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=-15.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 F3C06C49EA4 for ; Thu, 17 Jun 2021 10:09:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DF2CD613F1 for ; Thu, 17 Jun 2021 10:09:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232148AbhFQKLw (ORCPT ); Thu, 17 Jun 2021 06:11:52 -0400 Received: from www62.your-server.de ([213.133.104.62]:41760 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231346AbhFQKLv (ORCPT ); Thu, 17 Jun 2021 06:11:51 -0400 Received: from sslproxy06.your-server.de ([78.46.172.3]) by www62.your-server.de with esmtpsa (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92.3) (envelope-from ) id 1ltoxu-000BTK-0B; Thu, 17 Jun 2021 12:09:38 +0200 Received: from [85.7.101.30] (helo=linux.home) by sslproxy06.your-server.de with esmtpsa (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1ltoxt-000DB3-IW; Thu, 17 Jun 2021 12:09:37 +0200 Subject: Re: [PATCH v5] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run To: Eric Biggers Cc: Edward Cree , Kurt Manucredo , syzbot+bed360704c521841c85d@syzkaller.appspotmail.com, keescook@chromium.org, yhs@fb.com, dvyukov@google.com, andrii@kernel.org, ast@kernel.org, bpf@vger.kernel.org, davem@davemloft.net, hawk@kernel.org, john.fastabend@gmail.com, kafai@fb.com, kpsingh@kernel.org, kuba@kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, songliubraving@fb.com, syzkaller-bugs@googlegroups.com, nathan@kernel.org, ndesaulniers@google.com, clang-built-linux@googlegroups.com, kernel-hardening@lists.openwall.com, kasan-dev@googlegroups.com References: <1aaa2408-94b9-a1e6-beff-7523b66fe73d@fb.com> <202106101002.DF8C7EF@keescook> <85536-177443-curtm@phaethon> <4713f6e9-2cfb-e2a6-c42d-b2a62f035bf2@iogearbox.net> From: Daniel Borkmann Message-ID: <845ad31f-ca3f-0326-e64b-423a09ea4bea@iogearbox.net> Date: Thu, 17 Jun 2021 12:09:36 +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: 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 0.103.2/26203/Wed Jun 16 13:07:58 2021) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/16/21 12:07 AM, Eric Biggers wrote: > On Tue, Jun 15, 2021 at 11:54:41PM +0200, Daniel Borkmann wrote: >> On 6/15/21 11:38 PM, Eric Biggers wrote: >>> On Tue, Jun 15, 2021 at 02:32:18PM -0700, Eric Biggers wrote: >>>> On Tue, Jun 15, 2021 at 11:08:18PM +0200, Daniel Borkmann wrote: >>>>> On 6/15/21 9:33 PM, Eric Biggers wrote: >>>>>> On Tue, Jun 15, 2021 at 07:51:07PM +0100, Edward Cree wrote: >>>>>>> >>>>>>> As I understand it, the UBSAN report is coming from the eBPF interpreter, >>>>>>> which is the *slow path* and indeed on many production systems is >>>>>>> compiled out for hardening reasons (CONFIG_BPF_JIT_ALWAYS_ON). >>>>>>> Perhaps a better approach to the fix would be to change the interpreter >>>>>>> to compute "DST = DST << (SRC & 63);" (and similar for other shifts and >>>>>>> bitnesses), thus matching the behaviour of most chips' shift opcodes. >>>>>>> This would shut up UBSAN, without affecting JIT code generation. >>>>>> >>>>>> Yes, I suggested that last week >>>>>> (https://lkml.kernel.org/netdev/YMJvbGEz0xu9JU9D@gmail.com). The AND will even >>>>>> get optimized out when compiling for most CPUs. >>>>> >>>>> Did you check if the generated interpreter code for e.g. x86 is the same >>>>> before/after with that? >>>> >>>> Yes, on x86_64 with gcc 10.2.1, the disassembly of ___bpf_prog_run() is the same >>>> both before and after (with UBSAN disabled). Here is the patch I used: >>>> >>>> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c >>>> index 5e31ee9f7512..996db8a1bbfb 100644 >>>> --- a/kernel/bpf/core.c >>>> +++ b/kernel/bpf/core.c >>>> @@ -1407,12 +1407,30 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn) >>>> DST = (u32) DST OP (u32) IMM; \ >>>> CONT; >>>> + /* >>>> + * Explicitly mask the shift amounts with 63 or 31 to avoid undefined >>>> + * behavior. Normally this won't affect the generated code. >> >> The last one should probably be more specific in terms of 'normally', e.g. that >> it is expected that the compiler is optimizing this away for archs like x86. Is >> arm64 also covered by this ... do you happen to know on which archs this won't >> be the case? >> >> Additionally, I think such comment should probably be more clear in that it also >> needs to give proper guidance to JIT authors that look at the interpreter code to >> see what they need to implement, in other words, that they don't end up copying >> an explicit AND instruction emission if not needed there. > > Same result on arm64 with gcc 10.2.0. > > On arm32 it is different, probably because the 64-bit shifts aren't native in > that case. I don't know about other architectures. But there aren't many ways > to implement shifts, and using just the low bits of the shift amount is the most > logical way. > > Please feel free to send out a patch with whatever comment you want. The diff I > gave was just an example and I am not an expert in BPF. > >> >>>> + */ >>>> +#define ALU_SHIFT(OPCODE, OP) \ >>>> + ALU64_##OPCODE##_X: \ >>>> + DST = DST OP (SRC & 63);\ >>>> + CONT; \ >>>> + ALU_##OPCODE##_X: \ >>>> + DST = (u32) DST OP ((u32)SRC & 31); \ >>>> + CONT; \ >>>> + ALU64_##OPCODE##_K: \ >>>> + DST = DST OP (IMM & 63); \ >>>> + CONT; \ >>>> + ALU_##OPCODE##_K: \ >>>> + DST = (u32) DST OP ((u32)IMM & 31); \ >>>> + CONT; >> >> For the *_K cases these are explicitly rejected by the verifier already. Is this >> required here nevertheless to suppress UBSAN false positive? > > No, I just didn't know that these constants are never out of range. Please feel > free to send out a patch that does this properly. Summarized and fixed via: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=28131e9d933339a92f78e7ab6429f4aaaa07061c Thanks everyone, Daniel