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. -Kees -- Kees Cook Chrome OS Security _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
