Messages in this thread | | | Date | Wed, 15 Feb 2012 20:27:52 +0400 | From | Cyrill Gorcunov <> | Subject | Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree |
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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
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