Messages in this thread | | | Date | Fri, 1 Aug 2008 14:10:50 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [regression] nf_iterate(), BUG: unable to handle kernel NULL pointer dereference |
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On Thu, Jul 24, 2008 at 02:13:42PM -0700, Linus Torvalds wrote: > > > On Thu, 24 Jul 2008, Patrick McHardy wrote: > > > > To fix this I think we need a __krealloc() that doesn't > > free the old memory, especially since it must not be > > freed immediately because it may still be used in a RCU > > read side (see the last part in the patch attached to > > this mail (based on a kernel without your patch)). > > Hmm. Don't you need to fix some of the ordering of the initialization too? > > If there are possible readers that happen in parallel with changing this > thing, don't you need to protect the update of "ext->len" against the > actual changes? And the readers should probably have a read barrier > between checking "len" and actually looking at the values? Finally, why do > the "ct->ext" dereference thing, when we know it has to be equal to "new"? > > ie something like this on the writing side (in _addition_ to both the > patches already seen), but I didn't do the reading side (ie there are no > "smp_rmb()"'s on the reading side) > > And no, I don't know the code, so I don't know who/what can read those > things with RCU, so maybe there is some reason why the actual data doesn't > need protecting. But I somehow doubt it.
There certainly needs to be an rcu_assign_pointer() or a smp_wmb() in there somewhere -- whenever it is that the new structure becomes accessible to RCU readers. I looked around a bit, but couldn't tell when this stuff becomes accessible to readers...
Thanx, Paul
> Linus > > --- > net/netfilter/nf_conntrack_extend.c | 9 +++++---- > 1 files changed, 5 insertions(+), 4 deletions(-) > > diff --git a/net/netfilter/nf_conntrack_extend.c b/net/netfilter/nf_conntrack_extend.c > index 3469bc7..135e095 100644 > --- a/net/netfilter/nf_conntrack_extend.c > +++ b/net/netfilter/nf_conntrack_extend.c > @@ -115,10 +115,11 @@ void *__nf_ct_ext_add(struct nf_conn *ct, enum nf_ct_ext_id id, gfp_t gfp) > ct->ext = new; > } > > - ct->ext->offset[id] = newoff; > - ct->ext->len = newlen; > - memset((void *)ct->ext + newoff, 0, newlen - newoff); > - return (void *)ct->ext + newoff; > + new->offset[id] = newoff; > + memset((void *)new + newoff, 0, newlen - newoff); > + smp_wmb(); > + new->len = newlen; > + return (void *)new + newoff; > } > EXPORT_SYMBOL(__nf_ct_ext_add); > > -- > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html
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