In the Linux kernel, the following vulnerability has been resolved:
ext4: fix rbtree traversal bug in ext4_mb_use_preallocated
During allocations, while looking for preallocations(PA) in the per inode rbtree, we can't do a direct traversal of the tree because ext4_mb_discard_group_preallocation() can paralelly mark the pa deleted and that can cause direct traversal to skip some entries. This was leading to a BUG_ON() being hit [1] when we missed a PA that could satisfy our request and ultimately tried to create a new PA that would overlap with the missed one.
To makes sure we handle that case while still keeping the performance of the rbtree, we make use of the fact that the only pa that could possibly overlap the original goal start is the one that satisfies the below conditions:
- It must have it's logical start immediately to the left of
- It must not be deleted
To find this pa we use the following traversal method:
- Descend into the rbtree normally to find the immediate neighboring
- If the found PA is on right of original goal, use rb_prev() to find
- Check if this PA is deleted and keep moving left with rb_prev() until
- This is the PA we are looking for. Now we can check if it can satisfy
This approach also takes care of having deleted PAs in the tree.
(While we are at it, also fix a possible overflow bug in calculating the end of a PA)
[1] https://lore.kernel.org/linux-ext4/CA+G9fYv2FRpLqBZf34ZinR8bU2_ZRAUOjKAD3+tKRFaEQHtt8Q@mail.gmail.com/