atm: atmtcp: Prevent arbitrary write in atmtcp_recv_control().

syzbot reported the splat below. [0]

When atmtcp_v_open() or atmtcp_v_close() is called via connect() or close(), atmtcp_send_control() is called to send an in-kernel special message.

The message has ATMTCP_HDR_MAGIC in atmtcp_control.hdr.length. Also, a pointer of struct atm_vcc is set to atmtcp_control.vcc.

The notable thing is struct atmtcp_control is uAPI but has a space for an in-kernel pointer.

  struct atmtcp_control {
  	struct atmtcp_hdr hdr;	/* must be first */
  ...
  	atm_kptr_t vcc;		/* both directions */
  ...
  } __ATM_API_ALIGN;

  typedef struct { unsigned char _[8]; } __ATM_API_ALIGN atm_kptr_t;

The special message is processed in atmtcp_recv_control() called from atmtcp_c_send().

atmtcp_c_send() is vcc->dev->ops->send() and called from 2 paths:

1. .ndo_start_xmit() (vcc->send() == atm_send_aal0())
2. vcc_sendmsg()

The problem is sendmsg() does not validate the message length and userspace can abuse atmtcp_recv_control() to overwrite any kptr by atmtcp_control.

Let's add a new ->pre_send() hook to validate messages from sendmsg().

[0]:

Oops: general protection fault, probably for non-canonical address 0xdffffc00200000ab: 0000 [#1] SMP KASAN PTI
KASAN: probably user-memory-access in range [0x0000000100000558-0x000000010000055f]
CPU: 0 UID: 0 PID: 5865 Comm: syz-executor331 Not tainted 6.17.0-rc1-syzkaller-00215-gbab3ce404553 #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
RIP: 0010:atmtcp_recv_control drivers/atm/atmtcp.c:93 [inline]
RIP: 0010:atmtcp_c_send+0x1da/0x950 drivers/atm/atmtcp.c:297
Code: 4d 8d 75 1a 4c 89 f0 48 c1 e8 03 42 0f b6 04 20 84 c0 0f 85 15 06 00 00 41 0f b7 1e 4d 8d b7 60 05 00 00 4c 89 f0 48 c1 e8 03 <42> 0f b6 04 20 84 c0 0f 85 13 06 00 00 66 41 89 1e 4d 8d 75 1c 4c
RSP: 0018:ffffc90003f5f810 EFLAGS: 00010203
RAX: 00000000200000ab RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffff88802a510000 RSI: 00000000ffffffff RDI: ffff888030a6068c
RBP: ffff88802699fb40 R08: ffff888030a606eb R09: 1ffff1100614c0dd
R10: dffffc0000000000 R11: ffffffff8718fc40 R12: dffffc0000000000
R13: ffff888030a60680 R14: 000000010000055f R15: 00000000ffffffff
FS:  00007f8d7e9236c0(0000) GS:ffff888125c1c000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000000045ad50 CR3: 0000000075bde000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 vcc_sendmsg+0xa10/0xc60 net/atm/common.c:645
 sock_sendmsg_nosec net/socket.c:714 [inline]
 __sock_sendmsg+0x219/0x270 net/socket.c:729
 ____sys_sendmsg+0x505/0x830 net/socket.c:2614
 ___sys_sendmsg+0x21f/0x2a0 net/socket.c:2668
 __sys_sendmsg net/socket.c:2700 [inline]
 __do_sys_sendmsg net/socket.c:2705 [inline]
 __se_sys_sendmsg net/socket.c:2703 [inline]
 __x64_sys_sendmsg+0x19b/0x260 net/socket.c:2703
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f8d7e96a4a9
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 51 18 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f8d7e923198 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f8d7e9f4308 RCX: 00007f8d7e96a4a9
RDX: 0000000000000000 RSI: 0000200000000240 RDI: 0000000000000005
RBP: 00007f8d7e9f4300 R08: 65732f636f72702f R09: 65732f636f72702f
R10: 65732f636f72702f R11: 0000000000000246 R12: 00007f8d7e9c10ac
R13: 00007f8d7e9231a0 R14: 0000200000000200 R15: 0000200000000250
 </TASK>
Modules linked in:

---
 drivers/atm/atmtcp.c   | 17 ++++++++++++++---
 include/linux/atmdev.h |  1 +
 net/atm/common.c       | 15 ++++++++++++---
 3 files changed, 27 insertions(+), 6 deletions(-)

index eeae160c898d..fa3c76a2b49d 100644
@@ -279,6 +279,19 @@ static struct atm_vcc *find_vcc(struct atm_dev *dev, short vpi, int vci)
         return NULL;
 }
 
+static int atmtcp_c_pre_send(struct atm_vcc *vcc, struct sk_buff *skb)
+{
+	struct atmtcp_hdr *hdr;
+
+	if (skb->len < sizeof(struct atmtcp_hdr))
+		return -EINVAL;
+
+	hdr = (struct atmtcp_hdr *)skb->data;
+	if (hdr->length == ATMTCP_HDR_MAGIC)
+		return -EINVAL;
+
+	return 0;
+}
 
 static int atmtcp_c_send(struct atm_vcc *vcc,struct sk_buff *skb)
 {

@@ -288,9 +301,6 @@ static int atmtcp_c_send(struct atm_vcc *vcc,struct sk_buff *skb)
 	struct sk_buff *new_skb;
 	int result = 0;
 
-	if (skb->len < sizeof(struct atmtcp_hdr))
-		goto done;
-
 	dev = vcc->dev_data;
 	hdr = (struct atmtcp_hdr *) skb->data;
 	if (hdr->length == ATMTCP_HDR_MAGIC) {

@@ -347,6 +357,7 @@ static const struct atmdev_ops atmtcp_v_dev_ops = {
 
 static const struct atmdev_ops atmtcp_c_dev_ops = {
 	.close		= atmtcp_c_close,
+	.pre_send	= atmtcp_c_pre_send,
 	.send		= atmtcp_c_send
 };
 

index 45f2f278b50a..70807c679f1a 100644
@@ -185,6 +185,7 @@ struct atmdev_ops { /* only send is required */
 	int (*compat_ioctl)(struct atm_dev *dev,unsigned int cmd,
 			    void __user *arg);
 #endif
+	int (*pre_send)(struct atm_vcc *vcc, struct sk_buff *skb);
 	int (*send)(struct atm_vcc *vcc,struct sk_buff *skb);
 	int (*send_bh)(struct atm_vcc *vcc, struct sk_buff *skb);
 	int (*send_oam)(struct atm_vcc *vcc,void *cell,int flags);

index d7f7976ea13a..881c7f259dbd 100644
@@ -635,18 +635,27 @@ int vcc_sendmsg(struct socket *sock, struct msghdr *m, size_t size)
 
 	skb->dev = NULL; /* for paths shared with net_device interfaces */
 	if (!copy_from_iter_full(skb_put(skb, size), size, &m->msg_iter)) {
-		atm_return_tx(vcc, skb);
-		kfree_skb(skb);
 		error = -EFAULT;
-		goto out;
+		goto free_skb;
 	}
 	if (eff != size)
 		memset(skb->data + size, 0, eff-size);
+
+	if (vcc->dev->ops->pre_send) {
+		error = vcc->dev->ops->pre_send(vcc, skb);
+		if (error)
+			goto free_skb;
+	}
+
 	error = vcc->dev->ops->send(vcc, skb);
 	error = error ? error : size;
 out:
 	release_sock(sk);
 	return error;
+free_skb:
+	atm_return_tx(vcc, skb);
+	kfree_skb(skb);
+	goto out;
 }
 
 __poll_t vcc_poll(struct file *file, struct socket *sock, poll_table *wait)

2.51.0.rc1.193.gad69d77794-goog