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

Geert Uytterhoeven
2017-12-05T22:57:47Z

Hi Tobin,

On Tue, Dec 5, 2017 at 9:44 PM, Tobin C. Harding <me@tobin.cc> wrote:
> On Tue, Dec 05, 2017 at 09:20:57PM +0100, Geert Uytterhoeven wrote:
>> 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.

Lowest 3 is good enough for all natural types, up to long long.
We may still receive complaints from people who care about seeing if
a pointer is cacheline-aligned or not. Fixing that may need up to 7 bits, I'm
afraid, which is a bit too much to give up.

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds