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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 48BB6C38159 for ; Fri, 20 Jan 2023 05:27:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231128AbjATF1k (ORCPT ); Fri, 20 Jan 2023 00:27:40 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56196 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231428AbjATF1Y (ORCPT ); Fri, 20 Jan 2023 00:27:24 -0500 Received: from mail-pj1-x1042.google.com (mail-pj1-x1042.google.com [IPv6:2607:f8b0:4864:20::1042]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2B7772D7A; Thu, 19 Jan 2023 21:21:05 -0800 (PST) Received: by mail-pj1-x1042.google.com with SMTP id z1-20020a17090a66c100b00226f05b9595so3902083pjl.0; Thu, 19 Jan 2023 21:21:05 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=BU8B2SB14Z6ZiYODQrWdR57MbN6lG8lvjV5NsFbBUic=; b=YJg5U5BwBM46xaurgJGanFwE5EoWzCswgWMMaFfWrewzypGOh7FGM3byTxlhFoHCmH cVMETyezTIa0lPyMBbmCaJ/4It7aWDKz2WcqjBK2a1TlHheqRvQO+rcQ/Gci/QUyqBu6 aj8Zs6/xcOpb9blzvcU1zHx86gMpIYU0X9MGExU4iqEn2U6vEMew9S17D3iJXsHvORmA 0SWZKQcu9nPZ/WUUSpu/bF+ieEAed9PsSC4nqE2frOC/iE8P8A343TlV/F3IFpYYxChk HbhyDuDSNFqlYaRbUArfJ5UlgBcY7smkpZG5a3/bmMtjOXTUNeJQytdcLc6XXHN+7Jp5 vOGQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=BU8B2SB14Z6ZiYODQrWdR57MbN6lG8lvjV5NsFbBUic=; b=V4UYdNu1+oaKpKsC8UGrJ+EiiWuKADDgPl2MruVlm1gg+ws/XqOgEKVLJ3xoiKg0nU EJStsbpytiAIR1NX76CQR6P/pvTcpRCe/rzD3mKuQLBGtSSRLomWnpuGXj45H3kkgaEd K1Gsvo1tkjyPr55a5ABTIumUB+xoRsrNqQr/tTiYvocXDwhPFnBVn7bGmhyMVdwiD7SK PzFOSGYM3SOVzuHyfoTNuJjI8+4LJA2QqBDXwRg4uzFAyqncEPEgQq28NWScB4X/OvJU NcjFDRnc0misCtXphzsu4Opvw7jspG5yCOIWeJgNwj59eQnmtPunMP/aQ01nF5hexyaJ kpfA== X-Gm-Message-State: AFqh2koLVns3nIgnDeORSFg7PiMfJ8x/S0Rl635qGi+HHuB0XxPNnmzD ZgkFcp4Y4lVugXb+fKTp++dOv99jKCA= X-Google-Smtp-Source: AMrXdXsjzJEh5ZJAFX5DlvMyjrvo5UIiDllgPlJZQD6NYO63UWpuz6C2iGhulJtGFD7QHEiW7+0fcg== X-Received: by 2002:a17:90b:4b8a:b0:229:f5a2:80c9 with SMTP id lr10-20020a17090b4b8a00b00229f5a280c9mr3212910pjb.49.1674192064630; Thu, 19 Jan 2023 21:21:04 -0800 (PST) Received: from localhost ([2405:201:6014:dae3:7dbb:8857:7c39:bb2a]) by smtp.gmail.com with ESMTPSA id w19-20020a17090aea1300b0020dc318a43esm554490pjy.25.2023.01.19.21.21.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Jan 2023 21:21:04 -0800 (PST) Date: Fri, 20 Jan 2023 10:51:01 +0530 From: Kumar Kartikeya Dwivedi To: David Vernet Cc: bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, song@kernel.org, yhs@meta.com, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@google.com, haoluo@google.com, jolsa@kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, tj@kernel.org Subject: Re: [PATCH bpf-next 3/8] bpf: Disallow NULL PTR_TO_MEM for trusted kfuncs Message-ID: <20230120052101.sevhc4jybcm6onu2@apollo> References: <20230119235833.2948341-1-void@manifault.com> <20230119235833.2948341-4-void@manifault.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230119235833.2948341-4-void@manifault.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 20, 2023 at 05:28:28AM IST, David Vernet wrote: > KF_TRUSTED_ARGS kfuncs currently have a subtle and insidious bug in > validating pointers to scalars. Say that you have a kfunc like the > following, which takes an array as the first argument: > > bool bpf_cpumask_empty(const struct cpumask *cpumask) > { > return cpumask_empty(cpumask); > } > > ... > BTF_ID_FLAGS(func, bpf_cpumask_empty, KF_TRUSTED_ARGS) > ... > This is known and expected. > If a BPF program were to invoke the kfunc with a NULL argument, it would > crash the kernel. The reason is that struct cpumask is defined as a > bitmap, which is itself defined as an array, and is accessed as a memory > address memory by bitmap operations. So when the verifier analyzes the > register, it interprets it as a pointer to a scalar struct, which is an > array of size 8. check_mem_reg() then sees that the register is NULL, > and returns 0, and the kfunc crashes when it passes it down to the > cpumask wrappers. > > To fix this, this patch adds a check for KF_ARG_PTR_TO_MEM which > verifies that the register doesn't contain a NULL pointer if the kfunc > is KF_TRUSTED_ARGS. > > This may or may not be desired behavior. Some kfuncs may want to > allow callers to pass NULL-able pointers. An alternative would be adding > a KF_NOT_NULL flag and leaving KF_TRUSTED_ARGS alone, though given that > a kfunc is saying it wants to "trust" an argument, it seems reasonable > to prevent NULL. > > Signed-off-by: David Vernet > --- > kernel/bpf/verifier.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 9fa101420046..28ccb92ebe65 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -9092,6 +9092,11 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_ > i, btf_type_str(ref_t), ref_tname, PTR_ERR(resolve_ret)); > return -EINVAL; > } > + if (is_kfunc_trusted_args(meta) && register_is_null(reg)) { > + verbose(env, "NULL pointer passed to trusted arg%d\n", i); > + return -EACCES; > + } > + Current patch looks like a stop gap solution. Just checking for register_is_null is not enough, what about PTR_MAYBE_NULL? That can also be passed. Some arguments can be both PTR_TO_BTF_ID and PTR_TO_MEM, so it will be bypassed in the other case because this check is limited to KF_ARG_PTR_TO_MEM. It would probably be better to disallow NULL by default and explicitly tag the argument with __or_null to indicate that NULL is accepted. Seems like a much better default to me.