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

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Message-ID: <20140912160345.GF5532@arm.com>
Date:	Fri, 12 Sep 2014 17:03:45 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	Will Deacon <Will.Deacon@....com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"zlim.lnx@...il.com" <zlim.lnx@...il.com>,
	"ast@...mgrid.com" <ast@...mgrid.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH arm64-next] net: bpf: arm64: address randomize and write
 protect JIT code

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

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