Re: [PATCH v2] printk: hash addresses printed with %p

["Tobin C. Harding" <me@tobin.cc>]

On Tue, Oct 17, 2017 at 05:13:10PM -0700, Kees Cook wrote:
> On Tue, Oct 17, 2017 at 4:15 PM, Tobin C. Harding <me@tobin.cc> wrote:
> > On Tue, Oct 17, 2017 at 09:31:19AM -0400, Steven Rostedt wrote:
> >> On Tue, 17 Oct 2017 15:52:51 +1100
> >> "Tobin C. Harding" <me@tobin.cc> wrote:
> >>
> >> > Currently there are many places in the kernel where addresses are being
> >> > printed using an unadorned %p. Kernel pointers should be printed using
> >> > %pK allowing some control via the kptr_restrict sysctl. Exposing addresses
> >> > gives attackers sensitive information about the kernel layout in memory.
> >> >
> >> > We can reduce the attack surface by hashing all addresses printed with
> >> > %p. This will of course break some users, forcing code printing needed
> >> > addresses to be updated.
> >> >
> >> > For what it's worth, usage of unadorned %p can be broken down as follows
> >> >
> >> >     git grep '%p[^KFfSsBRrbMmIiEUVKNhdDgCGO]' | wc -l
> >>
> >> Does %p[FfSs] leak addresses? Well, I guess it does if they are not
> >> found in kallsyms, but otherwise you have:
> >>
> >>   function+0x<offset>
> >
> > You are correct %pF and %pS print an offset. Does this provide an attack vector,
> > I didn't think so but I'm no security expert. If they do then we need to amend
> > those calls also.
> 
> They haven't traditionally been a big deal. If they turn out to be
> problematic, we can deal with it then, IMO.

Thanks Kees,
Tobin.