From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-182.mta0.migadu.com (out-182.mta0.migadu.com [91.218.175.182]) (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 A353B3B19A5 for ; Wed, 3 Jun 2026 14:37:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780497474; cv=none; b=W3FnvBUCioM0rVP2oaAKluMqpzUyHUzjgJIb4R8c7xflix6Mdz1m+sVW1LYqyOUsUPn1FF19t9UePVP9DsELucSlo4NB8NIOuLj+mep1Jg1SUHXxK/xiOtx5cv58vrvZDJGx2utrL1wSCJwunoiQjti0QBBnaZ7TARdWWKANgrE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780497474; c=relaxed/simple; bh=83/CcjA7R6FlA91QsIqzRpUpMxfqD4LBKq0WnzuPEgo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=m7WqW/0abBOvCKQBzELE5oCbVUirzt9Rb6CQ0SnyUaILxMHL9TjxxeYi66gx+s1FMynfd8h615wGb3TCAEnIwlqi3PnjcftsVaDf6Q4gDARCFqZqnJaYgYtwedO0AVeMld1pAgYdzUkCLUX2S2sHgRpS+MrAZTGom7EYC4POj5c= 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=TKhLIJXM; arc=none smtp.client-ip=91.218.175.182 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="TKhLIJXM" Message-ID: <961fbada-5ca3-46c2-a5df-51bb006f9cb6@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780497460; 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=BqNdllQBqgAnWlE1ZNMFWUVUiaoezQRXBATxxDjsNRY=; b=TKhLIJXM6TWK6wMHejbD63m1HHx/rym3Gfwiew8hh+HqPckxVy1J+hxg7WDLbFqfenI5w2 6vX4UNfEzETLmluoxnAqqPdAzRB5npKfTzRbYunVpZkml3BsS8MG6uZ3Y0xJKqIuuU2qzG neQ1geHWPhTxj6pbvnUqjlwpduaqjd4= Date: Wed, 3 Jun 2026 07:37:33 -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: Dawei Feng , martin.lau@linux.dev Cc: emil@etsalapatis.com, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, eddyz87@gmail.com, memxor@gmail.com, song@kernel.org, jolsa@kernel.org, kees@kernel.org, joel.granados@kernel.org, bpf@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, jianhao.xu@seu.edu.cn, Zilin Guan References: <20260603105317.944304-1-dawei.feng@seu.edu.cn> <20260603105317.944304-2-dawei.feng@seu.edu.cn> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song In-Reply-To: <20260603105317.944304-2-dawei.feng@seu.edu.cn> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT 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? > ctx->new_len = buf_len; > ctx->new_updated = 1; >