From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-181.mta0.migadu.com (out-181.mta0.migadu.com [91.218.175.181]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 492433A7F72 for ; Thu, 4 Jun 2026 19:36:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.181 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780601770; cv=none; b=SoRx0TqDhGbHVleS+MKmcLKDzMg4VckP87JhgclurPd9PXvzTzfErzflYFKjlm/5N2vtqKAcgqQu9lOniSSzBnfdv1edzZMwYcLpdTuUTQTKkSymfoyY3LBZAhUUkd8GoSLYDSrTdQDbIfDfsF9Zi/q3nA1LAYvgJ6yl0LrW1rs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780601770; c=relaxed/simple; bh=EAD9/bXJ4tnJuAwrhyAdMrCQIQ55jfIqEKAmPGt31sQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=loZrJuWu0kHat4Zi3FSCBAkxjdhPi/3Hwyx4KeqNg9QxYUovdNsffl6RO+Nli4n2iVQhSTaoNQP7kfm2ndrVK8qgSJqQJ0FM0RLw9c2VWkYe9oA61e+OB2QOMr5ppsrIudl8PNecNAB+swtv6A3+kcQMhE4J6RBnbYICF2352h4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=DJouXcY5; arc=none smtp.client-ip=91.218.175.181 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="DJouXcY5" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780601756; 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=PNm9t5e8lAXGoW/IVeDw2m+9fidHrJXDk+58Ty0jXuo=; b=DJouXcY5+AmQMI5KNgzKE2sPEBtR6yIR7QUGT7yRsww8JuRKFwMvKRmTwlXu8LF+6RAiOL EudhekB9R3I/CLuC0enZj0B/SzSANeahh3PqMeszRkgSvSQp4cl0YzgD+LL/hBgPbHweB5 KXK2HN2I3m4gkuJFHTd0skO5UC2N8vM= Date: Thu, 4 Jun 2026 12:35:37 -0700 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v3 1/3] bpf: NUL-terminate replaced sysctl value Content-Language: en-GB To: Alexei Starovoitov Cc: Dawei Feng , Martin KaFai Lau , Emil Tsalapatis , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Eduard , Kumar Kartikeya Dwivedi , Song Liu , Jiri Olsa , Kees Cook , joel.granados@kernel.org, bpf , LKML , Linux-Fsdevel , jianhao.xu@seu.edu.cn, Zilin Guan References: <20260603105317.944304-1-dawei.feng@seu.edu.cn> <20260603105317.944304-2-dawei.feng@seu.edu.cn> <961fbada-5ca3-46c2-a5df-51bb006f9cb6@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 6/3/26 4:23 PM, Alexei Starovoitov wrote: > On Wed, Jun 3, 2026 at 7:37 AM Yonghong Song wrote: >> >> >> On 6/3/26 3:53 AM, Dawei Feng wrote: >>> When writing to sysctls, proc_sys_call_handler() guarantees that the >>> buffer passed to proc handlers is NUL-terminated. If >>> bpf_sysctl_set_new_value() replaces the pending sysctl value, it can >>> hand a replacement buffer directly to proc handlers. However, the >>> helper currently copies only buf_len bytes into that buffer without >>> appending a NUL terminator, leaving downstream parsers vulnerable to >>> out-of-bounds access. >>> >>> Fix this by appending a '\0' after the replaced value to restore the >>> expected sysctl semantics. Since the helper already rejects buf_len >>> greater than PAGE_SIZE - 1, there is always room for the extra byte. >>> >>> Reproduced in a QEMU x86_64 guest booted with KASAN while exercising >>> the sysctl replacement path with a cgroup/sysctl BPF program. The >>> reproducer targets `/proc/sys/net/core/flow_limit_cpu_bitmap`, fills >>> the original user write buffer with non-zero bytes, and overrides the >>> sysctl value so the replacement buffer lacks a terminating NUL. Under >>> that setup, the pre-fix kernel reported: >>> >>> BUG: KASAN: slab-out-of-bounds in strnchrnul+0x72/0x90 >>> Read of size 1 at addr ffff88800de57000 by task repro_patch3/66 >>> CPU: 0 UID: 0 PID: 66 Comm: repro_patch3 Not tainted 7.1.0-rc3-00269-g8370ca1f87cc #6 PREEMPT(lazy) >>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014 >>> Call Trace: >>> >>> dump_stack_lvl+0x68/0xa0 >>> print_report+0xcb/0x5e0 >>> ? __virt_addr_valid+0x21d/0x3f0 >>> ? strnchrnul+0x72/0x90 >>> ? strnchrnul+0x72/0x90 >>> kasan_report+0xca/0x100 >>> ? strnchrnul+0x72/0x90 >>> strnchrnul+0x72/0x90 >>> bitmap_parse+0x37/0x2e0 >>> flow_limit_cpu_sysctl+0xc6/0x840 >>> ? __pfx_flow_limit_cpu_sysctl+0x10/0x10 >>> ? __kvmalloc_node_noprof+0x5ba/0x870 >>> proc_sys_call_handler+0x31d/0x480 >>> ? __pfx_proc_sys_call_handler+0x10/0x10 >>> ? selinux_file_permission+0x39f/0x500 >>> ? lock_is_held_type+0x9e/0x120 >>> vfs_write+0x98e/0x1000 >>> ... >>> >>> The buggy address is located 0 bytes to the right of >>> allocated 4096-byte region [ffff88800de56000, ffff88800de57000) >>> With this fix applied, rerunning the same sysctl-targeted path yields >>> no corresponding KASAN reports. >>> >>> Signed-off-by: Zilin Guan >>> Signed-off-by: Dawei Feng >>> --- >>> kernel/bpf/cgroup.c | 1 + >>> 1 file changed, 1 insertion(+) >>> >>> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c >>> index 876f6a81a9b6..2c7f72d3fb11 100644 >>> --- a/kernel/bpf/cgroup.c >>> +++ b/kernel/bpf/cgroup.c >>> @@ -2342,6 +2342,7 @@ BPF_CALL_3(bpf_sysctl_set_new_value, struct bpf_sysctl_kern *, ctx, >>> return -E2BIG; >>> >>> memcpy(ctx->new_val, buf, buf_len); >>> + ((char *)ctx->new_val)[buf_len] = '\0'; >> In v2 (https://lore.kernel.org/bpf/bf25d653-d856-4ad7-a751-b97d38f38892@linux.dev/) >> I suggested >> memcpy(ctx->new_val, buf, buf_len + 1); >> Does it work? > may be it should be strscpy()? > The input is a string, right? The following is the bpf_sysctl_set_new_value proto: static const struct bpf_func_proto bpf_sysctl_set_new_value_proto = { .func = bpf_sysctl_set_new_value, .gpl_only = false, .ret_type = RET_INTEGER, .arg1_type = ARG_PTR_TO_CTX, .arg2_type = ARG_PTR_TO_MEM | MEM_RDONLY, .arg3_type = ARG_CONST_SIZE, }; So the input may not be a string.