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.”