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

[Harshit Mogalapalli]

Hi Greg,

On 01/01/24 7:25 pm, Greg Kroah-Hartman wrote:
> 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?

I haven't done any performance tests on this.

I tried to look at the diff in assembly code but couldn't comment on 
performance from that. Also, gustavo suggested to do this: instead of 
two memcpy()'s; a direct assignment and memcpy() for the payload part.

Is there a way to do perf analysis based on code without access to hardware?

Thanks,
Harshit

> thanks,
>
> greg k-h