From 46b0408d527e682aa5a5ab4f7c01ef5f281ca20f Mon Sep 17 00:00:00 2001 From: Ross Zwisler Date: Sat, 10 Sep 2016 20:34:19 +1000 Subject: [PATCH] radix-tree: 'slot' can be NULL in radix_tree_next_slot() There are four cases I can see where we could end up with a NULL 'slot' in radix_tree_next_slot(). Yet radix_tree_next_slot() never actually checks whether 'slot' is NULL. It just happens that for the cases where 'slot' is NULL, some other combination of factors prevents us from dereferencing it. It would be very easy for someone to unwittingly change one of these factors without realizing that we are implicitly depending on it to save us from a NULL pointer dereference. Add a comment documenting the things that allow 'slot' to be safely passed as NULL to radix_tree_next_slot(). Here are details on the four cases: 1) radix_tree_iter_retry() via a non-tagged iteration like radix_tree_for_each_slot(). In this case we currently aren't seeing a bug because radix_tree_iter_retry() sets iter->next_index = iter->index; which means that in in the else case in radix_tree_next_slot(), 'count' is zero, so we skip over the while() loop and effectively just return NULL without ever dereferencing 'slot'. 2) radix_tree_iter_retry() via tagged iteration like radix_tree_for_each_tagged(). This case was giving us NULL pointer dereferences in testing, and was fixed with this commit: commit 3cb9185c6730 ("radix-tree: fix radix_tree_iter_retry() for tagged iterators.") This fix doesn't explicitly check for 'slot' being NULL, though, it works around the NULL pointer dereference by instead zeroing iter->tags in radix_tree_iter_retry(), which makes us bail out of the if() case in radix_tree_next_slot() before we dereference 'slot'. 3) radix_tree_iter_next() via via a non-tagged iteration like radix_tree_for_each_slot(). This currently happens in shmem_tag_pins() and shmem_partial_swap_usage(). As with non-tagged iteration, 'count' in the else case of radix_tree_next_slot() is zero, so we skip over the while() loop and effectively just return NULL without ever dereferencing 'slot'. 4) radix_tree_iter_next() via tagged iteration like radix_tree_for_each_tagged(). This happens in shmem_wait_for_pins(). radix_tree_iter_next() zeros out iter->tags, so we end up exiting radix_tree_next_slot() here: if (flags & RADIX_TREE_ITER_TAGGED) { void *canon = slot; iter->tags >>= 1; if (unlikely(!iter->tags)) return NULL; Link: http://lkml.kernel.org/r/20160815194237.25967-2-ross.zwisler@linux.intel.com Signed-off-by: Ross Zwisler Cc: Konstantin Khlebnikov Cc: Andrey Ryabinin Cc: Dmitry Vyukov Cc: Shuah Khan Signed-off-by: Andrew Morton --- include/linux/radix-tree.h | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h index 4c45105dece3..4613bf35c311 100644 --- a/include/linux/radix-tree.h +++ b/include/linux/radix-tree.h @@ -461,6 +461,14 @@ static inline struct radix_tree_node *entry_to_node(void *ptr) * * This function updates @iter->index in the case of a successful lookup. * For tagged lookup it also eats @iter->tags. + * + * There are several cases where 'slot' can be passed in as NULL to this + * function. These cases result from the use of radix_tree_iter_next() or + * radix_tree_iter_retry(). In these cases we don't end up dereferencing + * 'slot' because either: + * a) we are doing tagged iteration and iter->tags has been set to 0, or + * b) we are doing non-tagged iteration, and iter->index and iter->next_index + * have been set up so that radix_tree_chunk_size() returns 1 or 0. */ static __always_inline void ** radix_tree_next_slot(void **slot, struct radix_tree_iter *iter, unsigned flags) -- 2.34.1