Re: [PATCH 1/5] radix-tree: Fix race in gang lookup

[Konstantin Khlebnikov]

On Thu, Feb 4, 2016 at 12:37 AM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote:
> On Thu, Jan 28, 2016 at 12:17 AM, Matthew Wilcox
> <matthew.r.wilcox@xxxxxxxxx> wrote:
>> From: Matthew Wilcox <willy@xxxxxxxxxxxxxxx>
>>
>> If the indirect_ptr bit is set on a slot, that indicates we need to
>> redo the lookup.  Introduce a new function radix_tree_iter_retry()
>> which forces the loop to retry the lookup by setting 'slot' to NULL and
>> turning the iterator back to point at the problematic entry.
>>
>> This is a pretty rare problem to hit at the moment; the lookup has to
>> race with a grow of the radix tree from a height of 0.  The consequences
>> of hitting this race are that gang lookup could return a pointer to a
>> radix_tree_node instead of a pointer to whatever the user had inserted
>> in the tree.
>>
>> Fixes: cebbd29e1c2f ("radix-tree: rewrite gang lookup using iterator")
>> Signed-off-by: Matthew Wilcox <willy@xxxxxxxxxxxxxxx>
>> Cc: stable@xxxxxxxxxxxxxxx
>> ---
>>  include/linux/radix-tree.h | 16 ++++++++++++++++
>>  lib/radix-tree.c           | 12 ++++++++++--
>>  2 files changed, 26 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
>> index f9a3da5bf892..db0ed595749b 100644
>> --- a/include/linux/radix-tree.h
>> +++ b/include/linux/radix-tree.h
>> @@ -387,6 +387,22 @@ void **radix_tree_next_chunk(struct radix_tree_root *root,
>>                              struct radix_tree_iter *iter, unsigned flags);
>>
>>  /**
>> + * radix_tree_iter_retry - retry this chunk of the iteration
>> + * @iter:      iterator state
>> + *
>> + * If we iterate over a tree protected only by the RCU lock, a race
>> + * against deletion or creation may result in seeing a slot for which
>> + * radix_tree_deref_retry() returns true.  If so, call this function
>> + * and continue the iteration.
>> + */
>> +static inline __must_check
>> +void **radix_tree_iter_retry(struct radix_tree_iter *iter)
>> +{
>> +       iter->next_index = iter->index;
>> +       return NULL;
>> +}
>> +
>> +/**
>>   * radix_tree_chunk_size - get current chunk size
>>   *
>>   * @iter:      pointer to radix tree iterator
>> diff --git a/lib/radix-tree.c b/lib/radix-tree.c
>> index a25f635dcc56..65422ac17114 100644
>> --- a/lib/radix-tree.c
>> +++ b/lib/radix-tree.c
>> @@ -1105,9 +1105,13 @@ radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
>>                 return 0;
>>
>>         radix_tree_for_each_slot(slot, root, &iter, first_index) {
>> -               results[ret] = indirect_to_ptr(rcu_dereference_raw(*slot));
>> +               results[ret] = rcu_dereference_raw(*slot);
>>                 if (!results[ret])
>>                         continue;
>> +               if (radix_tree_is_indirect_ptr(results[ret])) {
>> +                       slot = radix_tree_iter_retry(&iter);
>> +                       continue;
>> +               }
>>                 if (++ret == max_items)
>>                         break;
>>         }
>
> Looks like your fix doesn't work.
>
> After radix_tree_iter_retry: radix_tree_for_each_slot will call
> radix_tree_next_slot which isn't safe to call for NULL slot.
>
> #define radix_tree_for_each_slot(slot, root, iter, start) \
> for (slot = radix_tree_iter_init(iter, start) ; \
>     slot || (slot = radix_tree_next_chunk(root, iter, 0)) ; \
>     slot = radix_tree_next_slot(slot, iter, 0))
>
> tagged iterator works becase restart happens only at root - tags
> filled with single bit.
>
> quick (untested) fix for that
>
> --- a/include/linux/radix-tree.h
> +++ b/include/linux/radix-tree.h
> @@ -457,9 +457,9 @@ radix_tree_next_slot(void **slot, struct
> radix_tree_iter *iter, unsigned flags)
>                         return slot + offset + 1;
>                 }
>         } else {
> -               unsigned size = radix_tree_chunk_size(iter) - 1;
> +               int size = radix_tree_chunk_size(iter) - 1;
>
> -               while (size--) {
> +               while (size-- > 0) {
>                         slot++;
>                         iter->index++;
>                         if (likely(*slot))
>
>

Yep. Kernel crashes. Test in attachment.

fix: 145457528789.31321.4441662473067711123.stgit@zurg">https://lkml.kernel.org/r/145457528789.31321.4441662473067711123.stgit@zurg

>> @@ -1184,9 +1188,13 @@ radix_tree_gang_lookup_tag(struct radix_tree_root *root, void **results,
>>                 return 0;
>>
>>         radix_tree_for_each_tagged(slot, root, &iter, first_index, tag) {
>> -               results[ret] = indirect_to_ptr(rcu_dereference_raw(*slot));
>> +               results[ret] = rcu_dereference_raw(*slot);
>>                 if (!results[ret])
>>                         continue;
>> +               if (radix_tree_is_indirect_ptr(results[ret])) {
>> +                       slot = radix_tree_iter_retry(&iter);
>> +                       continue;
>> +               }
>>                 if (++ret == max_items)
>>                         break;
>>         }
>> --
>> 2.7.0.rc3
>>
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