Re: [PATCH net-next] x86: bpf_jit_comp: secure bpf jit against spraying attacks

[Daniel Borkmann <dborkman@redhat.com>]

On 05/20/2013 10:51 AM, David Laight wrote:
>> hpa bringed into my attention some security related issues
>> with BPF JIT on x86.
>>
>> This patch makes sure the bpf generated code is marked read only,
>> as other kernel text sections.
>>
>> It also splits the unused space (we vmalloc() and only use a fraction of
>> the page) in two parts, so that the generated bpf code not starts at a
>> known offset in the page, but a pseudo random one.
> ...
>> +static struct bpf_binary_header *bpf_alloc_binary(unsigned int proglen,
>> +						  u8 **image_ptr)
> ...
>> +	/* insert a random number of int3 instructions before BPF code */
>> +	*image_ptr = &header->image[prandom_u32() % hole];
>> +	return header;
>> +}
>
> Hmmm.... anyone looking to overwrite kernel code will then start
> looking for blocks of 0xcc bytes and know that what follows
> is the beginning of a function.
> That isn't any harder than random writes.
>
> Copying a random part of .rodata might be better - especially
> if you can find part of .rodata.str*.

Here seems also to be another approach ...

   http://grsecurity.net/~spender/jit_prot.diff

via: http://www.reddit.com/r/netsec/comments/13dzhx/linux_kernel_jit_spray_for_smep_kernexec_bypass/