From: Lee Jones
> Sent: 17 December 2021 14:35
>
> On Fri, 17 Dec 2021, David Laight wrote:
>
> > From: Lee Jones
> > > Sent: 17 December 2021 13:46
> > >
> > > net/sctp/diag.c for instance is built into its own separate module
> > > (sctp_diag.ko) and requires the use of sctp_endpoint_{hold,put}() in
> > > order to prevent a recently found use-after-free issue.
> > >
> > > In order to prevent data corruption of the pointer used to take a
> > > reference on a specific endpoint, between the time of calling
> > > sctp_endpoint_hold() and it returning, the API now returns a pointer
> > > to the exact endpoint that was incremented.
> > >
> > > For example, in sctp_sock_dump(), we could have the following hunk:
> > >
> > > sctp_endpoint_hold(tsp->asoc->ep);
> > > ep = tsp->asoc->ep;
> > > sk = ep->base.sk
> > > lock_sock(ep->base.sk);
> > >
> > > It is possible for this task to be swapped out immediately following
> > > the call into sctp_endpoint_hold() that would change the address of
> > > tsp->asoc->ep to point to a completely different endpoint. This means
> > > a reference could be taken to the old endpoint and the new one would
> > > be processed without a reference taken, moreover the new endpoint
> > > could then be freed whilst still processing as a result, causing a
> > > use-after-free.
> > >
> > > If we return the exact pointer that was held, we ensure this task
> > > processes only the endpoint we have taken a reference to. The
> > > resultant hunk now looks like this:
> > >
> > > ep = sctp_endpoint_hold(tsp->asoc->ep);
> > > sk = ep->base.sk
> > > lock_sock(sk);
> >
> > Isn't that just the same as doing things in the other order?
> > ep = tsp->asoc->ep;
> > sctp_endpoint_hold(ep);
>
> Sleep for a few milliseconds between those lines and see what happens.
>
> 'ep' could still be freed between the assignment and the call.
It can also be freed half way through setting up the arguments to the call.
So any call:
xxx(tsp->asoc->ep);
is only really valid if both tsp->asoc and asoc->ep are stable.
So it is exactly the same as doing:
ep = tsp->asoc->ep;
xxx(ep);
Returning the value of the argument doesn't help if any of the pointed-to
items can get freed.
David
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