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

    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

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