On Thu, 16 Dec 2021, Xin Long wrote:
> On Thu, Dec 16, 2021 at 11:39 AM Lee Jones <lee.jones@xxxxxxxxxx> wrote:
> >
> > On Thu, 16 Dec 2021, Xin Long wrote:
> >
> > > On Wed, Dec 15, 2021 at 8:48 PM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
> > > >
> > > > On Tue, 14 Dec 2021 21:57:32 +0000 Lee Jones wrote:
> > > > > The cause of the resultant dump_stack() reported below is a
> > > > > dereference of a freed pointer to 'struct sctp_endpoint' in
> > > > > sctp_sock_dump().
> > > > >
> > > > > This race condition occurs when a transport is cached into its
> > > > > associated hash table followed by an endpoint/sock migration to a new
> > > > > association in sctp_assoc_migrate() prior to their subsequent use in
> > > > > sctp_diag_dump() which uses sctp_for_each_transport() to walk the hash
> > > > > table calling into sctp_sock_dump() where the dereference occurs.
> >
> > > in sctp_sock_dump():
> > > struct sock *sk = ep->base.sk;
> > > ... <--[1]
> > > lock_sock(sk);
> > >
> > > Do you mean in [1], the sk is peeled off and gets freed elsewhere?
> >
> > 'ep' and 'sk' are both switched out for new ones in sctp_sock_migrate().
> >
> > > if that's true, it's still late to do sock_hold(sk) in your this patch.
> >
> > No, that's not right.
> >
> > The schedule happens *inside* the lock_sock() call.
> Sorry, I don't follow this.
> We can't expect when the schedule happens, why do you think this
> can never be scheduled before the lock_sock() call?
True, but I've had this running for hours and it hasn't reproduced.
Without this patch, I can reproduce this in around 2 seconds.
The C-repro for this is pretty intense!
If you want to be *sure* that a schedule will never happen, we can
take a reference directly with:
ep = sctp_endpoint_hold(tsp->asoc->ep);
sk = sock_hold(ep->base.sk);
Which was my original plan before I soak tested this submitted patch
for hours without any sign of reproducing the issue.
> If the sock is peeled off or is being freed, we shouldn't dump this sock,
> and it's better to skip it.
I guess we can do that too.
Are you suggesting sctp_sock_migrate() as the call site?
--
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
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