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

From: David Laight
Date: Mon May 20 2013 - 05:03:25 EST

> 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*.


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