From patchwork Sat Jan 21 00:22:32 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Kumar Kartikeya Dwivedi X-Patchwork-Id: 13110783 X-Patchwork-Delegate: bpf@iogearbox.net 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 918F2C05027 for ; Sat, 21 Jan 2023 00:26:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229542AbjAUA0t (ORCPT ); Fri, 20 Jan 2023 19:26:49 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35728 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230310AbjAUAY7 (ORCPT ); Fri, 20 Jan 2023 19:24:59 -0500 Received: from mail-pj1-x1043.google.com (mail-pj1-x1043.google.com [IPv6:2607:f8b0:4864:20::1043]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 92C90C13DA for ; Fri, 20 Jan 2023 16:24:21 -0800 (PST) Received: by mail-pj1-x1043.google.com with SMTP id b10so7100260pjo.1 for ; Fri, 20 Jan 2023 16:24:21 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=2H/54ZqUIjU3VYoXRIgXxyEiI8YMEx+pUQ0BaawvGCY=; b=iOFVvDBKnQJSkUi+dAHuhUyrKgrH0YBL0gekJv0qfcKQPMSFSbg9Vc0zQ5AdAhBUqM deUzxo5xP4Fy3TBCiq5jKTHeYtLkKHJz9TQRrd3fEfjQLCMvB3EO+h6YnSgKk9P2hiZX GDsRE0NKvcRfecFquU7hy/xSGjHJrDxoHxdQfeeF9p37bJfAdcp4howypukHq3dXbfkj 1Dj6BaVXY2+UHCI6YyZaBe+Gv+3YhTPX3Lv/lp174pmsxq3LGku3sleVDkTc42bUgos0 Px0wir+Ji5oKzEogl4MnjD1y+dQocQb0jeRLE9ZURqOlOlgXtEGSjO7RLBZcLHB8fXUC YMdA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=2H/54ZqUIjU3VYoXRIgXxyEiI8YMEx+pUQ0BaawvGCY=; b=6wwTTmFiTOy54nJjqU8bPKbP7/F+GAAmnY4jELpSTWsBIe8Z7entfw3xcWpEkV2wtx zauihQrbwrKHcgKGs36ne/X72ebGd+yio3sndMYxLi2YGybDfZw9KQ6JcWaMCah6x4GB MEuGO1moOVOWoQ3Yn7eKqzlsCn4FK8EMZTXrO6GZAvvpLHeE8i7pMiBKUSOKPjzrT5nl Y78AKlxkUVwxdceLtk2PzVVlKjRD8FY/MW+9+tO0lq+U0xUnKTQftC4GbctLW3ZteL5U aXBZ/349bh7jo+T6iBgCSgCAhMmztwQlMa6SCyVxtiTUWAJcLPalL52/zd24qRCJJgOO nsmQ== X-Gm-Message-State: AFqh2kp80zDrGX314EgVMY+4hKvQTMklhuQ32KlKHvWQcvELJQJ+zK3t Z9pr43D7rv/L7HCW68gOoi5VOxaakOM= X-Google-Smtp-Source: AMrXdXvQdFOM7pHzBq8FoHpSvdfXZ2EB4E5bSjY3WKkx/lRg4oDk9nhAoLxoe96Fruw5G+M90zvV+w== X-Received: by 2002:a17:902:8c94:b0:193:ab5:39c7 with SMTP id t20-20020a1709028c9400b001930ab539c7mr17171468plo.11.1674260575973; Fri, 20 Jan 2023 16:22:55 -0800 (PST) Received: from localhost ([2405:201:6014:dae3:7dbb:8857:7c39:bb2a]) by smtp.gmail.com with ESMTPSA id i11-20020a17090332cb00b001949c0d7a33sm9585695plr.7.2023.01.20.16.22.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 20 Jan 2023 16:22:55 -0800 (PST) From: Kumar Kartikeya Dwivedi To: bpf@vger.kernel.org Cc: Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Martin KaFai Lau Subject: [PATCH bpf-next v5 03/12] bpf: Fix partial dynptr stack slot reads/writes Date: Sat, 21 Jan 2023 05:52:32 +0530 Message-Id: <20230121002241.2113993-4-memxor@gmail.com> X-Mailer: git-send-email 2.39.1 In-Reply-To: <20230121002241.2113993-1-memxor@gmail.com> References: <20230121002241.2113993-1-memxor@gmail.com> MIME-Version: 1.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=8936; i=memxor@gmail.com; h=from:subject; bh=88XPzRu2Z9EO4iSWHshfPh1pJZo++IffnWh/36lilyY=; b=owEBbQKS/ZANAwAIAUzgyIZIvxHKAcsmYgBjyzAjDrwrMkcLcpOoo/SrtdbpZSaCSGvVeLRH4RUd 8HAwUXWJAjMEAAEIAB0WIQRLvip+Buz51YI8YRFM4MiGSL8RygUCY8swIwAKCRBM4MiGSL8RyhnKEA CTRy7imY7aMKqthGbXj4MxfEnR2ZdK1SrX/D8wn65kZTltEWlJWqM9dLjuNNYDFws3gE+1RcFn5x2y bmbhy9nQxbzBDiOUdG6kMxst74YRoA2jYb0EwJ1JgvCa9RiAWOouYc8Eg87DdzHFsmO0mWgM6vxnWA 2xD/gx8GrQ2ifdlPJnuJaYsjorrkG4/Ot8us9m6GIchhwmT4LcsarWNVIBLXRPnxQc5j1Gb12+Jbge fLlBgMZ4GHhJK/nqeJLLozL/3ANXc7IGaeiYkvHME68cYD23a6Fg9PruiXWC5PCaTK7jSKlbz5F2ms YPUClaYE7tsN/ApNbG7sbz6FAToZ5oqxyvtGpO5q/tEyFsupopCDRNOTv317TnrELm6SN6N8OARMsj cZsV0Ec2Hi8tg/U+l0kBKzJIaPCOjoiVtS0fbn2qI8RjBZR/x3wRoHeu2tycOzoaoXzOEj99I8zebF uDq/VRvS6U6mmVCPTy4tXWJ0cpGsTTLidj1hC6Mk4twnpGKPTqeKxYtPPPDbKs0RGjexPRLzhsvrr5 ETiyXTlr9u+Jzoz9tCFS/wrY6lj7+aCRxlpr9mMbgwOHSPTeosoAHEY4XieUC6gcra0uopR1A7vjVa tJdbj4DPqHtgN17lhK0DChFUa/vK/G5KnpAA4qTVQ2Q2ZtWdwLk/MVlLGITQ== X-Developer-Key: i=memxor@gmail.com; a=openpgp; fpr=4BBE2A7E06ECF9D5823C61114CE0C88648BF11CA Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org X-Patchwork-Delegate: bpf@iogearbox.net Currently, while reads are disallowed for dynptr stack slots, writes are not. Reads don't work from both direct access and helpers, while writes do work in both cases, but have the effect of overwriting the slot_type. While this is fine, handling for a few edge cases is missing. Firstly, a user can overwrite the stack slots of dynptr partially. Consider the following layout: spi: [d][d][?] 2 1 0 First slot is at spi 2, second at spi 1. Now, do a write of 1 to 8 bytes for spi 1. This will essentially either write STACK_MISC for all slot_types or STACK_MISC and STACK_ZERO (in case of size < BPF_REG_SIZE partial write of zeroes). The end result is that slot is scrubbed. Now, the layout is: spi: [d][m][?] 2 1 0 Suppose if user initializes spi = 1 as dynptr. We get: spi: [d][d][d] 2 1 0 But this time, both spi 2 and spi 1 have first_slot = true. Now, when passing spi 2 to dynptr helper, it will consider it as initialized as it does not check whether second slot has first_slot == false. And spi 1 should already work as normal. This effectively replaced size + offset of first dynptr, hence allowing invalid OOB reads and writes. Make a few changes to protect against this: When writing to PTR_TO_STACK using BPF insns, when we touch spi of a STACK_DYNPTR type, mark both first and second slot (regardless of which slot we touch) as STACK_INVALID. Reads are already prevented. Second, prevent writing to stack memory from helpers if the range may contain any STACK_DYNPTR slots. Reads are already prevented. For helpers, we cannot allow it to destroy dynptrs from the writes as depending on arguments, helper may take uninit_mem and dynptr both at the same time. This would mean that helper may write to uninit_mem before it reads the dynptr, which would be bad. PTR_TO_MEM: [?????dd] Depending on the code inside the helper, it may end up overwriting the dynptr contents first and then read those as the dynptr argument. Verifier would only simulate destruction when it does byte by byte access simulation in check_helper_call for meta.access_size, and fail to catch this case, as it happens after argument checks. The same would need to be done for any other non-trivial objects created on the stack in the future, such as bpf_list_head on stack, or bpf_rb_root on stack. A common misunderstanding in the current code is that MEM_UNINIT means writes, but note that writes may also be performed even without MEM_UNINIT in case of helpers, in that case the code after handling meta && meta->raw_mode will complain when it sees STACK_DYNPTR. So that invalid read case also covers writes to potential STACK_DYNPTR slots. The only loophole was in case of meta->raw_mode which simulated writes through instructions which could overwrite them. A future series sequenced after this will focus on the clean up of helper access checks and bugs around that. Fixes: 97e03f521050 ("bpf: Add verifier support for dynptrs") Signed-off-by: Kumar Kartikeya Dwivedi --- kernel/bpf/verifier.c | 88 +++++++++++++++++++ .../testing/selftests/bpf/progs/dynptr_fail.c | 6 +- 2 files changed, 91 insertions(+), 3 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 76afdbea425a..5c7f29ca94ec 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -769,6 +769,8 @@ static void mark_dynptr_cb_reg(struct bpf_reg_state *reg, __mark_dynptr_reg(reg, type, true); } +static int destroy_if_dynptr_stack_slot(struct bpf_verifier_env *env, + struct bpf_func_state *state, int spi); static int mark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_reg_state *reg, enum bpf_arg_type arg_type, int insn_idx) @@ -863,6 +865,55 @@ static int unmark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_re return 0; } +static void __mark_reg_unknown(const struct bpf_verifier_env *env, + struct bpf_reg_state *reg); + +static int destroy_if_dynptr_stack_slot(struct bpf_verifier_env *env, + struct bpf_func_state *state, int spi) +{ + int i; + + /* We always ensure that STACK_DYNPTR is never set partially, + * hence just checking for slot_type[0] is enough. This is + * different for STACK_SPILL, where it may be only set for + * 1 byte, so code has to use is_spilled_reg. + */ + if (state->stack[spi].slot_type[0] != STACK_DYNPTR) + return 0; + + /* Reposition spi to first slot */ + if (!state->stack[spi].spilled_ptr.dynptr.first_slot) + spi = spi + 1; + + if (dynptr_type_refcounted(state->stack[spi].spilled_ptr.dynptr.type)) { + verbose(env, "cannot overwrite referenced dynptr\n"); + return -EINVAL; + } + + mark_stack_slot_scratched(env, spi); + mark_stack_slot_scratched(env, spi - 1); + + /* Writing partially to one dynptr stack slot destroys both. */ + for (i = 0; i < BPF_REG_SIZE; i++) { + state->stack[spi].slot_type[i] = STACK_INVALID; + state->stack[spi - 1].slot_type[i] = STACK_INVALID; + } + + /* TODO: Invalidate any slices associated with this dynptr */ + + /* Do not release reference state, we are destroying dynptr on stack, + * not using some helper to release it. Just reset register. + */ + __mark_reg_not_init(env, &state->stack[spi].spilled_ptr); + __mark_reg_not_init(env, &state->stack[spi - 1].spilled_ptr); + + /* Same reason as unmark_stack_slots_dynptr above */ + state->stack[spi].spilled_ptr.live |= REG_LIVE_WRITTEN; + state->stack[spi - 1].spilled_ptr.live |= REG_LIVE_WRITTEN; + + return 0; +} + static bool is_dynptr_reg_valid_uninit(struct bpf_verifier_env *env, struct bpf_reg_state *reg) { struct bpf_func_state *state = func(env, reg); @@ -3391,6 +3442,10 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env, env->insn_aux_data[insn_idx].sanitize_stack_spill = true; } + err = destroy_if_dynptr_stack_slot(env, state, spi); + if (err) + return err; + mark_stack_slot_scratched(env, spi); if (reg && !(off % BPF_REG_SIZE) && register_is_bounded(reg) && !register_is_null(reg) && env->bpf_capable) { @@ -3504,6 +3559,14 @@ static int check_stack_write_var_off(struct bpf_verifier_env *env, if (err) return err; + for (i = min_off; i < max_off; i++) { + int spi; + + spi = __get_spi(i); + err = destroy_if_dynptr_stack_slot(env, state, spi); + if (err) + return err; + } /* Variable offset writes destroy any spilled pointers in range. */ for (i = min_off; i < max_off; i++) { @@ -5531,6 +5594,31 @@ static int check_stack_range_initialized( } if (meta && meta->raw_mode) { + /* Ensure we won't be overwriting dynptrs when simulating byte + * by byte access in check_helper_call using meta.access_size. + * This would be a problem if we have a helper in the future + * which takes: + * + * helper(uninit_mem, len, dynptr) + * + * Now, uninint_mem may overlap with dynptr pointer. Hence, it + * may end up writing to dynptr itself when touching memory from + * arg 1. This can be relaxed on a case by case basis for known + * safe cases, but reject due to the possibilitiy of aliasing by + * default. + */ + for (i = min_off; i < max_off + access_size; i++) { + int stack_off = -i - 1; + + spi = __get_spi(i); + /* raw_mode may write past allocated_stack */ + if (state->allocated_stack <= stack_off) + continue; + if (state->stack[spi].slot_type[stack_off % BPF_REG_SIZE] == STACK_DYNPTR) { + verbose(env, "potential write to dynptr at off=%d disallowed\n", i); + return -EACCES; + } + } meta->access_size = access_size; meta->regno = regno; return 0; diff --git a/tools/testing/selftests/bpf/progs/dynptr_fail.c b/tools/testing/selftests/bpf/progs/dynptr_fail.c index 02d57b95cf6e..9dc3f23a8270 100644 --- a/tools/testing/selftests/bpf/progs/dynptr_fail.c +++ b/tools/testing/selftests/bpf/progs/dynptr_fail.c @@ -420,7 +420,7 @@ int invalid_write1(void *ctx) * offset */ SEC("?raw_tp") -__failure __msg("Expected an initialized dynptr as arg #3") +__failure __msg("cannot overwrite referenced dynptr") int invalid_write2(void *ctx) { struct bpf_dynptr ptr; @@ -444,7 +444,7 @@ int invalid_write2(void *ctx) * non-const offset */ SEC("?raw_tp") -__failure __msg("Expected an initialized dynptr as arg #1") +__failure __msg("cannot overwrite referenced dynptr") int invalid_write3(void *ctx) { struct bpf_dynptr ptr; @@ -476,7 +476,7 @@ static int invalid_write4_callback(__u32 index, void *data) * be invalidated as a dynptr */ SEC("?raw_tp") -__failure __msg("arg 1 is an unacquired reference") +__failure __msg("cannot overwrite referenced dynptr") int invalid_write4(void *ctx) { struct bpf_dynptr ptr;