[kernel-hardening] Re: [PATCH V11 3/5] printk: hash addresses printed with %p

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

On Tue, Dec 05, 2017 at 09:20:57PM +0100, Geert Uytterhoeven wrote:
> Hi Tobin,
> 
> On Wed, Nov 29, 2017 at 3:05 AM, Tobin C. Harding <me@tobin.cc> wrote:
> > Currently there exist approximately 14 000 places in the kernel where
> > addresses are being printed using an unadorned %p. This potentially
> > leaks sensitive information regarding the Kernel layout in memory. Many
> > of these calls are stale, instead of fixing every call lets hash the
> > address by default before printing. This will of course break some
> > users, forcing code printing needed addresses to be updated.
> >
> > Code that _really_ needs the address will soon be able to use the new
> > printk specifier %px to print the address.
> 
> > --- a/lib/vsprintf.c
> > +++ b/lib/vsprintf.c
> 
> > +/* Maps a pointer to a 32 bit unique identifier. */
> > +static char *ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec)
> > +{
> > +       unsigned long hashval;
> > +       const int default_width = 2 * sizeof(ptr);
> > +
> > +       if (unlikely(!have_filled_random_ptr_key)) {
> > +               spec.field_width = default_width;
> > +               /* string length must be less than default_width */
> > +               return string(buf, end, "(ptrval)", spec);
> > +       }
> > +
> > +#ifdef CONFIG_64BIT
> > +       hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);
> > +       /*
> > +        * Mask off the first 32 bits, this makes explicit that we have
> > +        * modified the address (and 32 bits is plenty for a unique ID).
> > +        */
> > +       hashval = hashval & 0xffffffff;
> > +#else
> > +       hashval = (unsigned long)siphash_1u32((u32)ptr, &ptr_key);
> > +#endif
> 
> Would it make sense to keep the 3 lowest bits of the address?
> 
> Currently printed pointers no longer have any correlation with the actual
> alignment in memory of the object, which is a typical cause of a class of bugs.

We'd have to keep the lowest 4 since we are printing in hex, right? This
is easy enough to add. I wasn't the architect behind the hashing but I
can do up a patch and see if anyone who knows crypto objects.

thanks,
Tobin.