From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 749AF36D4E1; Wed, 30 Sep 2026 18:13:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790791994; cv=none; b=EZdfWbdIgA1LG3AjnaftIFicZuB0acAkn/ZYdliM+icNtkz+VvK3LnHXI+IuMy6LDTJpfbLHplcfz4yoKuh578jaZWwIpP+gEagSMIiDRb/3ny9apneRMQ0g0BL2mVEXWHdDdEESjoogpmi32PAEt5fSWUIhdZ+POvRzv0cIX/A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790791994; c=relaxed/simple; bh=WD+Y6TsSnzCwjX8CggdwgRDyDoiPCwDIkmrZ+7AHGYA=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=J27LIZFq33KNerIURGXPUbMujq4USprYO83uevK2/QcWx/ECzojO/qkFIj3y9OjFSd6qx9pcrlxCvQuCuw/Phaqj94GNYUTCnuLb9ZWcnOYlcRh+YXZj9/PP1I3DAQEqA1OzxRqbUNk4IsKFlOf8yiyRrugsYxoSXMUt5vVGj28= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=FGlPf9/D; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="FGlPf9/D" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CFCDC1F000FF; Wed, 30 Sep 2026 18:13:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1790791993; bh=MtoEypbh1dy/5uXg7F1273NlsOzxk+k8vROx7i/gju0=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=FGlPf9/DWgnLy6xmOcqCSFXcVeoH//eV0ZHOC7QY1q6YoplVr17GsicRER2VK3rvv xvtGPtQptUNvMxQjSh8rWvhXh0QvaDcAplP0JwUR+12kuTuMrBUn5VMdiVgD1J6haL sBc6m8EeLWWiJ+fcuA6O2Li+afUVsJBuJyiNXhBc= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Eric Dumazet , Simon Horman , Paolo Abeni , Sasha Levin Subject: [PATCH 5.15 497/752] tcp: do not let tcp_rmem be set below 4096 Date: Wed, 30 Sep 2026 17:26:07 +0200 Message-ID: <20260930152409.017144991@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260930152358.131179731@linuxfoundation.org> References: <20260930152358.131179731@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 5.15-stable review patch. If anyone has any objections, please let me know. ------------------ From: Eric Dumazet [ Upstream commit 83a945a529d6e002dd7339c532288a931f463dba ] We can hit a division by zero crash in tcp_rcvbuf_grow() and tcp_rcv_space_adjust(): divide error: 0000 [#1] PREEMPT SMP RIP: 0010:tcp_rcvbuf_grow+0x187/0x450 net/ipv4/tcp_input.c:939 ... grow = div_u64(((u64)rcvwin << 1) * (newval - oldval), oldval); The division uses oldval = tp->rcvq_space.space as divisor. When tp->rcvq_space.space is zero, this leads to a divide-by-zero exception. tp->rcvq_space.space is initialized in tcp_init_buffer_space(): tp->rcvq_space.space = min3(tp->rcv_ssthresh, tp->rcv_wnd, (u32)TCP_INIT_CWND * tp->advmss); If tcp_rmem[1] is configured to very small values (such as 1), sk->sk_rcvbuf is initialized to 1. Then tcp_full_space(sk), which computes (sk->sk_rcvbuf * scaling_ratio) >> 8, truncates to 0. This sets tp->window_clamp = 0, tp->rcv_ssthresh = 0, and tp->rcvq_space.space = 0. Later, when data arrives and DRS is invoked, tcp_rcvbuf_grow() divides by oldval == 0. Back in 2015, commit b1cb59cf2efe ("net: sysctl_net_core: check SNDBUF and RCVBUF for min length") ensured that net.core.rmem_default and net.core.rmem_max cannot be set below SOCK_MIN_RCVBUF. Similarly, SO_RCVBUF setsockopt enforces max_t(int, val * 2, SOCK_MIN_RCVBUF). However, net.ipv4.tcp_rmem still had .extra1 = SYSCTL_ONE, allowing arbitrarily small values. Because SOCK_MIN_RCVBUF depends on sizeof(struct sk_buff) and cacheline alignment, its value varies across architectures and configuration options. Using a fixed constant of 4096 ensures a predictable, architecture- independent lower bound that is safely above SOCK_MIN_RCVBUF everywhere and matches the documented 4K default. Fix this by setting tcp_rmem.extra1 to 4096 and updating the documentation. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Signed-off-by: Eric Dumazet Reviewed-by: Simon Horman Link: https://patch.msgid.link/20260912144848.3448026-1-edumazet@google.com Signed-off-by: Paolo Abeni Signed-off-by: Sasha Levin --- Documentation/networking/ip-sysctl.rst | 2 ++ net/ipv4/sysctl_net_ipv4.c | 4 +++- 2 files changed, 5 insertions(+), 1 deletion(-) diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst index 7f75767a24f1b..883a336309f9f 100644 --- a/Documentation/networking/ip-sysctl.rst +++ b/Documentation/networking/ip-sysctl.rst @@ -692,6 +692,8 @@ tcp_rmem - vector of 3 INTEGERs: min, default, max case this value is ignored. Default: between 131072 and 6MB, depending on RAM size. + Each of the three values cannot be set below 4096. + tcp_sack - BOOLEAN Enable select acknowledgments (SACKS). diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c index 1fc3b4ee2d436..f51e41ea46549 100644 --- a/net/ipv4/sysctl_net_ipv4.c +++ b/net/ipv4/sysctl_net_ipv4.c @@ -54,6 +54,8 @@ static int one_day_secs = 24 * 3600; static u32 fib_multipath_hash_fields_all_mask __maybe_unused = FIB_MULTIPATH_HASH_FIELD_ALL_MASK; +static int tcp_min_rcvbuf = 4096; + /* obsolete */ static int sysctl_tcp_low_latency __read_mostly; @@ -1307,7 +1309,7 @@ static struct ctl_table ipv4_net_table[] = { .maxlen = sizeof(init_net.ipv4.sysctl_tcp_rmem), .mode = 0644, .proc_handler = proc_dointvec_minmax, - .extra1 = SYSCTL_ONE, + .extra1 = &tcp_min_rcvbuf, }, { .procname = "tcp_comp_sack_delay_ns", -- 2.53.0