CVE-2024-50200

In the Linux kernel, the following vulnerability has been resolved:

maple_tree: correct tree corruption on spanning store

Patch series "maple_tree: correct tree corruption on spanning store", v3.

There has been a nasty yet subtle maple tree corruption bug that appears to have been in existence since the inception of the algorithm.

This bug seems far more likely to happen since commit f8d112a4e657 ("mm/mmap: avoid zeroing vma tree in mmap_region()"), which is the point at which reports started to be submitted concerning this bug.

We were made definitely aware of the bug thanks to the kind efforts of Bert Karwatzki who helped enormously in my being able to track this down and identify the cause of it.

The bug arises when an attempt is made to perform a spanning store across two leaf nodes, where the right leaf node is the rightmost child of the shared parent, AND the store completely consumes the right-mode node.

This results in maswrspanning_store() mitakenly duplicating the new and existing entries at the maximum pivot within the range, and thus maple tree corruption.

The fix patch corrects this by detecting this scenario and disallowing the mistaken duplicate copy.

The fix patch commit message goes into great detail as to how this occurs.

This series also includes a test which reliably reproduces the issue, and asserts that the fix works correctly.

Bert has kindly tested the fix and confirmed it resolved his issues. Also Mikhail Gavrilov kindly reported what appears to be precisely the same bug, which this fix should also resolve.

This patch (of 2):

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 maswrspanning_store().

In order to reach this stage, masstoregfp() invokes maswrpreallocate(), maswrstoretype() and maswr_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 maswrstoretype() marks the mas->storetype as wrspanningstore to denote this fact.

When we go to perform the store in maswrspanningstore(), 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. rmas.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 masstoreb_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. rmas.offset <= rmas.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 /
---truncated---