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* Re: [PATCH] mptcp: fix KMSAN: uninit-value in mptcp_established_options
  2026-05-04 16:22 [PATCH] mptcp: fix " Paolo Abeni
@ 2026-05-04 17:31 Paolo Abeni
  2026-05-04 18:20 ` [syzbot] [mptcp?] " syzbot
  0 siblings, 1 reply; 7+ messages in thread
From: Paolo Abeni @ 2026-05-04 17:31 UTC (permalink / raw)
  To: mptcp
  Cc: syzbot+ff020673c5e3d94d9478, Kuniyuki Iwashima, syzkaller-bugs,
	linux-kernel, Matthieu Baerts

[-- Attachment #1: Type: text/plain, Size: 447 bytes --]

On 5/4/26 6:22 PM, Paolo Abeni wrote:
> On 5/4/26 11:59 AM, Matthieu Baerts wrote:
>>
>> Sorry for the noise: I forgot to add the syzbot instruction... (and I
>> forgot to remove the MPTCP ML from the sendmail.to option).
> 
> I did not take in account all the possible corner cases.
> 
> Let's be a little more conservative.
Darn... the last upgrade here broke the line (un)wrap extension. Let me
attach the patch. Sorry for the spam.

#syz test

[-- Attachment #2: zero_opt_flags.patch --]
[-- Type: text/x-patch, Size: 1455 bytes --]

diff --git a/include/net/mptcp.h b/include/net/mptcp.h
index f7263fe2a2e4..0763fd6f7758 100644
--- a/include/net/mptcp.h
+++ b/include/net/mptcp.h
@@ -27,6 +27,9 @@ struct mptcp_ext {
 	u32		subflow_seq;
 	u16		data_len;
 	__sum16		csum;
+
+	struct_group(flags,
+
 	u8		use_map:1,
 			dsn64:1,
 			data_fin:1,
@@ -38,6 +41,8 @@ struct mptcp_ext {
 	u8		reset_reason:4,
 			csum_reqd:1,
 			infinite_map:1;
+
+	); /* end of flags group */
 };
 
 #define MPTCPOPT_HMAC_LEN	20
diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 8a1c5698983c..3fd40dbff82b 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -572,6 +572,11 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb,
 	bool ret = false;
 	u64 ack_seq;
 
+	/* Zero `can_ack` and `use_map` flags with one shot. */
+	BUILD_BUG_ON(sizeof_field(struct mptcp_ext, flags) != sizeof(u16));
+	BUILD_BUG_ON(!IS_ALIGNED(offsetof(struct mptcp_ext, flags),
+				 sizeof(u16)));
+	*(u16 *)&opts->ext_copy.flags = 0;
 	opts->csum_reqd = READ_ONCE(msk->csum_enabled);
 	mpext = skb ? mptcp_get_ext(skb) : NULL;
 
@@ -595,7 +600,6 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb,
 	/* passive sockets msk will set the 'can_ack' after accept(), even
 	 * if the first subflow may have the already the remote key handy
 	 */
-	opts->ext_copy.use_ack = 0;
 	if (!READ_ONCE(msk->can_ack)) {
 		*size = ALIGN(dss_size, 4);
 		return ret;

^ permalink raw reply related	[flat|nested] 7+ messages in thread
* Re: [PATCH] mptcp: fix KMSAN: uninit-value in mptcp_established_options
  2026-05-04  9:59 [PATCH] mptcp: fix " Matthieu Baerts
@ 2026-05-04 16:22 Paolo Abeni
  2026-05-04 17:14 ` [syzbot] [mptcp?] " syzbot
  0 siblings, 1 reply; 7+ messages in thread
From: Paolo Abeni @ 2026-05-04 16:22 UTC (permalink / raw)
  To: mptcp
  Cc: syzbot+ff020673c5e3d94d9478, Kuniyuki Iwashima, syzkaller-bugs,
	linux-kernel, Matthieu Baerts

On 5/4/26 11:59 AM, Matthieu Baerts wrote:
> 
> Sorry for the noise: I forgot to add the syzbot instruction... (and I
> forgot to remove the MPTCP ML from the sendmail.to option).

I did not take in account all the possible corner cases.

Let's be a little more conservative.

#syz test
---
diff --git a/include/net/mptcp.h b/include/net/mptcp.h
index f7263fe2a2e4..0763fd6f7758 100644
--- a/include/net/mptcp.h
+++ b/include/net/mptcp.h
@@ -27,6 +27,9 @@ struct mptcp_ext {
 	u32		subflow_seq;
 	u16		data_len;
 	__sum16		csum;
+
+	struct_group(flags,
+
 	u8		use_map:1,
 			dsn64:1,
 			data_fin:1,
@@ -38,6 +41,8 @@ struct mptcp_ext {
 	u8		reset_reason:4,
 			csum_reqd:1,
 			infinite_map:1;
+
+	); /* end of flags group */
 };

 #define MPTCPOPT_HMAC_LEN	20
diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 8a1c5698983c..3fd40dbff82b 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -572,6 +572,11 @@ static bool mptcp_established_options_dss(struct
sock *sk, struct sk_buff *skb,
 	bool ret = false;
 	u64 ack_seq;

+	/* Zero `can_ack` and `use_map` flags with one shot. */
+	BUILD_BUG_ON(sizeof_field(struct mptcp_ext, flags) != sizeof(u16));
+	BUILD_BUG_ON(!IS_ALIGNED(offsetof(struct mptcp_ext, flags),
+				 sizeof(u16)));
+	*(u16 *)&opts->ext_copy.flags = 0;
 	opts->csum_reqd = READ_ONCE(msk->csum_enabled);
 	mpext = skb ? mptcp_get_ext(skb) : NULL;

@@ -595,7 +600,6 @@ static bool mptcp_established_options_dss(struct
sock *sk, struct sk_buff *skb,
 	/* passive sockets msk will set the 'can_ack' after accept(), even
 	 * if the first subflow may have the already the remote key handy
 	 */
-	opts->ext_copy.use_ack = 0;
 	if (!READ_ONCE(msk->can_ack)) {
 		*size = ALIGN(dss_size, 4);
 		return ret;


^ permalink raw reply related	[flat|nested] 7+ messages in thread
* Re: [PATCH] mptcp: fix KMSAN: uninit-value in mptcp_established_options
@ 2026-05-04  9:59 Matthieu Baerts
  2026-05-04 11:14 ` [syzbot] [mptcp?] " syzbot
  0 siblings, 1 reply; 7+ messages in thread
From: Matthieu Baerts @ 2026-05-04  9:59 UTC (permalink / raw)
  To: mptcp
  Cc: syzbot+ff020673c5e3d94d9478, Paolo Abeni, Kuniyuki Iwashima,
	syzkaller-bugs, linux-kernel

Hello,

On 04/05/2026 11:51, Matthieu Baerts (NGI0) wrote:
> Just to let syzbot testing it.

Sorry for the noise: I forgot to add the syzbot instruction... (and I
forgot to remove the MPTCP ML from the sendmail.to option).

Hopefully now the following is correct:
#syz test

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 8a1c5698983c..24903a12a4e0 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -583,6 +583,8 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb,
 				map_size += TCPOLEN_MPTCP_DSS_CHECKSUM;
 
 			opts->ext_copy = *mpext;
+		} else {
+			opts->ext_copy.use_map = 0;
 		}
 
 		dss_size = map_size;

^ permalink raw reply related	[flat|nested] 7+ messages in thread
[parent not found: <20260503145319.7726-1-kartikey406@gmail.com>]
* [syzbot] [mptcp?] KMSAN: uninit-value in mptcp_established_options
@ 2026-05-01  6:15 syzbot
  2026-05-03 13:01 ` syzbot
  0 siblings, 1 reply; 7+ messages in thread
From: syzbot @ 2026-05-01  6:15 UTC (permalink / raw)
  To: davem, edumazet, geliang, horms, kuba, linux-kernel, martineau,
	matttbe, mptcp, netdev, pabeni, syzkaller-bugs

Hello,

syzbot found the following issue on:

HEAD commit:    08d0d3466664 Merge tag 'net-7.1-rc2' of git://git.kernel.o..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15f6976c580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=1c3f61154f3bb7e5
dashboard link: https://syzkaller.appspot.com/bug?extid=ff020673c5e3d94d9478
compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
userspace arch: i386

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/8688db3eb9ae/disk-08d0d346.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/72c570681077/vmlinux-08d0d346.xz
kernel image: https://storage.googleapis.com/syzbot-assets/21669c9bddcd/bzImage-08d0d346.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+ff020673c5e3d94d9478@syzkaller.appspotmail.com

=====================================================
BUG: KMSAN: uninit-value in mptcp_write_data_fin net/mptcp/options.c:542 [inline]
BUG: KMSAN: uninit-value in mptcp_established_options_dss net/mptcp/options.c:590 [inline]
BUG: KMSAN: uninit-value in mptcp_established_options+0x112f/0x3530 net/mptcp/options.c:874
 mptcp_write_data_fin net/mptcp/options.c:542 [inline]
 mptcp_established_options_dss net/mptcp/options.c:590 [inline]
 mptcp_established_options+0x112f/0x3530 net/mptcp/options.c:874
 tcp_established_options+0x312/0xcc0 net/ipv4/tcp_output.c:1192
 __tcp_transmit_skb+0x5dc/0x5fe0 net/ipv4/tcp_output.c:1575
 __tcp_send_ack+0x967/0xad0 net/ipv4/tcp_output.c:4499
 tcp_send_ack+0x3d/0x60 net/ipv4/tcp_output.c:4505
 mptcp_subflow_shutdown+0x164/0x690 net/mptcp/protocol.c:3137
 mptcp_check_send_data_fin+0x31b/0x3d0 net/mptcp/protocol.c:3218
 __mptcp_wr_shutdown net/mptcp/protocol.c:3234 [inline]
 __mptcp_close+0x860/0x1360 net/mptcp/protocol.c:3313
 mptcp_close+0x42/0x260 net/mptcp/protocol.c:3367
 inet_release+0x1ee/0x2a0 net/ipv4/af_inet.c:442
 __sock_release net/socket.c:722 [inline]
 sock_close+0xd6/0x2f0 net/socket.c:1514
 __fput+0x60e/0x1010 fs/file_table.c:510
 ____fput+0x25/0x30 fs/file_table.c:538
 task_work_run+0x208/0x2b0 kernel/task_work.c:233
 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
 __exit_to_user_mode_loop kernel/entry/common.c:67 [inline]
 exit_to_user_mode_loop+0x306/0x1b60 kernel/entry/common.c:98
 __exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
 syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:238 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:318 [inline]
 __do_fast_syscall_32+0x2c7/0x460 arch/x86/entry/syscall_32.c:310
 do_fast_syscall_32+0x37/0x80 arch/x86/entry/syscall_32.c:332
 do_SYSENTER_32+0x1f/0x30 arch/x86/entry/syscall_32.c:370
 entry_SYSENTER_compat_after_hwframe+0x84/0x8e

Local variable opts created at:
 __tcp_transmit_skb+0x4d/0x5fe0 net/ipv4/tcp_output.c:1536
 __tcp_send_ack+0x967/0xad0 net/ipv4/tcp_output.c:4499

CPU: 0 UID: 0 PID: 24584 Comm: syz.4.6812 Tainted: G        W           syzkaller #0 PREEMPT(full) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
=====================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

^ permalink raw reply	[flat|nested] 7+ messages in thread
end of thread, other threads:[~2026-05-04 18:20 UTC | newest]

Thread overview: 7+ messages (download: mbox.gz follow: Atom feed
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     [not found] <20260503232040.9451-1-kartikey406@gmail.com>
2026-05-04  0:08 ` [syzbot] [mptcp?] KMSAN: uninit-value in mptcp_established_options syzbot
2026-05-04 17:31 [PATCH] mptcp: fix " Paolo Abeni
2026-05-04 18:20 ` [syzbot] [mptcp?] " syzbot
  -- strict thread matches above, loose matches on Subject: below --
2026-05-04 16:22 [PATCH] mptcp: fix " Paolo Abeni
2026-05-04 17:14 ` [syzbot] [mptcp?] " syzbot
2026-05-04  9:59 [PATCH] mptcp: fix " Matthieu Baerts
2026-05-04 11:14 ` [syzbot] [mptcp?] " syzbot
     [not found] <20260503145319.7726-1-kartikey406@gmail.com>
2026-05-03 15:47 ` syzbot
2026-05-01  6:15 syzbot
2026-05-03 13:01 ` syzbot

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