linux-kernel - Re: [PATCH arm64-next v3] net: bpf: arm64: address randomize and write protect JIT code

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Date:	Mon, 15 Sep 2014 23:46:55 +0200
From:	Daniel Borkmann <dborkman@...hat.com>
To:	Will Deacon <will.deacon@....com>
CC:	Catalin Marinas <Catalin.Marinas@....com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Zi Shen Lim <zlim.lnx@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>
Subject: Re: [PATCH arm64-next v3] net: bpf: arm64: address randomize and
 write protect JIT code

On 09/15/2014 11:30 PM, Will Deacon wrote:
> Hi Daniel,
>
> One small comment than I forgot to mention before (see below).
>
> On Mon, Sep 15, 2014 at 09:20:23PM +0100, Daniel Borkmann wrote:
>> This is the ARM64 variant for 314beb9bcab ("x86: bpf_jit_comp: secure bpf
>> jit against spraying attacks").
>>
>> Thanks to commit 11d91a770f1f ("arm64: Add CONFIG_DEBUG_SET_MODULE_RONX
>> support") which added necessary infrastructure, we can now implement
>> RO marking of eBPF generated JIT image pages and randomize start offset
>> for the JIT code, so that it does not reside directly on a page boundary
>> anymore. Likewise, the holes are filled with illegal instructions: here
>> we use BRK #0x100 (opcode 0xd4202000) to trigger a fault in the kernel
>> (unallocated BRKs would trigger a fault through do_debug_exception). This
>> seems more reliable as we don't have a guaranteed undefined instruction
>> space on ARM64.
>>
>> This is basically the ARM64 variant of what we already have in ARM via
>> commit 55309dd3d4cd ("net: bpf: arm: address randomize and write protect
>> JIT code"). Moreover, this commit also presents a merge resolution due to
>> conflicts with commit 60a3b2253c41 ("net: bpf: make eBPF interpreter images
>> read-only") as we don't use kfree() in bpf_jit_free() anymore to release
>> the locked bpf_prog structure, but instead bpf_prog_unlock_free() through
>> a different allocator.
>>
>> JIT tested on aarch64 with BPF test suite.
>>
>> Reference: http://mainisusuallyafunction.blogspot.com/2012/11/attacking-hardened-linux-systems-with.html
>> Signed-off-by: Daniel Borkmann <dborkman@...hat.com>
>> Cc: Zi Shen Lim <zlim.lnx@...il.com>
>> Cc: Will Deacon <will.deacon@....com>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: David S. Miller <davem@...emloft.net>
>> Cc: Alexei Starovoitov <ast@...mgrid.com>
>> ---
>>   v2->v3:
>>    - Use cpu_to_le32() as suggested by Zi/Will
>>   v1->v2:
>>    - Use brk insn as suggested by Catalin
>>   Note:
>>    - This patch depends on net-next being merged to mainline due
>>      to the mentioned merge conflict.
>>
>>   arch/arm64/net/bpf_jit_comp.c | 38 +++++++++++++++++++++++++++++---------
>>   1 file changed, 29 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
>> index 7ae3354..4b71779 100644
>> --- a/arch/arm64/net/bpf_jit_comp.c
>> +++ b/arch/arm64/net/bpf_jit_comp.c
>> @@ -19,7 +19,6 @@
>>   #define pr_fmt(fmt) "bpf_jit: " fmt
>>
>>   #include <linux/filter.h>
>> -#include <linux/moduleloader.h>
>>   #include <linux/printk.h>
>>   #include <linux/skbuff.h>
>>   #include <linux/slab.h>
>> @@ -119,6 +118,15 @@ static inline int bpf2a64_offset(int bpf_to, int bpf_from,
>>   	return to - from;
>>   }
>>
>> +static void jit_fill_hole(void *area, unsigned int size)
>> +{
>> +	/* We use brk #0x100 to trigger a fault. */
>> +	u32 *ptr;
>> +	/* We are guaranteed to have aligned memory. */
>> +	for (ptr = area; size >= sizeof(u32); size -= sizeof(u32))
>> +		*ptr++ = cpu_to_le32(0xd4202000);
>> +}
>
> Please can you use the existing macros in debug-monitors.h for constructing
> this value? We have AARCH64_BREAK_MON to get the BRK encoding, then you can
> add an immediate to that file to reserve something for bpf. That way, we
> make sure that other parts of the kernel don't accidentally repurpose your
> fault code.

Okay, no problem, I'll do that and respin tomorrow.

Thanks,

Daniel
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