Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

Cyrill Gorcunov
2012-02-15T16:27:59Z

On Wed, Feb 15, 2012 at 05:06:52PM +0100, Oleg Nesterov wrote:
> Not a comment, but the question. I am just curious...
>
> > +/*
> > + * We don't expose real in-memory order of objects for security
> > + * reasons, still the comparison results should be suitable for
> > + * sorting. Thus, we obfuscate kernel pointers values and compare
> > + * the production instead.
> > + */
> > +static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
> > +
> > +static long kptr_obfuscate(long v, int type)
> > +{
> > + return (v ^ cookies[type][0]) * cookies[type][1];
> > +}
>
> OK, but why do we need this per type? Just to add more obfuscation
> or there is another reason?

Just to add more obfuscation.

>
> > +static __init int kcmp_cookies_init(void)
> > +{
> > + int i;
> > +
> > + get_random_bytes(cookies, sizeof(cookies));
> > +
> > + for (i = 0; i < KCMP_TYPES; i++)
> > + cookies[i][1] |= (~(~0UL >> 1) | 1);
>
> I am puzzled, help ;) this is equal to
>
> cookies[i][1] |= -LONG_MAX;
> or
> cookies[i][1] |= (LONG_MIN | 1);
>
> for what? why do we want to set these 2 bits (MSB and LSB) ?

Letme quote hpa@ here :)

| This code is wrong. You will have a zero cookie, legitimately, once in
| 2^32 or 2^64 attempts, depending on the bitness.
|
| The other thing is that for the multiplicative cookie you should OR in
| the value (~(~0UL >> 1) | 1) in order to make sure that the value is (a)
| large and (b) odd.

Cyrill