RE: [PATCH] Drivers: hv: vmbus: Fix variable assignments in hv_ringbuffer_read()

[Haiyang Zhang]

> -----Original Message-----
> From: Andres Beltran <lkmlabelt@xxxxxxxxx>
> Sent: Friday, July 24, 2020 7:04 PM
> To: Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx>
> Cc: KY Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang Zhang
> <haiyangz@xxxxxxxxxxxxx>; Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>;
> Wei Liu <wei.liu@xxxxxxxxxx>; linux-hyperv@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; Michael Kelley <mikelley@xxxxxxxxxxxxx>; Andrea
> Parri <parri.andrea@xxxxxxxxx>; Saruhan Karademir
> <skarade@xxxxxxxxxxxxx>
> Subject: Re: [PATCH] Drivers: hv: vmbus: Fix variable assignments in
> hv_ringbuffer_read()
> 
> On Fri, Jul 24, 2020 at 1:10 PM Stephen Hemminger
> <stephen@xxxxxxxxxxxxxxxxxx> wrote:
> > What is the rationale for this change, it may break other code.
> >
> > A common API model in Windows world where this originated
> > is to have a call where caller first
> > makes request and then if the requested buffer is not big enough the
> > caller look at the actual length and allocate a bigger buffer.
> >
> > Did you audit all the users of this API to make sure they aren't doing that.
> >
> 
> The rationale for the change was to solve instances like the one
> @Haiyang Zhang pointed out, especially in hv_utils, which needs
> additional hardening. Unfortunately, there is an instance in
> hv_pci_onchannelcallback() that does what you just described. Thus,
> the fix will have to be made to all the callers of vmbus_recvpacket()
> and vmbus_recvpacket_raw() to make sure they check the return value,
> which most callers are not doing now. Thanks for pointing out this
> behavior. I was not aware that the length can be checked by callers to
> allocate a bigger buffer.

To prevent future coding error, please add code comments for 
hv_ringbuffer_read() to indicate that the buffer_actual_len may be 
nonzero when the function fails, and should not be used to 
determine if the function succeeds or not.

Thanks,
- Haiyang