netdev - Re: [PATCH v6 net-next 1/6] net: filter: add "load 64-bit immediate" eBPF instruction

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Date:	Mon, 25 Aug 2014 19:02:52 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	David Miller <davem@...emloft.net>, Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Chema Gonzalez <chema@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Brendan Gregg <brendan.d.gregg@...il.com>,
	Namhyung Kim <namhyung@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 net-next 1/6] net: filter: add "load 64-bit immediate"
 eBPF instruction

On Mon, Aug 25, 2014 at 6:54 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Mon, Aug 25, 2014 at 6:53 PM, Alexei Starovoitov <ast@...mgrid.com> wrote:
>> On Mon, Aug 25, 2014 at 6:38 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>> On Mon, Aug 25, 2014 at 6:35 PM, Alexei Starovoitov <ast@...mgrid.com> wrote:
>>>> On Mon, Aug 25, 2014 at 6:06 PM, David Miller <davem@...emloft.net> wrote:
>>>>> From: Alexei Starovoitov <ast@...mgrid.com>
>>>>> Date: Mon, 25 Aug 2014 18:00:53 -0700
>>>>>
>>>>>> add BPF_LD_IMM64 instruction to load 64-bit immediate value into a register.
>>>>>
>>>>> I think you need to rethink this.
>>>>>
>>>>> I understand that you want to be able to compile arbitrary C code into
>>>>> eBPF, but you have to restrict strongly what data the eBPF code can get
>>>>> to.
>>>>
>>>> I believe verifier already does restrict it. I don't see any holes in
>>>> the architecture. I'm probably not explaining it clearly though :(
>>>>
>>>>> Arbitrary pointer loads is asking for trouble.
>>>>
>>>> Of course.
>>>> There is no arbitrary pointer from user space.
>>>> Verifier checks all pointers.
>>>> I guess this commit log description is confusing.
>>>> It says:
>>>> BPF_LD_IMM64(R1, const_imm_map_ptr)
>>>> that's what appears in the program _after_ it goes through verifier.
>>>> User space cannot pass a pointer into the kernel.
>>>
>>> If you don't intend for userspace to load a program that contains this
>>> instruction, then why does it need to be an instruction that the
>>> verifier rewrites?  Why not have an instruction "load immediate
>>
>> user space use _pseudo_ bpf_ld_imm64 instruction.
>> _pseudo_ stands for using 'map_fd' as imm instead of pointer.
>>
>>> relocated pointer" that contains a reference to a relocation table and
>>
>> Andy, I guess you missed explanation in:
>> https://lkml.org/lkml/2014/8/13/111
>> "
>> Obviously user space doesn't know what kernel map pointer is associated
>> with process-local map-FD.
>> So it's using pseudo BPF_LD_IMM64 instruction.
>> BPF_LD_IMM64 with src_reg == 0 -> generic move 64-bit immediate into dst_reg
>> BPF_LD_IMM64 with src_reg == BPF_PSEUDO_MAP_FD -> mov map_fd into dst_reg
>> Other values are reserved for now. (They will be used to implement
>> global variables, strings and other constants and per-cpu areas in the future)
>> So the programs look like:
>>   BPF_LD_MAP_FD(BPF_REG_1, process_local_map_fd),
>>   BPF_CALL(BPF_FUNC_map_lookup_elem),
>> eBPF verifier scans the program for such pseudo instructions, converts
>> process_local_map_fd -> in-kernel map pointer
>> and drops 'pseudo' flag of BPF_LD_IMM64 instruction.
>> "
>
> Will a program that uses BPF_LD_IMM64 w/o the FPG_REG_1 thing be accepted?

If you mean the program like:
BPF_LD_IMM64(BPF_REG_1, 0xdead),
BPF_CALL(BPF_FUNC_map_lookup_elem),
yes, it will be rejected, because type of R1 will not match
map_lookup() argument
constraints.
See check_ld_imm() in verifier.c where it assigns the type during verification.
There are 5 tests in verifier testsuite that test things around bpf_ld_imm64
and 2 tests around _pseudo_ bpf_ld_imm64.
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