Re: [2.6 patch] crypto/aes.c: array overrun

[Pavel Machek <pavel@ucw.cz>]

On So 11-03-06 13:41:16, Herbert Xu wrote:
> On Sat, Mar 11, 2006 at 02:03:39AM +0100, Adrian Bunk wrote:
> >
> > ...
> > #define loop8(i)                                    \
> 
> ...
> 
> >     t ^= E_KEY[8 * i + 7]; E_KEY[8 * i + 15] = t;   \
> > }
> > 
> > static int
> > aes_set_key(void *ctx_arg, const u8 *in_key, unsigned int key_len, u32 *flags)
> > {
> > ...
> >         case 32:
> > ...
> >                 for (i = 0; i < 7; ++i)
> >                         loop8 (i);
> 
> OK this is not pretty but it is actually correct.  Notice how we only
                                  ~~~~~~~~~~~~~~~~~
> overstep the mark for E_KEY but never for D_KEY.  Since D_KEY is only
> initialised after this, it is OK for us to trash the start of D_KEY.
> 
> It's just a trick that makes the code slightly nicer (and no I didn't
> write this nor am I necessarily condoning it :)

Overstepping array is not correct C. Even if gcc lays it out in order
where array-to-be-thrashed is after it, so it works in practice, it is
not okay. [Some kind of security-hardened-gcc may stop this as buffer
overflow, for example]
								Pavel
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