[PATCH] dcache: warn when a dentry is freed with a non-empty ->d_lru

[Jeff Layton <jlayton@kernel.org>]

We've had a number of panics that seem to occur on hosts with heavy
process churn. The symptoms are a panic when invalidating /proc entries
as a task is exiting:

queued_spin_lock_slowpath+0x153/0x270
shrink_dentry_list+0x11d/0x220
shrink_dcache_parent+0x68/0x110
d_invalidate+0x90/0x170
proc_invalidate_siblings_dcache+0xc8/0x140
release_task+0x41b/0x510
do_exit+0x3d8/0x9d0
do_group_exit+0x7d/0xa0
get_signal+0x2a9/0x6a0
arch_do_signal_or_restart+0x1a/0x1c0
syscall_exit_to_user_mode+0xe6/0x1c0
do_syscall_64+0x74/0x130
entry_SYSCALL_64_after_hwframe+0x4b/0x53

The problem appears to be a UAF. It's freeing a shrink list of
dentries, but one of the dentries on it has already been freed.

The d_lru field is always list_del_init()'ed, and so should be empty
whenever a dentry is freed. Add a WARN_ON_ONCE() whenever it isn't.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
We've had some of these panics internally for a while. Additionally,
Claude also noted that these syzbot reports may be related:

    https://syzbot.org/bug?extid=0aee5e8066eddbbe7397
    https://syzbot.org/bug?extid=e8b3520b53e78e90034e
    https://syzbot.org/bug?extid=ad14fd37e76c579511d0

So far, I've been unable to spot the bug. Hoping this will make it
easier.
---
 fs/dcache.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/dcache.c b/fs/dcache.c
index 7ba1801d8132..c6f475d940e3 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -429,6 +429,7 @@ static inline void __d_clear_type_and_inode(struct dentry *dentry)
 static void dentry_free(struct dentry *dentry)
 {
 	WARN_ON(!hlist_unhashed(&dentry->d_u.d_alias));
+	WARN_ON_ONCE(!list_empty(&dentry->d_lru));
 	if (unlikely(dname_external(dentry))) {
 		struct external_name *p = external_name(dentry);
 		if (likely(atomic_dec_and_test(&p->count))) {

---
base-commit: d8a9a4b11a137909e306e50346148fc5c3b63f9d
change-id: 20260403-dcache-warn-a493b0e3c877

Best regards,
-- 
Jeff Layton <jlayton@kernel.org>