From: Ross Zwisler ross.zwisler@linux.intel.com Subject: radix tree: fix multi-order iteration race

Fix a race in the multi-order iteration code which causes the kernel to hit a GP fault. This was first seen with a production v4.15 based kernel (4.15.6-300.fc27.x86_64) utilizing a DAX workload which used order 9 PMD DAX entries.

The race has to do with how we tear down multi-order sibling entries when we are removing an item from the tree. Remember for example that an order 2 entry looks like this:

struct radix_tree_node.slots[] = [entry][sibling][sibling][sibling]

where 'entry' is in some slot in the struct radix_tree_node, and the three slots following 'entry' contain sibling pointers which point back to 'entry.'

When we delete 'entry' from the tree, we call : radix_tree_delete() radix_tree_delete_item() __radix_tree_delete() replace_slot()

replace_slot() first removes the siblings in order from the first to the last, then at then replaces 'entry' with NULL. This means that for a brief period of time we end up with one or more of the siblings removed, so:

struct radix_tree_node.slots[] = [entry][NULL][sibling][sibling]

This causes an issue if you have a reader iterating over the slots in the tree via radix_tree_for_each_slot() while only under rcu_read_lock()/rcu_read_unlock() protection. This is a common case in mm/filemap.c.

The issue is that when __radix_tree_next_slot() => skip_siblings() tries to skip over the sibling entries in the slots, it currently does so with an exact match on the slot directly preceding our current slot. Normally this works:

V preceding slot struct radix_tree_node.slots[] = [entry][sibling][sibling][sibling] ^ current slot

This lets you find the first sibling, and you skip them all in order.

But in the case where one of the siblings is NULL, that slot is skipped and then our sibling detection is interrupted:

V preceding slot struct radix_tree_node.slots[] = [entry][NULL][sibling][sibling] ^ current slot

This means that the sibling pointers aren't recognized since they point all the way back to 'entry', so we think that they are normal internal radix tree pointers. This causes us to think we need to walk down to a struct radix_tree_node starting at the address of 'entry'.

In a real running kernel this will crash the thread with a GP fault when you try and dereference the slots in your broken node starting at 'entry'.

We fix this race by fixing the way that skip_siblings() detects sibling nodes. Instead of testing against the preceding slot we instead look for siblings via is_sibling_entry() which compares against the position of the struct radix_tree_node.slots[] array. This ensures that sibling entries are properly identified, even if they are no longer contiguous with the 'entry' they point to.

Link: http://lkml.kernel.org/r/20180503192430.7582-6-ross.zwisler@linux.intel.com Fixes: 148deab223b2 ("radix-tree: improve multiorder iterators") Signed-off-by: Ross Zwisler ross.zwisler@linux.intel.com Reported-by: CR, Sapthagirish sapthagirish.cr@intel.com Reviewed-by: Jan Kara jack@suse.cz Cc: Matthew Wilcox willy@infradead.org Cc: Christoph Hellwig hch@lst.de Cc: Dan Williams dan.j.williams@intel.com Cc: Dave Chinner david@fromorbit.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

lib/radix-tree.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-)

diff -puN lib/radix-tree.c~radix-tree-fix-multi-order-iteration-race lib/radix-tree.c --- a/lib/radix-tree.c~radix-tree-fix-multi-order-iteration-race +++ a/lib/radix-tree.c @@ -1612,11 +1612,9 @@ static void set_iter_tags(struct radix_t static void __rcu **skip_siblings(struct radix_tree_node **nodep, void __rcu **slot, struct radix_tree_iter *iter) { - void *sib = node_to_entry(slot - 1); - while (iter->index < iter->next_index) { *nodep = rcu_dereference_raw(*slot); - if (*nodep && *nodep != sib) + if (*nodep && !is_sibling_entry(iter->node, *nodep)) return slot; slot++; iter->index = __radix_tree_iter_add(iter, 1); @@ -1631,7 +1629,7 @@ void __rcu **__radix_tree_next_slot(void struct radix_tree_iter *iter, unsigned flags) { unsigned tag = flags & RADIX_TREE_ITER_TAG_MASK; - struct radix_tree_node *node = rcu_dereference_raw(*slot); + struct radix_tree_node *node;

slot = skip_siblings(&node, slot, iter);

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