CVE-2025-39828

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

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

syzbot reported the splat below. [0]

When atmtcpvopen() or atmtcpvclose() is called via connect() or close(), atmtcpsendcontrol() is called to send an in-kernel special message.

The message has ATMTCPHDRMAGIC in atmtcpcontrol.hdr.length. Also, a pointer of struct atmvcc 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 atmtcpcontrol { struct atmtcphdr hdr; /* must be first / ... atm_kptr_t vcc; / both directions */ ... } _ATMAPI_ALIGN;

typedef struct { unsigned char [8]; } _ATMAPIALIGN atmkptrt;

The special message is processed in atmtcprecvcontrol() called from atmtcpcsend().

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

1. .ndostartxmit() (vcc->send() == atmsendaal0())
2. vcc_sendmsg()

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

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

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:atmtcprecvcontrol drivers/atm/atmtcp.c:93 [inline] RIP: 0010:atmtcpcsend+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> vccsendmsg+0xa10/0xc60 net/atm/common.c:645 socksendmsgnosec net/socket.c:714 [inline] socksendmsg+0x219/0x270 net/socket.c:729 syssendmsg+0x505/0x830 net/socket.c:2614 _syssendmsg+0x21f/0x2a0 net/socket.c:2668 _syssendmsg net/socket.c:2700 [inline] _dosyssendmsg net/socket.c:2705 [inline] _sesyssendmsg net/socket.c:2703 [inline] _x64syssendmsg+0x19b/0x260 net/socket.c:2703 dosyscallx64 arch/x86/entry/syscall64.c:63 [inline] dosyscall64+0xfa/0x3b0 arch/x86/entry/syscall64.c:94 entrySYSCALL64afterhwframe+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 ORIGRAX: 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: