7.2-stable review patch. If anyone has any objections, please let me know. ------------------ From: Christian Brauner commit c17b44266a1f9fcb49b894d4611d1254bf1aa313 upstream. ext4_get_link() follows a symlink that keeps its target in a data block in rcu pathwalk mode with ext4_getblk(EXT4_GET_BLOCKS_CACHED_NOWAIT). That reads s_mount_state in the ASSERT() of ext4_getblk() and again in ext4_es_lookup_extent(), and the lookup bumps the extent status hit or miss counter, all through sb->s_fs_info. ext4_put_super() destroys those counters, clears sb->s_fs_info and frees the sbi without waiting for a grace period. A pathwalk that is still in ext4_get_link() when the last reference to the mount goes away then dereferences a NULL s_fs_info, the freed sbi or the destroyed percpu counters. With a pathwalk held up in ext4_get_link() while the last reference goes away: KASAN: null-ptr-deref in range [0x00000000000000a8-0x00000000000000af] RIP: 0010:ext4_getblk+0xf0/0x640 Call Trace: ext4_get_link+0x26c/0x410 pick_link+0xf96/0x1780 step_into_slowpath+0x9b8/0x1250 path_lookupat+0x484/0xb00 filename_lookup+0x1f1/0x5b0 vfs_statx+0xd7/0x3b0 vfs_fstatat+0x4d/0x90 __do_sys_newfstatat+0xa0/0x100 do_syscall_64+0x10e/0x5e0 entry_SYSCALL_64_after_hwframe+0x77/0x7f So far that has been hard to hit. umount(2) waits for a grace period in namespace_unlock() before it drops the mount, so only a mount whose last reference goes away elsewhere, like the close of a file or a chdir out of a lazily unmounted directory, can race a walker. Keep sb->s_fs_info and free the sbi together with the extent status counters after a grace period. Split the counters out of ext4_es_unregister_shrinker() for that. The shrinker still gets wasted in ext4_put_super(), the counters go with the sbi. Fixes: 6493792d3299 ("ext4: convert symlink external data block mapping to bdev") Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20261009-work-fixes-rcu-v1-1-7446d2d335e6@kernel.org Signed-off-by: Christian Brauner (Amutable) Signed-off-by: Greg Kroah-Hartman --- fs/ext4/ext4.h | 2 ++ fs/ext4/extents-test.c | 2 ++ fs/ext4/extents.c | 1 + fs/ext4/extents_status.c | 7 ++++++- fs/ext4/extents_status.h | 1 + fs/ext4/super.c | 13 +++++++++++-- 6 files changed, 23 insertions(+), 3 deletions(-) --- a/fs/ext4/ext4.h +++ b/fs/ext4/ext4.h @@ -1768,6 +1768,8 @@ struct ext4_sb_info { struct list_head s_es_list; /* List of inodes with reclaimable extents */ long s_es_nr_inode; struct ext4_es_stats s_es_stats; + /* ext4_get_link() reads the sbi in rcu pathwalk */ + struct rcu_head s_rcu; struct mb_cache *s_ea_block_cache; struct mb_cache *s_ea_inode_cache; spinlock_t s_es_lock ____cacheline_aligned_in_smp; --- a/fs/ext4/extents-test.c +++ b/fs/ext4/extents-test.c @@ -156,6 +156,7 @@ static void extents_kunit_exit(struct ku sbi = k_ctx.k_ei->vfs_inode.i_sb->s_fs_info; ext4_es_unregister_shrinker(sbi); + ext4_es_destroy_stats(sbi); deactivate_super(sbi->s_sb); kfree(sbi); kfree(k_ctx.k_ei); @@ -345,6 +346,7 @@ out: k_ctx.k_data = NULL; ext4_es_unregister_shrinker(sbi); + ext4_es_destroy_stats(sbi); out_deactivate: deactivate_locked_super(sb); kfree(sbi); --- a/fs/ext4/extents.c +++ b/fs/ext4/extents.c @@ -6297,6 +6297,7 @@ EXPORT_SYMBOL_FOR_EXT4_TEST(__ext4_ext_d EXPORT_SYMBOL_FOR_EXT4_TEST(ext4_ext_zeroout); EXPORT_SYMBOL_FOR_EXT4_TEST(ext4_es_register_shrinker); EXPORT_SYMBOL_FOR_EXT4_TEST(ext4_es_unregister_shrinker); +EXPORT_SYMBOL_FOR_EXT4_TEST(ext4_es_destroy_stats); EXPORT_SYMBOL_FOR_EXT4_TEST(ext4_map_create_blocks); EXPORT_SYMBOL_FOR_EXT4_TEST(ext4_es_init_tree); EXPORT_SYMBOL_FOR_EXT4_TEST(ext4_es_lookup_extent); --- a/fs/ext4/extents_status.c +++ b/fs/ext4/extents_status.c @@ -1882,11 +1882,16 @@ err1: void ext4_es_unregister_shrinker(struct ext4_sb_info *sbi) { + shrinker_free(sbi->s_es_shrinker); +} + +/* ext4_get_link() bumps the hit and miss counters in rcu pathwalk */ +void ext4_es_destroy_stats(struct ext4_sb_info *sbi) +{ percpu_counter_destroy(&sbi->s_es_stats.es_stats_cache_hits); percpu_counter_destroy(&sbi->s_es_stats.es_stats_cache_misses); percpu_counter_destroy(&sbi->s_es_stats.es_stats_all_cnt); percpu_counter_destroy(&sbi->s_es_stats.es_stats_shk_cnt); - shrinker_free(sbi->s_es_shrinker); } /* --- a/fs/ext4/extents_status.h +++ b/fs/ext4/extents_status.h @@ -238,6 +238,7 @@ static inline void ext4_es_store_pblock_ extern int ext4_es_register_shrinker(struct ext4_sb_info *sbi); extern void ext4_es_unregister_shrinker(struct ext4_sb_info *sbi); +extern void ext4_es_destroy_stats(struct ext4_sb_info *sbi); extern int ext4_seq_es_shrinker_info_show(struct seq_file *seq, void *v); --- a/fs/ext4/super.c +++ b/fs/ext4/super.c @@ -1279,6 +1279,14 @@ static void ext4_flex_groups_free(struct } } +static void ext4_put_sbi_rcu(struct rcu_head *head) +{ + struct ext4_sb_info *sbi = container_of(head, struct ext4_sb_info, s_rcu); + + ext4_es_destroy_stats(sbi); + kfree(sbi); +} + static void ext4_put_super(struct super_block *sb) { struct ext4_sb_info *sbi = EXT4_SB(sb); @@ -1372,7 +1380,6 @@ static void ext4_put_super(struct super_ ext4_stop_mmpd(sbi); brelse(sbi->s_sbh); - sb->s_fs_info = NULL; /* * Now that we are completely done shutting down the * superblock, we need to actually destroy the kobject. @@ -1385,7 +1392,8 @@ static void ext4_put_super(struct super_ #if IS_ENABLED(CONFIG_UNICODE) utf8_unload(sb->s_encoding); #endif - kfree(sbi); + /* ext4_get_link() reads the sbi and the counters in rcu pathwalk */ + call_rcu(&sbi->s_rcu, ext4_put_sbi_rcu); } static struct kmem_cache *ext4_inode_cachep; @@ -5778,6 +5786,7 @@ failed_mount_wq: } failed_mount3a: ext4_es_unregister_shrinker(sbi); + ext4_es_destroy_stats(sbi); failed_mount3: /* flush s_sb_upd_work before sbi destroy */ flush_work(&sbi->s_sb_upd_work);