In pick_link(), atime_needs_update() is called while the path walk may still be in RCU mode (LOOKUP_RCU set), meaning no reference is held on the inode. A concurrent iput() from another task can free the inode while atime_needs_update() reads inode->i_opflags (offset +2) inside current_time() -> is_mgtime(), causing a use-after-free. KASAN reports: BUG: KASAN: slab-use-after-free in is_mgtime include/linux/fs.h:2313 Read of size 2 at addr ffff8880407e4282 (offset +2 = i_opflags) The race: Task A (pick_link) Task B (unlinkat) ------------------ ----------------- atime_needs_update(inode) iput(inode) current_time(inode) evict() is_mgtime(inode) destroy_inode() inode->i_opflags UAF inode FREED Fix: call try_to_unlazy() before atime_needs_update() so that a proper inode reference is held before any inode fields are read. If try_to_unlazy() detects the inode is already gone, it returns false and we exit safely with -ECHILD without touching freed memory. Fixes: e45960c279b098ebc0c2 ("fs: unconditionally use atime_needs_update() in pick_link()") Reported-by: syzbot+36e50496c8ac4bcde3f9@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=36e50496c8ac4bcde3f9 Tested-by: syzbot+36e50496c8ac4bcde3f9@syzkaller.appspotmail.com Signed-off-by: Deepanshu Kartikey --- fs/namei.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/fs/namei.c b/fs/namei.c index c7fac83c9a85..eac0ec35be37 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -2037,11 +2037,11 @@ static noinline const char *pick_link(struct nameidata *nd, struct path *link, unlikely(link->mnt->mnt_flags & MNT_NOSYMFOLLOW)) return ERR_PTR(-ELOOP); + if (nd->flags & LOOKUP_RCU) { + if (!try_to_unlazy(nd)) + return ERR_PTR(-ECHILD); + } if (unlikely(atime_needs_update(&last->link, inode))) { - if (nd->flags & LOOKUP_RCU) { - if (!try_to_unlazy(nd)) - return ERR_PTR(-ECHILD); - } touch_atime(&last->link); cond_resched(); } -- 2.43.0