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

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Date:	Fri, 12 Sep 2014 17:46:57 +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

On Fri, Sep 12, 2014 at 05:21:27PM +0100, Daniel Borkmann wrote:
> On 09/12/2014 06:03 PM, Catalin Marinas wrote:
> > 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.

So, if I try this:

$ echo 0xffffffe7 | xxd -r > test.bin
$ arm-linux-gnueabihf-objdump -m arm -D -b binary test.bin
...
   0:   e7ffffff        udf     #65535  ; 0xffff

Do you use the same constant on arm32?

> > 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?

On current ARMv8 CPU implementations, the above constant is unallocated
in the A64 instruction space. But you never know, it may be allocated in
the future.

I think it's easier if you just use something like BRK #0x100 (opcode
0xd4202000) which would trigger a fault in the kernel (kgdb uses #imm
0x400 and 0x401).

An unallocated BRK would trigger a fault via do_debug_exception ->
brk_handler and panic the kernel.

-- 
Catalin

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