Re: Linux Kernel sctp_setsockopt() Integer Overflow

[Michael Tokarev <mjt () tls msk ru>]

Bringing up an old topic (discussed 15-May), because
it seems noone replied to my post, which contains er..
wrong claims.

Michael Tokarev wrote:
> Shaun Colley wrote:
> > ---
> > if (NULL == (tmp = kmalloc(optlen + 1, GFP_KERNEL))) {
> >                         retval = -ENOMEM;
> >                         goto out_unlock;
> >                 }
> > ---
> >
> > Because kmalloc() takes the 'count' variable as an
> > unsigned number, negative numbers are interpreted as
> > large unsigned numbers.  However, if -1 is passed as
> > 'optlen' (represented as 0xffffffff (hex) in unsigned
> > variables, which is the largest value an unsigned
>
> .....
> []
> > And thus, due to the integer overflow, 0 is passed to
> > kmalloc(), causing too little memory to be allocated
> > to hold 'optval'.

> But kmalloc(0) will return NULL, and the whole setsockopt
> will finish with errno set to ENOMEM.
>
>  From 2.4 mm/slab.c:
>
> void * kmalloc (size_t size, int flags)
> {
>         cache_sizes_t *csizep = cache_sizes;
>
>         for (; csizep->cs_size; csizep++) {
>                 if (size > csizep->cs_size)
>                         continue;
>                 return __kmem_cache_alloc(flags & GFP_DMA ?
>                          csizep->cs_dmacachep : csizep->cs_cachep, flags);
>         }
>         return NULL;
> }
>
> So, where's the bug?

I was wrong reading the above code, simple as that.
Sure, kmalloc(0) will NOT return NULL as I claimed.

                if (size > csizep->cs_size)
                        continue;
Here, when size == 0 (and csizep->cs_size is always > 0),
the condition is always false, so the next instruction
will be executed, which is:

                return __kmem_cache_alloc(flags & GFP_DMA ?
                         csizep->cs_dmacachep : csizep->cs_cachep, flags);

which will allocate either 32 or 64 bytes of memory (depending
on the arch) and return it to the caller.

So there IS a bug, exactly as described in the original advisory.

I wonder why noone replied... ;)

/mjt