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Subject: [PATCH] kernfs: fix missing kernfs_iattr_rwsem locking
From:   Ian Kent <raven@themaw.net>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tejun Heo <tj@kernel.org>
Cc:     Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Imran Khan <imran.f.khan@oracle.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Anders Roxell <anders.roxell@linaro.org>,
        Minchan Kim <minchan@kernel.org>,
        Eric Sandeen <sandeen@sandeen.net>
Date:   Sun, 06 Aug 2023 09:26:49 +0800
Message-ID: <169128520941.68052.15749253469930138901.stgit@donald.themaw.net>
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When the kernfs_iattr_rwsem was introduced a case was missed.

The update of the kernfs directory node child count was also protected
by the kernfs_rwsem and needs to be included in the change so that the
child count (and so the inode n_link attribute) does not change while
holding the rwsem for read.

Fixes: 9caf696142 (kernfs: Introduce separate rwsem to protect inode
attributes)

Signed-off-by: Ian Kent <raven@themaw.net>
Reviewed-By: Imran Khan <imran.f.khan@oracle.com>
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: Anders Roxell <anders.roxell@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Eric Sandeen <sandeen@sandeen.net>
---
 fs/kernfs/dir.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 5a1a4af9d3d2..bf243015834e 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -383,9 +383,11 @@ static int kernfs_link_sibling(struct kernfs_node *kn)
 	rb_insert_color(&kn->rb, &kn->parent->dir.children);
 
 	/* successfully added, account subdir number */
+	down_write(&kernfs_root(kn)->kernfs_iattr_rwsem);
 	if (kernfs_type(kn) == KERNFS_DIR)
 		kn->parent->dir.subdirs++;
 	kernfs_inc_rev(kn->parent);
+	up_write(&kernfs_root(kn)->kernfs_iattr_rwsem);
 
 	return 0;
 }
@@ -408,9 +410,11 @@ static bool kernfs_unlink_sibling(struct kernfs_node *kn)
 	if (RB_EMPTY_NODE(&kn->rb))
 		return false;
 
+	down_write(&kernfs_root(kn)->kernfs_iattr_rwsem);
 	if (kernfs_type(kn) == KERNFS_DIR)
 		kn->parent->dir.subdirs--;
 	kernfs_inc_rev(kn->parent);
+	up_write(&kernfs_root(kn)->kernfs_iattr_rwsem);
 
 	rb_erase(&kn->rb, &kn->parent->dir.children);
 	RB_CLEAR_NODE(&kn->rb);