CVE-2022-49850
nilfs2: fix deadlock in nilfs_count_free_blocks()
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nilfs2: fix deadlock in nilfs_count_free_blocks()
In the Linux kernel, the following vulnerability has been resolved: nilfs2: fix deadlock in nilfs_count_free_blocks() A semaphore deadlock can occur if nilfs_get_block() detects metadata corruption while locating data blocks and a superblock writeback occurs at the same time: task 1 task 2 ------ ------ * A file operation * nilfs_truncate() nilfs_get_block() down_read(rwsem A) <-- nilfs_bmap_lookup_contig() ... generic_shutdown_super() nilfs_put_super() * Prepare to write superblock * down_write(rwsem B) <-- nilfs_cleanup_super() * Detect b-tree corruption * nilfs_set_log_cursor() nilfs_bmap_convert_error() nilfs_count_free_blocks() __nilfs_error() down_read(rwsem A) <-- nilfs_set_error() down_write(rwsem B) <-- *** DEADLOCK *** Here, nilfs_get_block() readlocks rwsem A (= NILFS_MDT(dat_inode)->mi_sem) and then calls nilfs_bmap_lookup_contig(), but if it fails due to metadata corruption, __nilfs_error() is called from nilfs_bmap_convert_error() inside the lock section. Since __nilfs_error() calls nilfs_set_error() unless the filesystem is read-only and nilfs_set_error() attempts to writelock rwsem B (= nilfs->ns_sem) to write back superblock exclusively, hierarchical lock acquisition occurs in the order rwsem A -> rwsem B. Now, if another task starts updating the superblock, it may writelock rwsem B during the lock sequence above, and can deadlock trying to readlock rwsem A in nilfs_count_free_blocks(). However, there is actually no need to take rwsem A in nilfs_count_free_blocks() because it, within the lock section, only reads a single integer data on a shared struct with nilfs_sufile_get_ncleansegs(). This has been the case after commit aa474a220180 ("nilfs2: add local variable to cache the number of clean segments"), that is, even before this bug was introduced. So, this resolves the deadlock problem by just not taking the semaphore in nilfs_count_free_blocks().
Reserved 2025-05-01 | Published 2025-05-01 | Updated 2025-05-04 | Assigner Linuxgit.kernel.org/...c/3c89ca6d3dfa6c09c515807a7a97a521f5d5147e
git.kernel.org/...c/8b4506cff6630bb474bb46a2a75c31e533a756ba
git.kernel.org/...c/f0cc93080d4c09510b74ecba87fd778cca390bb1
git.kernel.org/...c/36ff974b0310771417c0be64b64aa221bd70d63d
git.kernel.org/...c/1d4ff73062096c21b47954d2996b4df259777bda
git.kernel.org/...c/abc082aac0d9b6b926038fc3adb7008306581be2
git.kernel.org/...c/cb029b54953420f7a2d65100f1c5107f14411bdc
git.kernel.org/...c/8ac932a4921a96ca52f61935dbba64ea87bbd5dc
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