Re: [PATCH hotfix 6.12 v3 1/2] maple_tree: correct tree corruption on spanning store

[Wei Yang]

On Mon, Oct 07, 2024 at 04:28:32PM +0100, Lorenzo Stoakes wrote:
>There has been a subtle bug present in the maple tree implementation from
>its inception.
>
>This arises from how stores are performed - when a store occurs, it will
>overwrite overlapping ranges and adjust the tree as necessary to
>accommodate this.
>
>A range may always ultimately span two leaf nodes. In this instance we walk
>the two leaf nodes, determine which elements are not overwritten to the
>left and to the right of the start and end of the ranges respectively and
>then rebalance the tree to contain these entries and the newly inserted
>one.
>
>This kind of store is dubbed a 'spanning store' and is implemented by
>mas_wr_spanning_store().
>
>In order to reach this stage, mas_store_gfp() invokes mas_wr_preallocate(),
>mas_wr_store_type() and mas_wr_walk() in turn to walk the tree and update
>the object (mas) to traverse to the location where the write should be
>performed, determining its store type.
>
>When a spanning store is required, this function returns false stopping at
>the parent node which contains the target range, and mas_wr_store_type()
>marks the mas->store_type as wr_spanning_store to denote this fact.
>
>When we go to perform the store in mas_wr_spanning_store(), we first
>determine the elements AFTER the END of the range we wish to store (that
>is, to the right of the entry to be inserted) - we do this by walking to
>the NEXT pivot in the tree (i.e. r_mas.last + 1), starting at the node we
>have just determined contains the range over which we intend to write.
>
>We then turn our attention to the entries to the left of the entry we are
>inserting, whose state is represented by l_mas, and copy these into a 'big
>node', which is a special node which contains enough slots to contain two
>leaf node's worth of data.
>
>We then copy the entry we wish to store immediately after this - the copy
>and the insertion of the new entry is performed by mas_store_b_node().
>
>After this we copy the elements to the right of the end of the range which
>we are inserting, if we have not exceeded the length of the node
>(i.e. r_mas.offset <= r_mas.end).
>
>Herein lies the bug - under very specific circumstances, this logic can
>break and corrupt the maple tree.
>
>Consider the following tree:
>
>Height
>  0                             Root Node
>                                 /      \
>                 pivot = 0xffff /        \ pivot = ULONG_MAX
>                               /          \
>  1                       A [-----]       ...
>                             /   \
>             pivot = 0x4fff /     \ pivot = 0xffff
>                           /       \
>  2 (LEAVES)          B [-----]  [-----] C
>                                      ^--- Last pivot 0xffff.
>
>Now imagine we wish to store an entry in the range [0x4000, 0xffff] (note
>that all ranges expressed in maple tree code are inclusive):
>
>1. mas_store_gfp() descends the tree, finds node A at <=0xffff, then
>   determines that this is a spanning store across nodes B and C. The mas
>   state is set such that the current node from which we traverse further
>   is node A.
>
>2. In mas_wr_spanning_store() we try to find elements to the right of pivot
>   0xffff by searching for an index of 0x10000:
>
>    - mas_wr_walk_index() invokes mas_wr_walk_descend() and
>      mas_wr_node_walk() in turn.
>
>        - mas_wr_node_walk() loops over entries in node A until EITHER it
>          finds an entry whose pivot equals or exceeds 0x10000 OR it
>          reaches the final entry.
>
>        - Since no entry has a pivot equal to or exceeding 0x10000, pivot
>          0xffff is selected, leading to node C.
>
>    - mas_wr_walk_traverse() resets the mas state to traverse node C. We
>      loop around and invoke mas_wr_walk_descend() and mas_wr_node_walk()
>      in turn once again.
>
>         - Again, we reach the last entry in node C, which has a pivot of
>           0xffff.
>
>3. We then copy the elements to the left of 0x4000 in node B to the big
>   node via mas_store_b_node(), and insert the new [0x4000, 0xffff] entry
>   too.
>
>4. We determine whether we have any entries to copy from the right of the
>   end of the range via - and with r_mas set up at the entry at pivot
>   0xffff, r_mas.offset <= r_mas.end, and then we DUPLICATE the entry at
>   pivot 0xffff.
>
>5. BUG! The maple tree is corrupted with a duplicate entry.
>
>This requires a very specific set of circumstances - we must be spanning
>the last element in a leaf node, which is the last element in the parent
>node.
>
>spanning store across two leaf nodes with a range that ends at that shared
>pivot.
>
>A potential solution to this problem would simply be to reset the walk each
>time we traverse r_mas, however given the rarity of this situation it seems
>that would be rather inefficient.
>
>Instead, this patch detects if the right hand node is populated, i.e. has
>anything we need to copy.
>
>We do so by only copying elements from the right of the entry being inserted
>when the maximum value present exceeds the last, rather than basing this on
>offset position.
>
>The patch also updates some comments and eliminates the unused bool return
>value in mas_wr_walk_index().
>
>The work performed in commit f8d112a4e657 ("mm/mmap: avoid zeroing vma tree
>in mmap_region()") seems to have made the probability of this event much
>more likely, which is the point at which reports started to be submitted
>concerning this bug.
>
>The motivation for this change arose from Bert Karwatzki's report of
>encountering mm instability after the release of kernel v6.12-rc1 which,
>after the use of CONFIG_DEBUG_VM_MAPLE_TREE and similar configuration
>options, was identified as maple tree corruption.
>
>After Bert very generously provided his time and ability to reproduce this
>event consistently, I was able to finally identify that the issue discussed
>in this commit message was occurring for him.
>
>Reported-and-tested-by: Bert Karwatzki <spasswolf@xxxxxx>
>Closes: https://lore.kernel.org/all/20241001023402.3374-1-spasswolf@xxxxxx/
>Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>
>Closes: https://lore.kernel.org/all/CABXGCsOPwuoNOqSMmAvWO2Fz4TEmPnjFj-b7iF+XFRu1h7-+Dg@xxxxxxxxxxxxxx/
>Fixes: 54a611b60590 ("Maple Tree: add new data structure")
>Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
>Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Reviewed-by: Wei Yang <richard.weiyang@xxxxxxxxx>

-- 
Wei Yang
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