Re: Linux Kernel sctp_setsockopt() Integer Overflow

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

Shaun Colley wrote:
[]
> Below is the vulnerable call:
>
> ---
> 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?

/mjt