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

From: Daniel Borkmann
Date: Mon May 20 2013 - 05:50:50 EST

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 ...

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