[PATCH net] tcp: do not let tcp_rmem be set below 4096

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 <edumazet@google.com>
---
 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 b05829e44d8fcb5efe1a0e27569a5ffa37915e49..f7af0286341c9b20047f4719d4c5b81d3a12a107 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst

@@ -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).
 

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 2f0363bca2a88d68276670cfce6fb04398f82bc5..e3760daa347064cf455d1e6e9bbee390dab930d9 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c

@@ -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",

-- 
2.55.0.1007.g17ff1f9808-goog