FAILED: patch "[PATCH] configfs: unhash the dentry before dropping the item in rmdir" failed to apply to 6.18-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x f06c2d26d1999d37e93299db0ecead04ca7d0b9f
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026091656-showman-zit-60ac@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From f06c2d26d1999d37e93299db0ecead04ca7d0b9f Mon Sep 17 00:00:00 2001
From: Vasileios Almpanis <vasilisalmpanis@gmail.com>
Date: Thu, 30 Jul 2026 11:30:25 +0200
Subject: [PATCH] configfs: unhash the dentry before dropping the item in rmdir

configfs_get_config_item() treats a hashed dentry as proof that
sd->s_element is a live config_item.  configfs_rmdir() breaks that:
simple_rmdir() leaves the dentry hashed, the last reference to the item is
dropped right after, and the dentry is only unhashed by d_delete() once
->rmdir() has returned.  configfs_symlink() resolves its target holding no
lock on it, so get_target() can land in that window:

  BUG: KASAN: slab-use-after-free in config_item_get+0x26/0x90
   get_target fs/configfs/symlink.c:128 [inline]
   configfs_symlink+0x4ab/0x1030 fs/configfs/symlink.c:185

Unhash in configfs_remove_dir(), while the item is still guaranteed to be
there.  A reference obtained just before that stays harmless, as
create_link() rechecks CONFIGFS_USET_DROPPING, already set by
configfs_detach_prep().  Both configfs_unregister_subsystem() paths
d_drop() after detaching, so this only makes rmdir match them.

Reported-by: syzbot+6b16e3d085833cbf3e25@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6b16e3d085833cbf3e25
Fixes: 7063fbf22611 ("[PATCH] configfs: User-driven configuration filesystem")
Cc: stable@vger.kernel.org
Signed-off-by: Vasileios Almpanis <vasilisalmpanis@gmail.com>
Tested-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260730093435.195441-3-vasilisalmpanis@gmail.com
Signed-off-by: Breno Leitao <leitao@debian.org>

diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index 3c88f13f1ca2..eda80c2a2d38 100644
--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -416,6 +416,15 @@ static void configfs_remove_dir(struct dentry *d)
 	if (d_really_is_positive(d)) {
 		if (unlikely(simple_rmdir(d_inode(parent), d)))
 			pr_warn("remove_dir (%pd): attributes remain", d);
+		else
+			/*
+			 * configfs_get_config_item() takes a hashed dentry as
+			 * proof that ->s_element is still alive.  Our caller
+			 * is about to drop the last reference to the item and
+			 * the VFS will not unhash until after we return, so
+			 * unhash it here.
+			 */
+			d_drop(d);
 	}
 
 	pr_debug(" o %pd removing done (%d)\n", d, d_count(d));