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

Harshit Mogalapalli
2024-01-02T18:34:58Z

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