On Wed, 2013-09-11 at 12:25 -0700, Kees Cook wrote:
> On Wed, Sep 11, 2013 at 12:09 PM, Joe Perches <j...@perches.com> 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.carpen...@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
> >
> > 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 <fweis...@gmail.com>
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.
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);
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