Re: [PATCH v1] usb: host: Fix possible kernel crash

[Venu Byravarasu]

On Tuesday 10 July 2012 10:05 PM, Scan Subscription wrote:
> Hi Greg/Venu/Alan and others,
>
> The defect discussed in this thread was found in 2006, and was marked in Coverity Scan as false positive - intentional ( by linux developer or coverity admin that we don't know)...
>
> As a general rule,
> 1. what was discussed with some of the Linux folks, Focus on NEW defects...
> 2. Do NOT fix anything that is already marked as Intentional /False Positive
>
>
> For any defects found by Covierty Scan there could be potential 3 actions
> 1. Review and Fix the defect
> 2. Mark it as Intentional in Coverity Scan, and it will not appear as Defect in the future
> 3. Contact Coverity Scan-admin, if you would like to understand it more why it was flagged as defect.
>
> • As we all know, inherent in the technology foundation, static analysis will report some false positives. We put a lot of effort into our product to drive this rate to a very low, acceptable limit (commonly between 5% and 25%)
> • In order to address FPs, the SCAN part of our product offers a triage process that utilizes a persistent database based on defect hashing. This gives developers the option to declare a defect as FP and then never have to look at it again.
>
> For instance, 3 weeks ago, Coverity reported 7 new defects introduced based on recent code changes,  and as we can see in the various threads almost everything was fixed and committed by maintainer.
> But a  week after that, out of 6 new defects reported based on latest code change, 1 was ignored stating False positive, Intentional...
>
> I hope this clarifies the issue that was discussed here.

Thanks for your detailed mail.


> Thanks
>
> Coverity Scan-admin scan-admin@xxxxxxxxxxxx
> Dakshesh Vyas | Technical Manager - SCAN
> Coverity | 185 Berry Street | Suite 6500, Lobby 3 | San Francisco, CA  94107
> Office: 415.935.2957 | dvyas@xxxxxxxxxxxx
>
>
> ________________________________________
> From: linux-kernel-owner@xxxxxxxxxxxxxxx [linux-kernel-owner@xxxxxxxxxxxxxxx] On Behalf Of gregkh@xxxxxxxxxxxxxxxxxxx [gregkh@xxxxxxxxxxxxxxxxxxx]
> Sent: Tuesday, July 10, 2012 7:45 AM
> To: Venu Byravarasu
> Cc: Alan Stern; linux-usb@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v1] usb: host: Fix possible kernel crash
>
> On Tue, Jul 10, 2012 at 09:56:39AM +0530, Venu Byravarasu wrote:
> > Thanks Alan for your comments.
> >
> > On Monday 09 July 2012 08:04 PM, Alan Stern wrote:
> > > On Mon, 9 Jul 2012, Venu Byravarasu wrote:
> > >
> > > > In functions itd_complete &  sitd_complete, a pointer
> > > > by name stream may get dereferenced after freeing it, when
> > > > iso_stream_put is called with stream->refcount = 2.
> > > I don't understand the problem.  Did you actually see this happen or is
> > > it only theoretical?
> > Yes it is a theoretical problem, as complained by Coverity.
> >
> > >      /* for each uframe with a packet */
> > >      for (uframe = 0; uframe < 8; uframe++) {
> > > @@ -1783,7 +1784,8 @@ itd_complete (
> > >                      dev->devpath, stream->bEndpointAddress & 0x0f,
> > >                      (stream->bEndpointAddress & USB_DIR_IN) ? "in" : "out");
> > >      }
> > > -    iso_stream_put (ehci, stream);
> > > +    stream_ref_count = stream->refcount;
> > > +    iso_stream_put(ehci, stream);
> > > This iso_stream_put removes the reference held by the URB.  Before it
> > > is called, stream->refcount must be >= 3:
> > >
> > >      refcount is set to 1 when the stream is created;
> > >
> > >      each active URB holds a reference;
> > >
> > >      each itd holds a reference.
> > >
> > > So after the call, the refcount value must be >= 2 and the stream could
> > > not have been deallocated.
> > >
> > > >   done:
> > > >     itd->urb = NULL;
> > > > @@ -1797,7 +1799,7 @@ done:
> > > >              * Move it to a safe place until a new frame starts.
> > > >              */
> > > >             list_move(&itd->itd_list, &ehci->cached_itd_list);
> > > > -           if (stream->refcount == 2) {
> > > > +           if (stream_ref_count == 3) {
> > > Therefore this seems unnecessary.
> > As per the logic you explained above, this change is not needed.
> > However coverity was complaining as below:
> >
> > /kernel/drivers/usb/host/ehci-sched.c 1777 USE_AFTER_FREE
> > Dereferencing freed pointer "stream"
> >
> > Hence to pacify coverity, this change is done.
> Why are you trying to "pacify" coverity, when the tool is wrong in this
> case?  Go poke the owners of that tool to get it to stop emitting this
> false warning.  Don't paper over it in the kernel.  Especially for a
> tool that none of us can run on our own.
>
> greg k-h
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