Re: [RFC PATCH] VMCI: Silence memcpy() run-time false positive warning

[Greg Kroah-Hartman]

On Mon, Jan 01, 2024 at 05:08:28AM -0800, Harshit Mogalapalli wrote:
> 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[], so there
> is no bug, but a run time warning is seen while fuzzing with Syzkaller.
> 
> One possible way to silence the warning is to split the memcpy() into
> two parts -- one -- copying the msg and second taking care of payload.

And what are the performance impacts of this?

thanks,

greg k-h