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

Daniel Borkmann
2014-09-12T16:22:10Z

On 09/12/2014 06:03 PM, Catalin Marinas wrote:
> 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.

Ok, yes, that would mitigate it a bit. Sounds fine to me.

> 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).

Hm, ok, the boards we've tried out and where Zi tested it too, it worked.

> 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).

The trade-off would be that if we align on 8, it would certainly increase
the probability to jump to the right offset. Note, on x86_64 we have no
alignment requirements, hence 1, and on s390x only alignment of 2.

So, on that few (?) boards where UND would be a valid instruction [ as
opposed to crash the kernel ], would it translate into a NOP and just
'walk' from there into the JIT image?