syzbot reported: BUG: KASAN: slab-use-after-free in config_item_put+0x26/0x90 fs/configfs/item.c:149 Call Trace: ... config_item_put+0x26/0x90 fs/configfs/item.c:149 configfs_symlink+0x6be/0x11d0 fs/configfs/symlink.c:225 vfs_symlink+0x18b/0x330 fs/namei.c:5823 filename_symlinkat+0x1cd/0x420 fs/namei.c:5848 Allocated by task 5712: ... nvmet_ports_make+0xe6/0xf00 drivers/nvme/target/configfs.c:2054 configfs_mkdir+0x4f6/0x9e0 fs/configfs/dir.c:1369 vfs_mkdir+0x408/0x620 fs/namei.c:5439 filename_mkdirat+0x289/0x520 fs/namei.c:5472 Freed by task 5712: ... config_item_cleanup fs/configfs/item.c:128 [inline] config_item_release+0x13a/0x2d0 fs/configfs/item.c:137 configfs_rmdir+0x885/0x950 fs/configfs/dir.c:1580 vfs_rmdir+0x3e9/0x6b0 fs/namei.c:5544 filename_rmdir+0x292/0x520 fs/namei.c:5601 configfs_symlink() takes a reference on parent_item, drops the directory inode lock around get_target(), and puts the reference at the end. If a concurrent rmdir removes the group in that window, and parent_item is a default group embedded in a larger object (e.g. nvmet_port.subsys_group), the reference does not keep the containing object alive. The object is freed, and the final config_item_put(parent_item) hits freed memory. The syzbot reproducer hits this by racing symlinkat() into ports/1/subsystems against rmdir of ports/1. Fix this by not holding parent_item across the unlocked window: put it before inode_unlock(), and look it up again from dentry->d_parent after inode_lock(), returning -ENOENT if the lookup fails. That is not enough on its own. configfs_get_config_item() only fails if the parent dentry was unhashed, but if simple_rmdir() fails, rmdir skips d_drop() and the dentry stays hashed even though the directory is already detached. Default groups are detached through configfs_detach_group(), which marks children S_DEAD, so also check IS_DEADDIR(dir) after retaking the lock and return -ENOENT. Reported-by: syzbot+63336d70e1af377d7388@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=63336d70e1af377d7388 Tested-by: syzbot+63336d70e1af377d7388@syzkaller.appspotmail.com Signed-off-by: Drif Abdelmalek Mohamed Said --- v3: - Removed redundant null assignment to parent_item - Simplified commit body to not rely heavily on repro explain bug and fix logic v2: https://lore.kernel.org/all/20261023162906.2938-1-drifabdelmalekmohamedsaid@gmail.com/T/ v1: https://lore.kernel.org/all/20261017123748.4430-1-drifabdelmalekmohamedsaid@gmail.com/T/ fs/configfs/symlink.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/fs/configfs/symlink.c b/fs/configfs/symlink.c index 89178f5d371a..62d13ce5b46a 100644 --- a/fs/configfs/symlink.c +++ b/fs/configfs/symlink.c @@ -196,12 +196,26 @@ int configfs_symlink(const struct mnt_idmap *idmap, struct inode *dir, * * AV, a thoroughly annoyed bastard. */ + + /* Put the reference back, retake and check after relocking */ + config_item_put(parent_item); inode_unlock(dir); ret = get_target(symname, &target_item, &target_sd, dentry->d_sb); inode_lock(dir); if (ret) goto out_put; + if (IS_DEADDIR(dir)) { + ret = -ENOENT; + goto out_put_target; + } + + parent_item = configfs_get_config_item(dentry->d_parent); + if (!parent_item) { + ret = -ENOENT; + goto out_put_target; + } + if (dentry->d_inode || d_unhashed(dentry)) ret = -EEXIST; else @@ -218,6 +232,7 @@ int configfs_symlink(const struct mnt_idmap *idmap, struct inode *dir, target_item); } +out_put_target: configfs_put(target_sd); config_item_put(target_item); -- 2.43.0