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

Tobin C. Harding
2017-12-05T20:44:12Z

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.