, Kees Cook wrote:
>> On Wed, Sep 11, 2013 at 2:31 AM, Dan Carpenter <dan.carpenter@oracle.com> 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
>>
>
> Hm... That's unfortunate. The comments were shifted around so it says
> it's ignored but it's not.

Outside of scanf, there are very few uses, though:

$ git grep %n | grep print
...
net/phonet/socket.c: seq_printf(seq, "%s%n", "pt loc rem
rs st tx_queue rx_queue "
net/phonet/socket.c: seq_printf(seq, "%s%n", "rs uid inode", &len);
net/phonet/socket.c: seq_printf(seq, "%02X %5u %lu%n",
net/sctp/objcnt.c: seq_printf(seq, "%s: %d%n", sctp_dbg_objcnt[i].label,

-Kees

--
Kees Cook
Chrome OS Security


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