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. 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); _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
