CVE-2026-93800: In the Linux kernel, the following vulnerability has been resolved: btrfs: fix use-after-free on reloc root after error in insert_dirty_subvol() If during relocation we fail in insert_dirty_subvol() because btrfs_update_

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In the Linux kernel, the following vulnerability has been resolved: btrfs: fix use-after-free on reloc root after error in insert_dirty_subvol() If during relocation we fail in insert_dirty_subvol() because btrfs_update_reloc_root() returned an error, we will leave a root's reloc_root field pointing to a reloc root that was freed instead of NULL, resulting later in a use-after-free, or double free attempt during unmount. The sequence of steps is this: 1) During relocation the call to btrfs_update_reloc_root() in insert_dirty_subvol() fails, so insert_dirty_subvol() returns the error to merge_reloc_root() without adding the root to the list rc->dirty_subvol_roots; 2) Then merge_reloc_root() aborts the current transaction because insert_dirty_subvol() returned an error; 3) Up the call chain, merge_reloc_roots() gets the error, adds the reloc root for root X to the local reloc_roots list and jumps to the 'out' label, where it calls free_reloc_roots() to free all the reloc roots in the local reloc_roots list. This frees the reloc root for root X; 4) We go up the call chain to relocate_block_group() which calls clean_dirty_subvols() to go over dirty roots and set their ->reloc_root field to NULL, but root X is not in the dirty_subvol_roots list, so its ->reloc_root still points to a reloc root; 5) Relocation finishes, with an error and a transaction abort, but the ->reloc_root field for root X still points to the reloc root that was freed in step 3; 6) When unmounting the fs we end up calling: btrfs_free_fs_roots() btrfs_drop_and_free_fs_root() --> calls btrfs_put_root() against root X's ->reloc_root which is not NULL and points to the already freed reloc root in step 4 above Resulting in a use-after-free to a double free attempt. Syzbot reported this with the following dmes