From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="rfmlhM8v" Received: from out-179.mta1.migadu.com (out-179.mta1.migadu.com [95.215.58.179]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5196E1708 for ; Thu, 7 Dec 2023 20:02:56 -0800 (PST) Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1702008174; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=lw/JRLEwZLfsHYtjrgpWrAOPjpvhQdaf7TT9U1MBUHg=; b=rfmlhM8vg0qeTmSb7wpFab+Mq4kpYi6g2zjwe/q2WCeudDH2W8smWh2Tgjsqyf/AqGD8Er 3FcxG1Qe5xKAQfH0m+ghaUDCapqGeHoSoIQ/1UmONOk1xJF27q2AsbeKOcEHHpIaoUNq+u FBpqkpUSiEZxuKDfOKmWruqQjYAVD9Y= Date: Thu, 7 Dec 2023 20:02:49 -0800 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH bpf-next v4] bpf: Fix a race condition between btf_put() and map_free() Content-Language: en-GB X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song To: Martin KaFai Lau Cc: Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , kernel-team@fb.com, Martin KaFai Lau , Hou Tao , bpf@vger.kernel.org References: <20231206210959.1035724-1-yonghong.song@linux.dev> <969852f3-34f8-45d9-bf2d-f6a4d5167e55@linux.dev> In-Reply-To: <969852f3-34f8-45d9-bf2d-f6a4d5167e55@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 12/7/23 7:59 PM, Yonghong Song wrote: > > On 12/7/23 5:23 PM, Martin KaFai Lau wrote: >> On 12/6/23 1:09 PM, Yonghong Song wrote: >>> When running `./test_progs -j` in my local vm with latest kernel, >>> I once hit a kasan error like below: >>> >>>    [ 1887.184724] BUG: KASAN: slab-use-after-free in >>> bpf_rb_root_free+0x1f8/0x2b0 >>>    [ 1887.185599] Read of size 4 at addr ffff888106806910 by task >>> kworker/u12:2/2830 >>>    [ 1887.186498] >>>    [ 1887.186712] CPU: 3 PID: 2830 Comm: kworker/u12:2 Tainted: >>> G           OEL 6.7.0-rc3-00699-g90679706d486-dirty #494 >>>    [ 1887.188034] Hardware name: QEMU Standard PC (i440FX + PIIX, >>> 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014 >>>    [ 1887.189618] Workqueue: events_unbound bpf_map_free_deferred >>>    [ 1887.190341] Call Trace: >>>    [ 1887.190666]  >>>    [ 1887.190949]  dump_stack_lvl+0xac/0xe0 >>>    [ 1887.191423]  ? nf_tcp_handle_invalid+0x1b0/0x1b0 >>>    [ 1887.192019]  ? panic+0x3c0/0x3c0 >>>    [ 1887.192449]  print_report+0x14f/0x720 >>>    [ 1887.192930]  ? preempt_count_sub+0x1c/0xd0 >>>    [ 1887.193459]  ? __virt_addr_valid+0xac/0x120 >>>    [ 1887.194004]  ? bpf_rb_root_free+0x1f8/0x2b0 >>>    [ 1887.194572]  kasan_report+0xc3/0x100 >>>    [ 1887.195085]  ? bpf_rb_root_free+0x1f8/0x2b0 >>>    [ 1887.195668]  bpf_rb_root_free+0x1f8/0x2b0 >>>    [ 1887.196183]  ? __bpf_obj_drop_impl+0xb0/0xb0 >>>    [ 1887.196736]  ? preempt_count_sub+0x1c/0xd0 >>>    [ 1887.197270]  ? preempt_count_sub+0x1c/0xd0 >>>    [ 1887.197802]  ? _raw_spin_unlock+0x1f/0x40 >>>    [ 1887.198319]  bpf_obj_free_fields+0x1d4/0x260 >>>    [ 1887.198883]  array_map_free+0x1a3/0x260 >>>    [ 1887.199380]  bpf_map_free_deferred+0x7b/0xe0 >>>    [ 1887.199943]  process_scheduled_works+0x3a2/0x6c0 >>>    [ 1887.200549]  worker_thread+0x633/0x890 >>>    [ 1887.201047]  ? __kthread_parkme+0xd7/0xf0 >>>    [ 1887.201574]  ? kthread+0x102/0x1d0 >>>    [ 1887.202020]  kthread+0x1ab/0x1d0 >>>    [ 1887.202447]  ? pr_cont_work+0x270/0x270 >>>    [ 1887.202954]  ? kthread_blkcg+0x50/0x50 >>>    [ 1887.203444]  ret_from_fork+0x34/0x50 >>>    [ 1887.203914]  ? kthread_blkcg+0x50/0x50 >>>    [ 1887.204397]  ret_from_fork_asm+0x11/0x20 >>>    [ 1887.204913]  >>>    [ 1887.204913]  >>>    [ 1887.205209] >>>    [ 1887.205416] Allocated by task 2197: >>>    [ 1887.205881]  kasan_set_track+0x3f/0x60 >>>    [ 1887.206366]  __kasan_kmalloc+0x6e/0x80 >>>    [ 1887.206856]  __kmalloc+0xac/0x1a0 >>>    [ 1887.207293]  btf_parse_fields+0xa15/0x1480 >>>    [ 1887.207836]  btf_parse_struct_metas+0x566/0x670 >>>    [ 1887.208387]  btf_new_fd+0x294/0x4d0 >>>    [ 1887.208851]  __sys_bpf+0x4ba/0x600 >>>    [ 1887.209292]  __x64_sys_bpf+0x41/0x50 >>>    [ 1887.209762]  do_syscall_64+0x4c/0xf0 >>>    [ 1887.210222]  entry_SYSCALL_64_after_hwframe+0x63/0x6b >>>    [ 1887.210868] >>>    [ 1887.211074] Freed by task 36: >>>    [ 1887.211460]  kasan_set_track+0x3f/0x60 >>>    [ 1887.211951]  kasan_save_free_info+0x28/0x40 >>>    [ 1887.212485]  ____kasan_slab_free+0x101/0x180 >>>    [ 1887.213027]  __kmem_cache_free+0xe4/0x210 >>>    [ 1887.213514]  btf_free+0x5b/0x130 >>>    [ 1887.213918]  rcu_core+0x638/0xcc0 >>>    [ 1887.214347]  __do_softirq+0x114/0x37e >>> >>> The error happens at bpf_rb_root_free+0x1f8/0x2b0: >>> >>>    00000000000034c0 : >>>    ; { >>>      34c0: f3 0f 1e fa                   endbr64 >>>      34c4: e8 00 00 00 00                callq   0x34c9 >>> >>>      34c9: 55                            pushq   %rbp >>>      34ca: 48 89 e5                      movq    %rsp, %rbp >>>    ... >>>    ;       if (rec && rec->refcount_off >= 0 && >>>      36aa: 4d 85 ed                      testq   %r13, %r13 >>>      36ad: 74 a9                         je      0x3658 >>> >>>      36af: 49 8d 7d 10                   leaq    0x10(%r13), %rdi >>>      36b3: e8 00 00 00 00                callq   0x36b8 >>> >>>                                          <==== kasan function >>>      36b8: 45 8b 7d 10                   movl    0x10(%r13), %r15d >>>                                          <==== use-after-free load >>>      36bc: 45 85 ff                      testl   %r15d, %r15d >>>      36bf: 78 8c                         js      0x364d >>> >>> >>> So the problem is at rec->refcount_off in the above. >>> >>> I did some source code analysis and find the reason. >>>                                    CPU A CPU B >>>    bpf_map_put: >>>      ... >>>      btf_put with rcu callback >>>      ... >>>      bpf_map_free_deferred >>>        with system_unbound_wq >>>      ...                          ... ... >>>      ...                          btf_free_rcu: ... >>>      ...                          ... bpf_map_free_deferred: >>>      ...                          ... >>>      ...         --------->       btf_struct_metas_free() >>>      ...         | race condition ... >>>      ... ---------> map->ops->map_free() >>>      ... >>>      ...                          btf->struct_meta_tab = NULL >>> >>> In the above, map_free() corresponds to array_map_free() and eventually >>> calling bpf_rb_root_free() which calls: >>>    ... >>>    __bpf_obj_drop_impl(obj, field->graph_root.value_rec, false); >>>    ... >>> >>> Here, 'value_rec' is assigned in btf_check_and_fixup_fields() with >>> following code: >>> >>>    meta = btf_find_struct_meta(btf, btf_id); >>>    if (!meta) >>>      return -EFAULT; >>>    rec->fields[i].graph_root.value_rec = meta->record; >>> >>> So basically, 'value_rec' is a pointer to the record in >>> struct_metas_tab. >>> And it is possible that that particular record has been freed by >>> btf_struct_metas_free() and hence we have a kasan error here. >>> >>> Actually it is very hard to reproduce the failure with current >>> bpf/bpf-next >>> code, I only got the above error once. To increase reproducibility, >>> I added >>> a delay in bpf_map_free_deferred() to delay map->ops->map_free(), which >>> significantly increased reproducibility. >>> >>>    diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c >>>    index 5e43ddd1b83f..aae5b5213e93 100644 >>>    --- a/kernel/bpf/syscall.c >>>    +++ b/kernel/bpf/syscall.c >>>    @@ -695,6 +695,7 @@ static void bpf_map_free_deferred(struct >>> work_struct *work) >>>          struct bpf_map *map = container_of(work, struct bpf_map, >>> work); >>>          struct btf_record *rec = map->record; >>> >>>    +     mdelay(100); >>>          security_bpf_map_free(map); >>>          bpf_map_release_memcg(map); >>>          /* implementation dependent freeing */ >>> >>> To fix the problem, we need to have a reference on btf in order to >>> safeguard accessing field->graph_root.value_rec in >>> map->ops->map_free(). >>> The function btf_parse_graph_root() is the place to get a btf >>> reference. >>> The following are rough call stacks reaching bpf_parse_graph_root(): >>> >>>     btf_parse >>>       ... >>>         btf_parse_fields >>>           ... >>>             btf_parse_graph_root >>> >>>     map_check_btf >>>       btf_parse_fields >>>         ... >>>           btf_parse_graph_root >>> >>> Looking at the above call stack, the btf_parse_graph_root() is >>> indirectly >>> called from btf_parse() or map_check_btf(). >>> >>> We cannot take a reference in btf_parse() case since at that moment, >>> btf is still in the middle to self-validation and initial reference >>> (refcount_set(&btf->refcnt, 1)) has not been triggered yet. >> >> Thanks for the details analysis and clear explanation. It helps a lot. >> >> Sorry for jumping in late. >> >> I am trying to avoid making a special case for "bool has_btf_ref;" >> and "bool from_map_check". It seems to a bit too much to deal with >> the error path for btf_parse(). >> >> Would doing the refcount_set(&btf->refcnt, 1) earlier in btf_parse help? > > No, it does not. The core reason is what Hao is mentioned in > https://lore.kernel.org/bpf/47ee3265-23f7-2130-ff28-27bfaf3f7877@huaweicloud.com/ > We simply cannot take btf reference if called from btf_parse(). > Let us say we move refcount_set(&btf->refcnt, 1) earlier in btf_parse() > so we take ref for btf during btf_parse_fields(), then we have >      btf_put <=== expect refcount == 0 to start the destruction process >        ... >          btf_record_free <=== in which if graph_root, a btf reference > will be hold > so btf_put will never be able to actually free btf data. > Yes, the kasan problem will be resolved but we leak memory. Let me send another version with better commit message. [...]