Re: [PATCH arm64-next] net: bpf: arm64: address randomize and write protect JIT code

Catalin Marinas
2014-09-12T16:03:51Z

Daniel,

On Fri, Sep 12, 2014 at 08:11:37AM +0100, Daniel Borkmann wrote:
> Will, Catalin, Dave, this is more or less a heads-up: when net-next and
> arm64-next tree will get both merged into Linus' tree, we will run into
> a 'silent' merge conflict until someone actually runs eBPF JIT on ARM64
> and might notice (I presume) an oops when JIT is freeing bpf_prog. I'd
> assume nobody actually _runs_ linux-next, but not sure about that though.

Some people do.

> How do we handle this? Would I need to resend this patch when the time
> comes or would you ARM64 guys take care of it automagically? ;)

I think we could disable BPF for arm64 until -rc1 and re-enable it
together with this patch.

One comment below:

> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
[...]
> +static void jit_fill_hole(void *area, unsigned int size)
> +{
> + /* Insert illegal UND instructions. */
> + u32 *ptr, fill_ins = 0xe7ffffff;

On arm64 we don't have a guaranteed undefined instruction space (and
Will tells me that on Thumb-2 for the 32-bit arm port it actually is a
valid instruction, it seems that you used the same value).

I think the only guaranteed way is to use the BRK #imm instruction but
it requires some changes to the handling code as it is currently used
for kgdb (unless you can use two instructions for filling in which could
generate a NULL pointer access).

--
Catalin