AI Trajectory

null

{
  "EnableConfigs": null,
  "FocusSymbols": [
    "tcp_reqsk_send_challenge_ack",
    "tcp_challenge_ack_allowed",
    "tcp_check_req"
  ],
  "Reasoning": "The patch modifies core TCP logic to send challenge ACKs for non-exact in-window RSTs in SYN-RECEIVED state, implementing RFC 5961. This is a functional change in network protocol handling that should be fuzzed.",
  "WorthFuzzing": true
}

null

{
  "PatchDiff": "commit ebb9e6c7291bd6b50e7a337cd9e7ce2531e4d961\nAuthor: syz-cluster \u003ctriage@syzkaller.com\u003e\nDate:   Fri Jul 17 10:16:28 2026 +0000\n\n    syz-cluster: applied patch under review\n\ndiff --git a/include/net/tcp.h b/include/net/tcp.h\nindex 6d376ea4d1c01d..2c5b889530b556 100644\n--- a/include/net/tcp.h\n+++ b/include/net/tcp.h\n@@ -1974,6 +1974,8 @@ static inline void tcp_fast_path_check(struct sock *sk)\n \n bool tcp_oow_rate_limited(struct net *net, const struct sk_buff *skb,\n \t\t\t  int mib_idx, u32 *last_oow_ack_time);\n+void tcp_reqsk_send_challenge_ack(struct sock *sk, struct sk_buff *skb,\n+\t\t\t\t  struct request_sock *req);\n \n static inline void tcp_mib_init(struct net *net)\n {\ndiff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c\nindex 61045a8886e485..daff93d5134287 100644\n--- a/net/ipv4/tcp_input.c\n+++ b/net/ipv4/tcp_input.c\n@@ -4038,24 +4038,17 @@ static void tcp_send_ack_reflect_ect(struct sock *sk, bool accecn_reflector)\n \t__tcp_send_ack(sk, tp-\u003ercv_nxt, flags);\n }\n \n-/* RFC 5961 7 [ACK Throttling] */\n-static void tcp_send_challenge_ack(struct sock *sk, bool accecn_reflector)\n+/* Consume one slot from the per-netns RFC 5961 challenge ACK quota.\n+ * Returns true if a challenge ACK may be sent.\n+ */\n+static bool tcp_challenge_ack_allowed(struct net *net)\n {\n-\tstruct tcp_sock *tp = tcp_sk(sk);\n-\tstruct net *net = sock_net(sk);\n \tu32 count, now, ack_limit;\n \n-\t/* First check our per-socket dupack rate limit. */\n-\tif (__tcp_oow_rate_limited(net,\n-\t\t\t\t   LINUX_MIB_TCPACKSKIPPEDCHALLENGE,\n-\t\t\t\t   \u0026tp-\u003elast_oow_ack_time))\n-\t\treturn;\n-\n \tack_limit = READ_ONCE(net-\u003eipv4.sysctl_tcp_challenge_ack_limit);\n \tif (ack_limit == INT_MAX)\n-\t\tgoto send_ack;\n+\t\treturn true;\n \n-\t/* Then check host-wide RFC 5961 rate limit. */\n \tnow = jiffies / HZ;\n \tif (now != READ_ONCE(net-\u003eipv4.tcp_challenge_timestamp)) {\n \t\tu32 half = (ack_limit + 1) \u003e\u003e 1;\n@@ -4067,12 +4060,49 @@ static void tcp_send_challenge_ack(struct sock *sk, bool accecn_reflector)\n \tcount = READ_ONCE(net-\u003eipv4.tcp_challenge_count);\n \tif (count \u003e 0) {\n \t\tWRITE_ONCE(net-\u003eipv4.tcp_challenge_count, count - 1);\n-send_ack:\n+\t\treturn true;\n+\t}\n+\treturn false;\n+}\n+\n+/* RFC 5961 7 [ACK Throttling] */\n+static void tcp_send_challenge_ack(struct sock *sk, bool accecn_reflector)\n+{\n+\tstruct tcp_sock *tp = tcp_sk(sk);\n+\tstruct net *net = sock_net(sk);\n+\n+\t/* First check our per-socket dupack rate limit. */\n+\tif (__tcp_oow_rate_limited(net,\n+\t\t\t\t   LINUX_MIB_TCPACKSKIPPEDCHALLENGE,\n+\t\t\t\t   \u0026tp-\u003elast_oow_ack_time))\n+\t\treturn;\n+\n+\t/* Then check the per-netns RFC 5961 rate limit. */\n+\tif (tcp_challenge_ack_allowed(net)) {\n \t\tNET_INC_STATS(net, LINUX_MIB_TCPCHALLENGEACK);\n \t\ttcp_send_ack_reflect_ect(sk, accecn_reflector);\n \t}\n }\n \n+/* Send a challenge ACK from a SYN-RECEIVED request socket. Uses\n+ * __tcp_oow_rate_limited() directly so that an RST carrying payload\n+ * cannot bypass the per-request rate limit.\n+ */\n+void tcp_reqsk_send_challenge_ack(struct sock *sk, struct sk_buff *skb,\n+\t\t\t\t  struct request_sock *req)\n+{\n+\tstruct net *net = sock_net(sk);\n+\n+\tif (__tcp_oow_rate_limited(net, LINUX_MIB_TCPACKSKIPPEDCHALLENGE,\n+\t\t\t\t   \u0026tcp_rsk(req)-\u003elast_oow_ack_time))\n+\t\treturn;\n+\n+\tif (tcp_challenge_ack_allowed(net)) {\n+\t\tNET_INC_STATS(net, LINUX_MIB_TCPCHALLENGEACK);\n+\t\treq-\u003ersk_ops-\u003esend_ack(sk, skb, req);\n+\t}\n+}\n+\n static void tcp_store_ts_recent(struct tcp_sock *tp)\n {\n \ttp-\u003erx_opt.ts_recent = tp-\u003erx_opt.rcv_tsval;\ndiff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c\nindex ddc4b17a826b52..6ab3e3a0b43173 100644\n--- a/net/ipv4/tcp_minisocks.c\n+++ b/net/ipv4/tcp_minisocks.c\n@@ -833,7 +833,7 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,\n \t * elsewhere and is checked directly against the child socket rather\n \t * than req because user data may have been sent out.\n \t */\n-\tif ((flg \u0026 TCP_FLAG_ACK) \u0026\u0026 !fastopen \u0026\u0026\n+\tif ((flg \u0026 TCP_FLAG_ACK) \u0026\u0026 !(flg \u0026 TCP_FLAG_RST) \u0026\u0026 !fastopen \u0026\u0026\n \t    (TCP_SKB_CB(skb)-\u003eack_seq !=\n \t     tcp_rsk(req)-\u003esnt_isn + 1))\n \t\treturn sk;\n@@ -872,6 +872,16 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,\n \t\tflg \u0026= ~TCP_FLAG_SYN;\n \t}\n \n+\t/* RFC 5961 section 3.2, as clarified by RFC 9293 section\n+\t * 3.10.7.4, requires a challenge ACK for a non-exact\n+\t * in-window RST in SYN-RECEIVED.\n+\t */\n+\tif ((flg \u0026 TCP_FLAG_RST) \u0026\u0026\n+\t    TCP_SKB_CB(skb)-\u003eseq != tcp_rsk(req)-\u003ercv_nxt) {\n+\t\ttcp_reqsk_send_challenge_ack(sk, skb, req);\n+\t\treturn NULL;\n+\t}\n+\n \t/* RFC793: \"second check the RST bit\" and\n \t *\t   \"fourth, check the SYN bit\"\n \t */\ndiff --git a/tools/testing/selftests/net/packetdrill/tcp_rfc5961_rst-syn-recv.pkt b/tools/testing/selftests/net/packetdrill/tcp_rfc5961_rst-syn-recv.pkt\nnew file mode 100644\nindex 00000000000000..3fc2de03658af0\n--- /dev/null\n+++ b/tools/testing/selftests/net/packetdrill/tcp_rfc5961_rst-syn-recv.pkt\n@@ -0,0 +1,61 @@\n+// SPDX-License-Identifier: GPL-2.0\n+//\n+// RFC 9293 Section 3.10.7.4: in SYN-RECEIVED, an exact RST resets\n+// the connection.  A non-exact in-window RST elicits a challenge ACK,\n+// while an out-of-window RST is silently discarded.\n+\n+`./defaults.sh`\n+\n+// An exact RST removes the request socket.\n+   0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3\n+  +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0\n+  +0 bind(3, ..., ...) = 0\n+  +0 listen(3, 1) = 0\n+  +0 \u003c S 0:0(0) win 1000 \u003cmss 1000,sackOK,nop,nop,nop,wscale 0\u003e\n+  +0 \u003e S. 0:0(0) ack 1 \u003c...\u003e\n+  +0 \u003c R 1:1(0) win 1000\n+ +.1 \u003c . 1:1(0) ack 1 win 1000\n+  +0 \u003e R 1:1(0)\n+  +0 close(3) = 0\n+\n+// A non-exact in-window RST gets a challenge ACK and the request survives.\n+  +0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3\n+  +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0\n+  +0 bind(3, ..., ...) = 0\n+  +0 listen(3, 1) = 0\n+  +0 \u003c S 0:0(0) win 1000 \u003cmss 1000,sackOK,nop,nop,nop,wscale 0\u003e\n+  +0 \u003e S. 0:0(0) ack 1 \u003c...\u003e\n+  +0 \u003c R 2:2(0) win 1000\n+  +0 \u003e . 1:1(0) ack 1\n+  +0 \u003c . 1:1(0) ack 1 win 1000\n+  +0 accept(3, ..., ...) = 4\n+  +0 close(4) = 0\n+  +0 close(3) = 0\n+\n+// RST sequence validation precedes ACK validation.  Even an RST|ACK\n+// with an unacceptable ACK value gets a challenge ACK.\n+  +0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3\n+  +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0\n+  +0 bind(3, ..., ...) = 0\n+  +0 listen(3, 1) = 0\n+  +0 \u003c S 0:0(0) win 1000 \u003cmss 1000,sackOK,nop,nop,nop,wscale 0\u003e\n+  +0 \u003e S. 0:0(0) ack 1 \u003c...\u003e\n+  +0 \u003c R. 2:2(0) ack 100 win 1000\n+  +0 \u003e . 1:1(0) ack 1\n+  +0 \u003c . 1:1(0) ack 1 win 1000\n+  +0 accept(3, ..., ...) = 4\n+  +0 close(4) = 0\n+  +0 close(3) = 0\n+\n+// An out-of-window RST is silent and does not remove the request.\n+  +0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3\n+  +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0\n+  +0 bind(3, ..., ...) = 0\n+  +0 listen(3, 1) = 0\n+  +0 \u003c S 0:0(0) win 1000 \u003cmss 1000,sackOK,nop,nop,nop,wscale 0\u003e\n+  +0 \u003e S. 0:0(0) ack 1 \u003c...\u003e\n+  +0 \u003c R 100001:100001(0) win 1000\n+ +.1 \u003c . 1:1(0) ack 1 win 1000\n+  +0 accept(3, ..., ...) = 4\n+  +0 close(4) = 0\n+  +0 close(3) = 0\n"
}

gemini-3.1-pro-preview

null

{
  "EnableConfigs": null,
  "FocusSymbols": [
    "tcp_reqsk_send_challenge_ack",
    "tcp_challenge_ack_allowed",
    "tcp_check_req"
  ],
  "Reasoning": "The patch modifies core TCP logic to send challenge ACKs for non-exact in-window RSTs in SYN-RECEIVED state, implementing RFC 5961. This is a functional change in network protocol handling that should be fuzzed.",
  "WorthFuzzing": true
}

You are an expert Linux kernel maintainer.
Your job is to review a provided patch series and determine
if it makes functional changes to the kernel that should be fuzzed.

IMPORTANT: The changes have ALREADY been applied and committed as the HEAD commit in
your workspace. Do NOT rely on your internal knowledge of the kernel. You must actively
use your code access tools to examine the actual source code and confirm any assumptions.

Return WorthFuzzing=false if the patch only contains:
- Modifications to Documentation/, Kconfig files, or code comments.
- Purely decorative changes, such as logging (e.g., pr_err, printk) or tracepoints.
- Changes to numeric constants or macros that do not functionally alter execution flow.
- Code paths that are impossible to reach in virtualized environments like GCE or QEMU,
even when utilizing software-emulated hardware (e.g., usb gadget, mac80211_hwsim).

If it modifies reachable core kernel logic, drivers, or architectures, use your code search
tools to verify the code can be executed, then return WorthFuzzing=true.

When returning WorthFuzzing=true, you MUST ALSO:
1. Extract any specific kernel functions that should be heavily fuzzed into FocusSymbols.
   Avoid listing generic hot-path functions to prevent skewed test distributions.
2. Identify any specific CONFIG_ options required to properly test this new/modified feature.
   Go and look into the Kconfig files and check for ifdefs around the code, do not make assumptions.
   Do not list too generic configs (we already have them enabled). Only list those that
   specifically cover the modified code. List them in the EnableConfigs output array,
   and DO NOT add a 'CONFIG_' prefix (e.g., return "NET_IPV4" instead of "CONFIG_NET_IPV4").
Prefer calling several tools at the same time to save round-trips.


Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.

For your convenience, here is the diff of the changes:
commit ebb9e6c7291bd6b50e7a337cd9e7ce2531e4d961
Author: syz-cluster <triage@syzkaller.com>
Date:   Fri Jul 17 10:16:28 2026 +0000

    syz-cluster: applied patch under review

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 6d376ea4d1c01d..2c5b889530b556 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1974,6 +1974,8 @@ static inline void tcp_fast_path_check(struct sock *sk)
 
 bool tcp_oow_rate_limited(struct net *net, const struct sk_buff *skb,
 			  int mib_idx, u32 *last_oow_ack_time);
+void tcp_reqsk_send_challenge_ack(struct sock *sk, struct sk_buff *skb,
+				  struct request_sock *req);
 
 static inline void tcp_mib_init(struct net *net)
 {
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 61045a8886e485..daff93d5134287 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4038,24 +4038,17 @@ static void tcp_send_ack_reflect_ect(struct sock *sk, bool accecn_reflector)
 	__tcp_send_ack(sk, tp->rcv_nxt, flags);
 }
 
-/* RFC 5961 7 [ACK Throttling] */
-static void tcp_send_challenge_ack(struct sock *sk, bool accecn_reflector)
+/* Consume one slot from the per-netns RFC 5961 challenge ACK quota.
+ * Returns true if a challenge ACK may be sent.
+ */
+static bool tcp_challenge_ack_allowed(struct net *net)
 {
-	struct tcp_sock *tp = tcp_sk(sk);
-	struct net *net = sock_net(sk);
 	u32 count, now, ack_limit;
 
-	/* First check our per-socket dupack rate limit. */
-	if (__tcp_oow_rate_limited(net,
-				   LINUX_MIB_TCPACKSKIPPEDCHALLENGE,
-				   &tp->last_oow_ack_time))
-		return;
-
 	ack_limit = READ_ONCE(net->ipv4.sysctl_tcp_challenge_ack_limit);
 	if (ack_limit == INT_MAX)
-		goto send_ack;
+		return true;
 
-	/* Then check host-wide RFC 5961 rate limit. */
 	now = jiffies / HZ;
 	if (now != READ_ONCE(net->ipv4.tcp_challenge_timestamp)) {
 		u32 half = (ack_limit + 1) >> 1;
@@ -4067,12 +4060,49 @@ static void tcp_send_challenge_ack(struct sock *sk, bool accecn_reflector)
 	count = READ_ONCE(net->ipv4.tcp_challenge_count);
 	if (count > 0) {
 		WRITE_ONCE(net->ipv4.tcp_challenge_count, count - 1);
-send_ack:
+		return true;
+	}
+	return false;
+}
+
+/* RFC 5961 7 [ACK Throttling] */
+static void tcp_send_challenge_ack(struct sock *sk, bool accecn_reflector)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct net *net = sock_net(sk);
+
+	/* First check our per-socket dupack rate limit. */
+	if (__tcp_oow_rate_limited(net,
+				   LINUX_MIB_TCPACKSKIPPEDCHALLENGE,
+				   &tp->last_oow_ack_time))
+		return;
+
+	/* Then check the per-netns RFC 5961 rate limit. */
+	if (tcp_challenge_ack_allowed(net)) {
 		NET_INC_STATS(net, LINUX_MIB_TCPCHALLENGEACK);
 		tcp_send_ack_reflect_ect(sk, accecn_reflector);
 	}
 }
 
+/* Send a challenge ACK from a SYN-RECEIVED request socket. Uses
+ * __tcp_oow_rate_limited() directly so that an RST carrying payload
+ * cannot bypass the per-request rate limit.
+ */
+void tcp_reqsk_send_challenge_ack(struct sock *sk, struct sk_buff *skb,
+				  struct request_sock *req)
+{
+	struct net *net = sock_net(sk);
+
+	if (__tcp_oow_rate_limited(net, LINUX_MIB_TCPACKSKIPPEDCHALLENGE,
+				   &tcp_rsk(req)->last_oow_ack_time))
+		return;
+
+	if (tcp_challenge_ack_allowed(net)) {
+		NET_INC_STATS(net, LINUX_MIB_TCPCHALLENGEACK);
+		req->rsk_ops->send_ack(sk, skb, req);
+	}
+}
+
 static void tcp_store_ts_recent(struct tcp_sock *tp)
 {
 	tp->rx_opt.ts_recent = tp->rx_opt.rcv_tsval;
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index ddc4b17a826b52..6ab3e3a0b43173 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -833,7 +833,7 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 	 * elsewhere and is checked directly against the child socket rather
 	 * than req because user data may have been sent out.
 	 */
-	if ((flg & TCP_FLAG_ACK) && !fastopen &&
+	if ((flg & TCP_FLAG_ACK) && !(flg & TCP_FLAG_RST) && !fastopen &&
 	    (TCP_SKB_CB(skb)->ack_seq !=
 	     tcp_rsk(req)->snt_isn + 1))
 		return sk;
@@ -872,6 +872,16 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 		flg &= ~TCP_FLAG_SYN;
 	}
 
+	/* RFC 5961 section 3.2, as clarified by RFC 9293 section
+	 * 3.10.7.4, requires a challenge ACK for a non-exact
+	 * in-window RST in SYN-RECEIVED.
+	 */
+	if ((flg & TCP_FLAG_RST) &&
+	    TCP_SKB_CB(skb)->seq != tcp_rsk(req)->rcv_nxt) {
+		tcp_reqsk_send_challenge_ack(sk, skb, req);
+		return NULL;
+	}
+
 	/* RFC793: "second check the RST bit" and
 	 *	   "fourth, check the SYN bit"
 	 */
diff --git a/tools/testing/selftests/net/packetdrill/tcp_rfc5961_rst-syn-recv.pkt b/tools/testing/selftests/net/packetdrill/tcp_rfc5961_rst-syn-recv.pkt
new file mode 100644
index 00000000000000..3fc2de03658af0
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_rfc5961_rst-syn-recv.pkt
@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: GPL-2.0
+//
+// RFC 9293 Section 3.10.7.4: in SYN-RECEIVED, an exact RST resets
+// the connection.  A non-exact in-window RST elicits a challenge ACK,
+// while an out-of-window RST is silently discarded.
+
+`./defaults.sh`
+
+// An exact RST removes the request socket.
+   0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3
+  +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+  +0 bind(3, ..., ...) = 0
+  +0 listen(3, 1) = 0
+  +0 < S 0:0(0) win 1000 <mss 1000,sackOK,nop,nop,nop,wscale 0>
+  +0 > S. 0:0(0) ack 1 <...>
+  +0 < R 1:1(0) win 1000
+ +.1 < . 1:1(0) ack 1 win 1000
+  +0 > R 1:1(0)
+  +0 close(3) = 0
+
+// A non-exact in-window RST gets a challenge ACK and the request survives.
+  +0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3
+  +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+  +0 bind(3, ..., ...) = 0
+  +0 listen(3, 1) = 0
+  +0 < S 0:0(0) win 1000 <mss 1000,sackOK,nop,nop,nop,wscale 0>
+  +0 > S. 0:0(0) ack 1 <...>
+  +0 < R 2:2(0) win 1000
+  +0 > . 1:1(0) ack 1
+  +0 < . 1:1(0) ack 1 win 1000
+  +0 accept(3, ..., ...) = 4
+  +0 close(4) = 0
+  +0 close(3) = 0
+
+// RST sequence validation precedes ACK validation.  Even an RST|ACK
+// with an unacceptable ACK value gets a challenge ACK.
+  +0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3
+  +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+  +0 bind(3, ..., ...) = 0
+  +0 listen(3, 1) = 0
+  +0 < S 0:0(0) win 1000 <mss 1000,sackOK,nop,nop,nop,wscale 0>
+  +0 > S. 0:0(0) ack 1 <...>
+  +0 < R. 2:2(0) ack 100 win 1000
+  +0 > . 1:1(0) ack 1
+  +0 < . 1:1(0) ack 1 win 1000
+  +0 accept(3, ..., ...) = 4
+  +0 close(4) = 0
+  +0 close(3) = 0
+
+// An out-of-window RST is silent and does not remove the request.
+  +0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3
+  +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+  +0 bind(3, ..., ...) = 0
+  +0 listen(3, 1) = 0
+  +0 < S 0:0(0) win 1000 <mss 1000,sackOK,nop,nop,nop,wscale 0>
+  +0 > S. 0:0(0) ack 1 <...>
+  +0 < R 100001:100001(0) win 1000
+ +.1 < . 1:1(0) ack 1 win 1000
+  +0 accept(3, ..., ...) = 4
+  +0 close(4) = 0
+  +0 close(3) = 0

gemini-3.1-pro-preview

null

null

							input: 4763
							output: 24
							thoughts: 414
						

**Fuzzing Kernel TCP Logic: Patch Evaluation**

Okay, so I've got this patch series to review. My immediate concern is whether it introduces functional changes that warrant fuzzing. The summary says it modifies `include/net/tcp.h`, `net/ipv4/tcp_input.c`, `net/ipv4/tcp_minisocks.c`, and `tools/testing/selftests/net/packetdrill/tcp_rfc5961_rst-syn-recv.pkt`. That's a decent spread. Reading through the details, it looks like it's impacting core TCP logic, specifically how the kernel handles RST packets in the SYN-RECEIVED state. Definitely a functional change! We need to fuzz this.

Now, let's identify the key functions to target. Based on the changes, it looks like `tcp_reqsk_send_challenge_ack`, `tcp_check_req`, and `tcp_challenge_ack_allowed` are the primary candidates. These are the entry points for fuzzing.

Next, I need to figure out if there are any specific kernel configurations we need to be aware of. The code is located in `net/ipv4/tcp_input.c` and `net/ipv4/tcp_minisocks.c`, which tells me this is core TCP/IPv4 code. Typically, that means `CONFIG_INET` is sufficient. Let's see... looking at the Kconfig files, I don't see any special options specific to this feature. `CONFIG_INET` is the broadest, and the more specific `CONFIG_TCP_CONG_CUBIC` is unrelated.

Alright, let's confirm this with the tools. This looks like a solid starting point for a fuzzing campaign!

{
  "Expression": "tcp_reqsk_send_challenge_ack"
}

{
  "Output": "include/net/tcp.h=1975=bool tcp_oow_rate_limited(struct net *net, const struct sk_buff *skb,\ninclude/net/tcp.h-1976-\t\t\t  int mib_idx, u32 *last_oow_ack_time);\ninclude/net/tcp.h:1977:void tcp_reqsk_send_challenge_ack(struct sock *sk, struct sk_buff *skb,\ninclude/net/tcp.h-1978-\t\t\t\t  struct request_sock *req);\n--\nnet/ipv4/tcp_input.c=4069=static void tcp_send_challenge_ack(struct sock *sk, bool accecn_reflector)\n--\nnet/ipv4/tcp_input.c-4090- */\nnet/ipv4/tcp_input.c:4091:void tcp_reqsk_send_challenge_ack(struct sock *sk, struct sk_buff *skb,\nnet/ipv4/tcp_input.c-4092-\t\t\t\t  struct request_sock *req)\n--\nnet/ipv4/tcp_minisocks.c=687=struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,\n--\nnet/ipv4/tcp_minisocks.c-880-\t    TCP_SKB_CB(skb)-\u003eseq != tcp_rsk(req)-\u003ercv_nxt) {\nnet/ipv4/tcp_minisocks.c:881:\t\ttcp_reqsk_send_challenge_ack(sk, skb, req);\nnet/ipv4/tcp_minisocks.c-882-\t\treturn NULL;\n"
}

gemini-3.1-pro-preview

null

null

							input: 6005
							output: 19
							thoughts: 112
						

**Analyzing the Code's Reachability and Functionality**

Okay, so I'm pretty confident the code I'm looking at is both reachable and working as expected. Now, to pinpoint the relevant symbols for this specific analysis. Let's see... I'll focus on these: `tcp_reqsk_send_challenge_ack`, and `tcp_challenge_ack_allowed`. While `tcp_check_req` has seen some modifications, it's potentially too broad for the immediate scope of my investigation. Those first two seem far more targeted and pertinent.

Right, time to dive into the code. I'll start by checking the definition of `tcp_check_req` in `net/ipv4/tcp_minisocks.c` to see how it might be related to these key functions. I'll review its interactions with those functions.

{
  "Expression": "tcp_check_req"
}

{
  "Output": "include/net/tcp.h=476=enum tcp_tw_status tcp_timewait_state_process(struct inet_timewait_sock *tw,\n--\ninclude/net/tcp.h-480-\t\t\t\t\t      enum skb_drop_reason *drop_reason);\ninclude/net/tcp.h:481:struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,\ninclude/net/tcp.h-482-\t\t\t   struct request_sock *req, bool fastopen,\n--\nnet/ipv4/tcp_input.c=7152=tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)\n--\nnet/ipv4/tcp_input.c-7218-\t\tSKB_DR_SET(reason, TCP_FASTOPEN);\nnet/ipv4/tcp_input.c:7219:\t\tif (!tcp_check_req(sk, skb, req, true, \u0026req_stolen, \u0026reason))\nnet/ipv4/tcp_input.c-7220-\t\t\tgoto discard;\n--\nnet/ipv4/tcp_ipv4.c=2067=int tcp_v4_rcv(struct sk_buff *skb)\n--\nnet/ipv4/tcp_ipv4.c-2162-\t\t\ttcp_v4_fill_cb(skb, iph, th);\nnet/ipv4/tcp_ipv4.c:2163:\t\t\tnsk = tcp_check_req(sk, skb, req, false, \u0026req_stolen,\nnet/ipv4/tcp_ipv4.c-2164-\t\t\t\t\t    \u0026drop_reason);\n--\nnet/ipv4/tcp_minisocks.c=546=struct sock *tcp_create_openreq_child(const struct sock *sk,\n--\nnet/ipv4/tcp_minisocks.c-686-\nnet/ipv4/tcp_minisocks.c:687:struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,\nnet/ipv4/tcp_minisocks.c-688-\t\t\t   struct request_sock *req,\n--\nnet/ipv6/tcp_ipv6.c=1728=INDIRECT_CALLABLE_SCOPE int tcp_v6_rcv(struct sk_buff *skb)\n--\nnet/ipv6/tcp_ipv6.c-1819-\t\t\ttcp_v6_fill_cb(skb, hdr, th);\nnet/ipv6/tcp_ipv6.c:1820:\t\t\tnsk = tcp_check_req(sk, skb, req, false, \u0026req_stolen,\nnet/ipv6/tcp_ipv6.c-1821-\t\t\t\t\t    \u0026drop_reason);\n"
}

gemini-3.1-pro-preview

null

null

							input: 7016
							output: 111
							thoughts: 185
						

**Analyzing the TCP Challenge ACK Patch**

Okay, so I'm digging into this patch. From what I can gather, `tcp_check_req` gets called from both `tcp_v4_rcv` and `tcp_v6_rcv` when dealing with SYN-RECEIVED state, specifically through request sockets. It seems relatively active, though not as intense as `tcp_v4_rcv` itself.

The key players here are definitely `tcp_reqsk_send_challenge_ack` and `tcp_challenge_ack_allowed`. Those are symbols I should really focus on. I'll make a note of those.

Now, let's see about any specific configurations. Hmm, just `CONFIG_INET`, which is expected. No specialized settings needed, which is a good sign. It's clean.

The core of this patch seems to be about modifying core TCP behavior. It's about sending challenge ACKs in SYN-RECEIVED for RST packets that don't perfectly align with the window. Basically, it's implementing RFC 5961. This is a real functional change to network protocol handling, so it's a prime target for fuzzing. This seems like a good target.

I'm ready to proceed with using `set-results`.

{
  "EnableConfigs": [],
  "FocusSymbols": [
    "tcp_reqsk_send_challenge_ack",
    "tcp_challenge_ack_allowed",
    "tcp_check_req"
  ],
  "Reasoning": "The patch modifies core TCP logic to send challenge ACKs for non-exact in-window RSTs in SYN-RECEIVED state, implementing RFC 5961. This is a functional change in network protocol handling that should be fuzzed.",
  "WorthFuzzing": true
}

{
  "EnableConfigs": null,
  "FocusSymbols": [
    "tcp_reqsk_send_challenge_ack",
    "tcp_challenge_ack_allowed",
    "tcp_check_req"
  ],
  "Reasoning": "The patch modifies core TCP logic to send challenge ACKs for non-exact in-window RSTs in SYN-RECEIVED state, implementing RFC 5961. This is a functional change in network protocol handling that should be fuzzed.",
  "WorthFuzzing": true
}