Re: [PATCH v2 2/2] VMCI: Fix memcpy() run-time warning in dg_dispatch_as_host()

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 1/5/24 10:40, 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[], 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."
> 
> Reported-by: syzkaller <syzkaller@googlegroups.com>
> Suggested-by: Vegard Nossum <vegard.nossum@oracle.com>
> Suggested-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Thanks!
-- 
Gustavo

> ---
> This patch is only tested with the C reproducer, not any testing
> specific to driver is done.
> 
> v1->v2: ( Suggestions from Gustavo )
>          1. Change the commit message false positive --> legitimate
>             warning.
>          2. Remove unneeded payload_size variable.
>          3. Replace first memcpy() with direct assignment.
> ---
>   drivers/misc/vmw_vmci/vmci_datagram.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/misc/vmw_vmci/vmci_datagram.c b/drivers/misc/vmw_vmci/vmci_datagram.c
> index ac6cb0c8d99b..ba379cd6d054 100644
> --- a/drivers/misc/vmw_vmci/vmci_datagram.c
> +++ b/drivers/misc/vmw_vmci/vmci_datagram.c
> @@ -234,7 +234,8 @@ static int dg_dispatch_as_host(u32 context_id, struct vmci_datagram *dg)
>   
>   			dg_info->in_dg_host_queue = true;
>   			dg_info->entry = dst_entry;
> -			memcpy(&dg_info->msg, dg, dg_size);
> +			dg_info->msg = *dg;
> +			memcpy(&dg_info->msg_payload, dg + 1, dg->payload_size);
>   
>   			INIT_WORK(&dg_info->work, dg_delayed_dispatch);
>   			schedule_work(&dg_info->work);