From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3A484ECE599 for ; Wed, 16 Oct 2019 21:22:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 197EA20854 for ; Wed, 16 Oct 2019 21:22:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389621AbfJPVWI (ORCPT ); Wed, 16 Oct 2019 17:22:08 -0400 Received: from www62.your-server.de ([213.133.104.62]:47806 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729231AbfJPVWI (ORCPT ); Wed, 16 Oct 2019 17:22:08 -0400 Received: from sslproxy01.your-server.de ([88.198.220.130]) by www62.your-server.de with esmtpsa (TLSv1.2:DHE-RSA-AES256-GCM-SHA384:256) (Exim 4.89_1) (envelope-from ) id 1iKqk2-0002cX-Hy; Wed, 16 Oct 2019 23:21:58 +0200 Received: from [178.197.249.55] (helo=pc-63.home) by sslproxy01.your-server.de with esmtpsa (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256) (Exim 4.89) (envelope-from ) id 1iKqk2-0001ke-8R; Wed, 16 Oct 2019 23:21:58 +0200 Subject: Re: [PATCH v3 bpf-next 06/11] bpf: implement accurate raw_tp context access via BTF To: Alexei Starovoitov , davem@davemloft.net Cc: x86@kernel.org, netdev@vger.kernel.org, bpf@vger.kernel.org, kernel-team@fb.com References: <20191016032505.2089704-1-ast@kernel.org> <20191016032505.2089704-7-ast@kernel.org> From: Daniel Borkmann Message-ID: <04fab556-9eda-87ec-8f8c-defcab25a80e@iogearbox.net> Date: Wed, 16 Oct 2019 23:21:57 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20191016032505.2089704-7-ast@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Authenticated-Sender: daniel@iogearbox.net X-Virus-Scanned: Clear (ClamAV 0.101.4/25604/Wed Oct 16 10:53:05 2019) Sender: bpf-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On 10/16/19 5:25 AM, Alexei Starovoitov wrote: > libbpf analyzes bpf C program, searches in-kernel BTF for given type name > and stores it into expected_attach_type. > The kernel verifier expects this btf_id to point to something like: > typedef void (*btf_trace_kfree_skb)(void *, struct sk_buff *skb, void *loc); > which represents signature of raw_tracepoint "kfree_skb". > > Then btf_ctx_access() matches ctx+0 access in bpf program with 'skb' > and 'ctx+8' access with 'loc' arguments of "kfree_skb" tracepoint. > In first case it passes btf_id of 'struct sk_buff *' back to the verifier core > and 'void *' in second case. > > Then the verifier tracks PTR_TO_BTF_ID as any other pointer type. > Like PTR_TO_SOCKET points to 'struct bpf_sock', > PTR_TO_TCP_SOCK points to 'struct bpf_tcp_sock', and so on. > PTR_TO_BTF_ID points to in-kernel structs. > If 1234 is btf_id of 'struct sk_buff' in vmlinux's BTF > then PTR_TO_BTF_ID#1234 points to one of in kernel skbs. > > When PTR_TO_BTF_ID#1234 is dereferenced (like r2 = *(u64 *)r1 + 32) > the btf_struct_access() checks which field of 'struct sk_buff' is > at offset 32. Checks that size of access matches type definition > of the field and continues to track the dereferenced type. > If that field was a pointer to 'struct net_device' the r2's type > will be PTR_TO_BTF_ID#456. Where 456 is btf_id of 'struct net_device' > in vmlinux's BTF. > > Such verifier analysis prevents "cheating" in BPF C program. > The program cannot cast arbitrary pointer to 'struct sk_buff *' > and access it. C compiler would allow type cast, of course, > but the verifier will notice type mismatch based on BPF assembly > and in-kernel BTF. > > Signed-off-by: Alexei Starovoitov Overall set looks great! [...] > +int btf_struct_access(struct bpf_verifier_log *log, > + const struct btf_type *t, int off, int size, > + enum bpf_access_type atype, > + u32 *next_btf_id) > +{ > + const struct btf_member *member; > + const struct btf_type *mtype; > + const char *tname, *mname; > + int i, moff = 0, msize; > + > +again: > + tname = __btf_name_by_offset(btf_vmlinux, t->name_off); More of a high-level question wrt btf_ctx_access(), is there a reason the ctx access is only done for raw_tp? I presume kprobes is still on todo (?), what about uprobes which also have pt_regs and could benefit from this work, but is not fixed to btf_vmlinux to search its ctx type. I presume BPF_LDX | BPF_PROBE_MEM | BPF_* would need no additional encoding, but JIT emission would have to differ depending on the prog type. > + if (!btf_type_is_struct(t)) { > + bpf_log(log, "Type '%s' is not a struct", tname); > + return -EINVAL; > + } > + > + for_each_member(i, t, member) { > + /* offset of the field in bits */ > + moff = btf_member_bit_offset(t, member); > + > + if (btf_member_bitfield_size(t, member)) > + /* bitfields are not supported yet */ > + continue; > + > + if (off + size <= moff / 8) > + /* won't find anything, field is already too far */ > + break; > + > + /* type of the field */ > + mtype = btf_type_by_id(btf_vmlinux, member->type); > + mname = __btf_name_by_offset(btf_vmlinux, member->name_off); > + > + /* skip modifiers */ [...]