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 41387C54E76 for ; Wed, 4 Jan 2023 22:35:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240595AbjADWf1 (ORCPT ); Wed, 4 Jan 2023 17:35:27 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50920 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235536AbjADWf0 (ORCPT ); Wed, 4 Jan 2023 17:35:26 -0500 Received: from mail-pj1-x102c.google.com (mail-pj1-x102c.google.com [IPv6:2607:f8b0:4864:20::102c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5E57C42E00 for ; Wed, 4 Jan 2023 14:35:25 -0800 (PST) Received: by mail-pj1-x102c.google.com with SMTP id o2so32291188pjh.4 for ; Wed, 04 Jan 2023 14:35:25 -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=16xp0Ny29Nkv4172hDYB/4erWPv+elMmk2a21V3qfrg=; b=IYQc6/BR8oV/io1GlaxTrpE2Vv59OuuI1IfzO1Uclad7LwuMGaY52zuEGIXmkRB2nx bBKn9+Yu5Kb1ykw4heVhPOVxwIwqpmXAHi02hmDrNTsxMgz6W9sWqxR9mV0l2uNvdHYO PRgrQLDM7GoWVmYIcThimdyniFLJlsFWNbvqlsYwsMprYWQ1FKJ7GRHMj0/7xS8+KKhK 4QNg8ul1awa2bXP95TPCFn4Q8MjAz3bxDW6kNYJpnhs7SvI7XhHgyU49brKfhvvh2pBA bG70oDMsXfuTZ+O6fTTxtkye8OVcZKRmhI62jBic38iptQKKdJ92cWCUQMengzbgiKp7 Hx5g== 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=16xp0Ny29Nkv4172hDYB/4erWPv+elMmk2a21V3qfrg=; b=Cd9syHV9qTn0boBmiksuU3teEPLBIsE4FydfvP+yW01h7emW/+mil31fgf6teEPqlk p3WGUb/BcpNwUKVsxqrUTERBeR6o5FsC9Si0iMY3zmvG/eaYGgBfpfgfzDLsYImkZDju C3n9II7zD1pUNk/A0bk9Vc76r64f/npWtf71z2YJ9GaokW5vAOSykS4Myn2f68LWy188 HaPDh9gxPrxJj6tf4i3WPT9v+3l8lYAh2jmQ5+0ZOGPz2dd6pjLIaV2SNfBu1rxvgLmL cSnOrbGrj/CheiYxDiFmglOsIHnV+ctSJWmJchcxSMtJcGsr71fwr/bFxxPkwk5qfq5k AbRg== X-Gm-Message-State: AFqh2kpQOKKuen2c44QJ/gzUtj5bEYSIzfQ9g97LtL9dxs2awUhsdYaS IrWt7g+f3HBforjlb1+jdXg= X-Google-Smtp-Source: AMrXdXt2FLeMFLp51KfoM2qLNv4ZXGF3a2Lc7vxTmKQL37tG0378FmUs963AQ5dCmcXFY9g19zrkdQ== X-Received: by 2002:a17:90a:d784:b0:226:a539:1dfe with SMTP id z4-20020a17090ad78400b00226a5391dfemr5593049pju.11.1672871724708; Wed, 04 Jan 2023 14:35:24 -0800 (PST) Received: from macbook-pro-6.dhcp.thefacebook.com ([2620:10d:c090:400::5:1385]) by smtp.gmail.com with ESMTPSA id f62-20020a17090a704400b00225f49bd4b6sm50492pjk.36.2023.01.04.14.35.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 Jan 2023 14:35:24 -0800 (PST) Date: Wed, 4 Jan 2023 14:35:21 -0800 From: Alexei Starovoitov To: Andrii Nakryiko Cc: Andrii Nakryiko , bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net, kernel-team@fb.com Subject: Re: [PATCH bpf-next 7/7] bpf: unify PTR_TO_MAP_{KEY,VALUE} with default case in regsafe() Message-ID: <20230104223521.hi2wvabfn7ldgh6o@macbook-pro-6.dhcp.thefacebook.com> References: <20221223054921.958283-1-andrii@kernel.org> <20221223054921.958283-8-andrii@kernel.org> <20221228020015.xquaykefotqmok7r@macbook-pro-6.dhcp.thefacebook.com> <20221230021917.yuvm4g7sjj7vy5qc@MacBook-Pro-6.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Tue, Jan 03, 2023 at 02:04:44PM -0800, Andrii Nakryiko wrote: > > It sounds logical, but it can get tricky with ranges and branch taken logic. > > Consider something like: > > R1=(min=2,max=8), R2=(min=1, max=10) > > if (R1 within R2) // bpf prog is doing its own 'within' > > a bit confused what is "R1 within R2" here and what you mean "bpf prog > is doing its own 'within'"? Any sort of `R1 < R2` checks (and any > other op: <=, >=, etc) can't really kick in branch elimination because > R2_min=1 < R1_max=8, so arithmetically speaking we can't conclude that > "R1 is always smaller than R2", so both branches would have to be > examined. Something like that. Details didn't matter to me. It was hypothetical 'within' operation just to illustrate the point. > But I probably misunderstood your example, sorry. > > > // branch taken kicks in > > else > > // issues that were never checked > > > > Now new state has: > > R1=(min=4,max=6), R2=(min=5, max=5) > > > > Both R1 and R2 of new state individually range_within of old safe state, > > but together the prog may go to the unverified path. > > Not sure whether it's practical today. > > You asked for hypothetical, so here it goes :) > > No problem with "hypothetical-ness". But my confusion and argument is > similarly "in principle"-like. Because if such an example above can be > constructed then this would be an issue for SCALAR as well, right? And > if you can bypass verifier's safety with SCALAR, you (hypothetically) > could use that SCALAR to do out-of-bounds memory access by adding this > SCALAR to some mem-like register. Correct. The issue would apply to regular scalar if such 'within' operation was available. > So that's my point and my source of confusion: if we don't trust > var_off+range_within() logic to handle *all* situations correctly, > then we should be worried about SCALARs just as much as anything else > (unless, as usual, I missed something). Yes. I personally don't believe that doing range_within for all regtypes by default is a safer way forward. The example wasn't real. It was trying to demonstrate a possible issue. You insist to see a real example with range_within. I don't have it. It's a gut feel that it could be there because I could construct it with fake 'within'. > > More gut feel than real issue. > > > > > > > > > > > > > SCALARS and PTR_TO_BTF_ID will likely dominate future bpf progs. > > > > Keeping default as regs_exact (that does ID match) is safer default. > > > > > > It's fine, though the point of this patch set was patch #7, enabling > > > logic similar to PTR_TO_MAP_VALUE for PTR_TO_MEM and PTR_TO_BUF. I can > > > send specific fixes for that, no problem. But as I said above, I'm > > > really curious to understand what kind of situations will lead to > > > unsafety if we do var_off+range_within checks. > > > > PTR_TO_MEM and PTR_TO_BUF explicitly are likely ok despite my convoluted > > example above. > > I'm less sure about PTR_TO_BTF_ID. It could be ok. > > Just feels safer to opt-in each type explicitly. > > Sure, I can just do a simple opt-in, no problem. As I said, mostly > trying to understand the issue overall. > > For PTR_TO_BTF_ID specifically, I can see how we can enable > var_off+range_within for cases when we access some array, right? But > then I think we'll be enforcing that we are staying within the > boundaries of a single array field, never crossing into another field. Likely yes, but why? You're trying hard to collapse the switch statement in regsafe() while claiming it's a safer way. I don't see it this way. For example the upcoming active_lock_id would need its own check_ids() call. It will be necessary for PTR_TO_BTF_ID only. Why collapse the switch into 'default:' just to bring some back? The default without checking active_lock_id through check_ids would be wrong, so collapsed switch doesn't make things safer. > But just to take a step back, from my perspective var_off and > range_within are complementary and solve slightly different uses, but > should be both satisfied: > - var_off is not precise with range boundaries (due to some bits too > coarsely marked as unknown), but it's useful to enforce having a value > being a multiple of some power-of-2 (e.g., knowing for sure that > lowest 2 bits are zero means that value is multiple of 4; I haven't > checked, but I assume we check with for various pointer accesses to > ensure we don't have misaligned reads). They can be only approximately > used for actual possible range of values. Right. var_off is used for alignment checking too. grep tnum_is_aligned. We have bare minimum of testing for that though. Only few tests in the test_verifier use BPF_F_STRICT_ALIGNMENT > - range_within() can and should be used for *precise* range of value > tracking, but it can't express that alignment restriction. Right. > So while I previously thought that we can do away without var_off, I > now think there are cases when it's necessary. But if we are sure that > we handle any SCALAR case correctly for any possible var_off + > range_within situation, it should be fine to do that for any mem-like > pointer just as much, as var_off+range_within is basically a MEM + > SCALAR combined case. Right. Likely true. > Anyways, I'm not blocked on this, but I think we'll benefit from > taking this discussion to its logical conclusion. Not sure what conclusion you're looking for.