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

[Geert Uytterhoeven]

Hi Tobin,

On Wed, Nov 29, 2017 at 3:05 AM, Tobin C. Harding <me@xxxxxxxx> 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.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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