{"id":"CVE-2021-47600","state":"PUBLISHED","title":"dm btree remove: fix use after free in rebalance_children()","description":"In the Linux kernel, the following vulnerability has been resolved:\n\ndm btree remove: fix use after free in rebalance_children()\n\nMove dm_tm_unlock() after dm_tm_dec().","publishedAt":"2024-06-19T14:54:00.981Z","updatedAt":"2026-08-05T08:48:32.894Z","severity":{"score":7.8,"label":"High","version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","source":"Linux"},"assigner":"Linux","products":["Linux 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btree remove: fix use after free in rebalance_children()","descriptions":[{"lang":"en","value":"In the Linux kernel, the following vulnerability has been resolved:\n\ndm btree remove: fix use after free in rebalance_children()\n\nMove dm_tm_unlock() after 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- The UAF is in dm-btree rebalance_children(), reached via local dm-thin metadata updates (discard → dm_thin_remove_range → dm_btree_remove/remove_leaves) or CAP_SYS_ADMIN dm-ioctl pool messages, not via network or adjacent protocols.\nAC:L - An attacker who can discard/unmap on a thin volume fully drives btree shrinkage into the single-child collapse path where unlock precedes dm_block_location(); no conditions outside attacker control are required, and UAFs are AC:L per guidance.\nPR:L - On hosts already using dm-thin/LVM thin pools, an unprivileged user with access to a thin-backed filesystem or device can issue discards (e.g. deletes under discard mounts or BLKDISCARD) that hit this path; creating the pool needs CAP_SYS_ADMIN but is a deployment precondition.\nUI:N - The attacker triggers the vulnerable remove/rebalance path themselves through their own I/O or ioctls; no separate victim action is required.\nS:U - This is a host-kernel heap use-after-free within the same OS security authority and does not cross a VM, IOMMU, or sandbox boundary.\nC:H - Unlocking the child block then reading dm_block_location(child) is a use-after-free on the bufio buffer; reclaim/spray of that object enables arbitrary kernel memory disclosure.\nI:H - The UAF can feed an attacker-controlled block number into dm_tm_dec(), corrupting the space map and yielding heap/metadata corruption primitives exploitable for arbitrary write and control-flow hijack.\nA:H - Any use-after-free can oops/panic the kernel (KASAN UAF or invalid access after reclaim), fully impacting availability even without successful code 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