On Mon, Apr 30, 2012 at 01:23:29PM +0200, Arne Jansen wrote:
> On 30.04.2012 13:11, Dan Carpenter wrote:
> > Hello Arne Jansen,
> >
> > The patch 8c9c2bf7a3c4: "btrfs: fix race in reada" from Feb 25, 2012,
> > leads to the following warning:
> > fs/btrfs/reada.c:308 reada_find_zone()
> > warn: 'zone' was already freed.
>
> Who emits this warning? It's bogus.
>
This is a Smatch warning, but it's not turned on by default because
a lot of place do this:
kfree(p);
printk(KERN_DEBUG "pointer was %p", p);
> >
> > @@ -307,13 +302,15 @@ again:
> > ret = radix_tree_insert(&dev->reada_zones,
> > (unsigned long)(zone->end >> PAGE_CACHE_SHIFT),
> > zone);
> > - spin_unlock(&fs_info->reada_lock);
> >
> > - if (ret) {
> > + if (ret == -EEXIST) {
> > kfree(zone);
> > ^^^^^^^^^^^
> > Freed here.
> >
> > - looped = 1;
> > - goto again;
> > + ret = radix_tree_gang_lookup(&dev->reada_zones, (void **)&zone,
> > ^^^^
> > Use after free inside radix_tree_gang_lookup() function.
>
> It's not used by radix_tree_gang_lookup, the second parameter is
> a pointer to the return value.
>
Uh... I just glanced at it the first time through, but looking but
look carefully, we're not using freed memory, we're scribbling over
stack memory. Eep!
990 unsigned int
991 radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
992 unsigned long first_index, unsigned int max_items)
993 {
994 struct radix_tree_iter iter;
995 void **slot;
996 unsigned int ret = 0;
997
998 if (unlikely(!max_items))
999 return 0;
1000
1001 radix_tree_for_each_slot(slot, root, &iter, first_index) {
1002 results[ret] = indirect_to_ptr(rcu_dereference_raw(*slot));
^^^^^^^^^^^^
The bug is right here. The first time through we write to
*(&zone + 0) which is just "zone = indirect_to_ptr();" and that's
fine. But the second time we write to "*(&zone + 1)" which
corrupts the cache pointer from the reada_find_zone() function.
My guess is that the normal use case is to only loop through here
one time or we would have caught this in testing. All the uses in
fs/btrfs/reada.c have same memory corruption issue.
1003 if (!results[ret])
1004 continue;
1005 if (++ret == max_items)
1006 break;
1007 }
1008
1009 return ret;
1010 }
regards,
dan carpenter
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