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
