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=-9.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_GIT 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 1C5D3C3E8A4 for ; Mon, 28 Jan 2019 20:23:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EA6DE217F5 for ; Mon, 28 Jan 2019 20:23:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726979AbfA1UXk (ORCPT ); Mon, 28 Jan 2019 15:23:40 -0500 Received: from www62.your-server.de ([213.133.104.62]:44682 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726719AbfA1UXj (ORCPT ); Mon, 28 Jan 2019 15:23:39 -0500 Received: from [178.197.249.35] (helo=localhost) by www62.your-server.de with esmtpsa (TLSv1.2:DHE-RSA-AES256-GCM-SHA384:256) (Exim 4.89_1) (envelope-from ) id 1goDRS-0003Yy-7r; Mon, 28 Jan 2019 21:23:38 +0100 From: Daniel Borkmann To: gregkh@linuxfoundation.org Cc: ast@kernel.org, stable@vger.kernel.org, Daniel Borkmann Subject: [PATCH stable 4.20 03/10] bpf: enable access to ax register also from verifier rewrite Date: Mon, 28 Jan 2019 21:23:23 +0100 Message-Id: <20190128202330.32664-4-daniel@iogearbox.net> X-Mailer: git-send-email 2.9.5 In-Reply-To: <20190128202330.32664-1-daniel@iogearbox.net> References: <20190128202330.32664-1-daniel@iogearbox.net> X-Authenticated-Sender: daniel@iogearbox.net X-Virus-Scanned: Clear (ClamAV 0.100.2/25339/Mon Jan 28 19:32:47 2019) Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org [ commit 9b73bfdd08e73231d6a90ae6db4b46b3fbf56c30 upstream ] Right now we are using BPF ax register in JIT for constant blinding as well as in interpreter as temporary variable. Verifier will not be able to use it simply because its use will get overridden from the former in bpf_jit_blind_insn(). However, it can be made to work in that blinding will be skipped if there is prior use in either source or destination register on the instruction. Taking constraints of ax into account, the verifier is then open to use it in rewrites under some constraints. Note, ax register already has mappings in every eBPF JIT. Signed-off-by: Daniel Borkmann Acked-by: Alexei Starovoitov Signed-off-by: Alexei Starovoitov Signed-off-by: Daniel Borkmann --- include/linux/filter.h | 7 +------ kernel/bpf/core.c | 20 ++++++++++++++++++++ 2 files changed, 21 insertions(+), 6 deletions(-) diff --git a/include/linux/filter.h b/include/linux/filter.h index 5728378182a1..c83230e2b26a 100644 --- a/include/linux/filter.h +++ b/include/linux/filter.h @@ -53,12 +53,7 @@ struct sock_reuseport; #define BPF_REG_D BPF_REG_8 /* data, callee-saved */ #define BPF_REG_H BPF_REG_9 /* hlen, callee-saved */ -/* Kernel hidden auxiliary/helper register for hardening step. - * Only used by eBPF JITs. It's nothing more than a temporary - * register that JITs use internally, only that here it's part - * of eBPF instructions that have been rewritten for blinding - * constants. See JIT pre-step in bpf_jit_blind_constants(). - */ +/* Kernel hidden auxiliary/helper register. */ #define BPF_REG_AX MAX_BPF_REG #define MAX_BPF_EXT_REG (MAX_BPF_REG + 1) #define MAX_BPF_JIT_REG MAX_BPF_EXT_REG diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c index ada876048ca4..ac44653025ad 100644 --- a/kernel/bpf/core.c +++ b/kernel/bpf/core.c @@ -727,6 +727,26 @@ static int bpf_jit_blind_insn(const struct bpf_insn *from, BUILD_BUG_ON(BPF_REG_AX + 1 != MAX_BPF_JIT_REG); BUILD_BUG_ON(MAX_BPF_REG + 1 != MAX_BPF_JIT_REG); + /* Constraints on AX register: + * + * AX register is inaccessible from user space. It is mapped in + * all JITs, and used here for constant blinding rewrites. It is + * typically "stateless" meaning its contents are only valid within + * the executed instruction, but not across several instructions. + * There are a few exceptions however which are further detailed + * below. + * + * Constant blinding is only used by JITs, not in the interpreter. + * The interpreter uses AX in some occasions as a local temporary + * register e.g. in DIV or MOD instructions. + * + * In restricted circumstances, the verifier can also use the AX + * register for rewrites as long as they do not interfere with + * the above cases! + */ + if (from->dst_reg == BPF_REG_AX || from->src_reg == BPF_REG_AX) + goto out; + if (from->imm == 0 && (from->code == (BPF_ALU | BPF_MOV | BPF_K) || from->code == (BPF_ALU64 | BPF_MOV | BPF_K))) { -- 2.17.1