CVE-2025-39828

In the Linux kernel, the following vulnerability has been resolved:

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>

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