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tcp: do not let tcp_rmem be set below 4096

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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 b1cb59cf2e ("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: 1da177e4c3 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260912144848.3448026-1-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Eric Dumazet 2026-09-12 14:48:48 +00:00 committed by Paolo Abeni
parent 8e759cd1f6
commit 83a945a529
2 changed files with 5 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
Documentation/networking/ip-sysctl.rst
View File

@ -874,6 +874,8 @@ tcp_rmem - vector of 3 INTEGERs: min, default, max
case this value is ignored.
Default: between 131072 and 32MB, depending on RAM size.
Each of the three values cannot be set below 4096.
tcp_sack - BOOLEAN
Enable select acknowledgments (SACKS).

4
net/ipv4/sysctl_net_ipv4.c
View File

@ -51,6 +51,8 @@ static int tcp_ecn_mode_max = 5;
static u32 icmp_errors_extension_mask_all =
GENMASK_U8(ICMP_ERR_EXT_COUNT - 1, 0);
static int tcp_min_rcvbuf = 4096;
/* obsolete */
static int sysctl_tcp_low_latency __read_mostly;
@ -1462,7 +1464,7 @@ static const 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",