From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-188.mta0.migadu.com (out-188.mta0.migadu.com [91.218.175.188]) (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 4F3DE34C124 for ; Mon, 1 Jun 2026 21:22:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.188 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780348943; cv=none; b=YX8ZV90hpY45W3ff50g1SVNukqj+7MwJii1UbIs+Sjpp8PVF+QXjSM2r8a+vqyZVyvWo8Ao+K7B9si2yc5v7NGzW9c/jZ1d/DSV8mSgzRUDlUd8HUeMmiNM9CrexEUhRmXIoJ+k9ii7BGKTU9Gz6f+iDu3vFFq5zOjFEz42Tn1U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780348943; c=relaxed/simple; bh=TOet9gHRAoWgXcK57IsHF+aGMxd4XyBmcv73npyP4bo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=prKsZHlG1S1zQdFf5RIQ5yFPifqYpb1ONc0yIH39j3kl7iNVvgwTTkEXUaFV7sUr09SO+//Hyf7UUxZNbAcBXS9bIGcjI5qpz7knGyIP8eKv445V9mgLj8p6vI6hJDTLr+KqQw8xcwgb0HfEa2+OjVWDSUIgd0Ql8l/bq/e4MW8= 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=MTGDR4w/; arc=none smtp.client-ip=91.218.175.188 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="MTGDR4w/" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780348938; 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=Yi/kLXPWTXDnXBwCzubmcWKzXu2wCsq25zFOF0fkSZQ=; b=MTGDR4w/IFz8uTMl3ssMh69s/YTySr/SiRKDomnTGpH6qm5pvickCNs3WZahjym/SWFHEG YuH/fI4XBLaiipPltU2q313frUWOx8JiLk6ReNLVW3rqAo1YfVI5iha8SIvzJr+55aIfDm 9BTslTIYhAcMWxvC0rWX9WZdP/q7EKs= Date: Mon, 1 Jun 2026 14:22:07 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v2 2/3] bpf: cgroup: 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: <20260529031026.2716641-1-dawei.feng@seu.edu.cn> <20260529031026.2716641-3-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: <20260529031026.2716641-3-dawei.feng@seu.edu.cn> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 5/28/26 8:10 PM, 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 > ? kmem_cache_free+0x308/0x550 > ? __pfx_vfs_write+0x10/0x10 > ? __pfx_do_sys_openat2+0x10/0x10 > ksys_write+0xf2/0x1d0 > ? __pfx_ksys_write+0x10/0x10 > ? trace_irq_enable.constprop.0+0x110/0x140 > do_syscall_64+0x115/0x690 > entry_SYSCALL_64_after_hwframe+0x77/0x7f > RIP: 0033:0x447f37 > Code: ff ff f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 00 f3 0f 1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 51 c3 48 83 ec 28 48 89 54 24 18 48 89 74 24 > RSP: 002b:00007fff01ade608 EFLAGS: 00000246 ORIG_RAX: 0000000000000001 > RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 0000000000447f37 > RDX: 0000000000001fff RSI: 00000000172b1780 RDI: 0000000000000005 > RBP: 00000000172b1780 R08: 00000000004ca1b0 R09: 00000000172b1780 > R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000001fff > R13: 0000000000000000 R14: 0000000000000005 R15: 0000000000000003 > > The buggy address is located 0 bytes to the right of > allocated 4096-byte region [ffff88800de56000, ffff88800de57000) The above log can be simplied. > > 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 faadcfb9b5e5..a0b5f8cd8b10 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'; Does memcpy(ctx->new_val, buf, buf_len + 1) work? > ctx->new_len = buf_len; > ctx->new_updated = 1; >