CVE-2024-35944

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

VMCI: Fix memcpy() run-time warning in dg_dispatch_as_host()

Syzkaller hit 'WARNING in dg_dispatch_as_host' bug.

memcpy: detected field-spanning write (size 56) of single field "&dg_info->msg"

at drivers/misc/vmw_vmci/vmci_datagram.c:237 (size 24)

WARNING: CPU: 0 PID: 1555 at drivers/misc/vmw_vmci/vmci_datagram.c:237

dg_dispatch_as_host+0x88e/0xa60 drivers/misc/vmw_vmci/vmci_datagram.c:237

Some code commentry, based on my understanding:

544 #define VMCI_DG_SIZE(_dg) (VMCI_DG_HEADERSIZE + (size_t)(_dg)->payload_size)

/// This is 24 + payload_size

memcpy(&dg_info->msg, dg, dg_size);

Destination = dg_info->msg ---> this is a 24 byte

structure(struct vmci_datagram)

Source = dg --> this is a 24 byte structure (struct vmci_datagram)

Size = dg_size = 24 + payload_size

{payload_size = 56-24 =32} -- Syzkaller managed to set payload_size to 32.

35 struct delayed_datagram_info {

36 struct datagram_entry *entry;

37 struct work_struct work;

38 bool in_dg_host_queue;

39 /* msg and msg_payload must be together. */

40 struct vmci_datagram msg;

41 u8 msg_payload[];

42 };

So those extra bytes of payload are copied into msg_payload[], a run time

warning is seen while fuzzing with Syzkaller.

One possible way to fix the warning is to split the memcpy() into

two parts -- one -- direct assignment of msg and second taking care of payload.

Gustavo quoted:

"Under FORTIFY_SOURCE we should not copy data across multiple members

in a structure."