From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-183.mta1.migadu.com (out-183.mta1.migadu.com [95.215.58.183]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0388B3A3831 for ; Thu, 19 Mar 2026 07:37:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.183 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773905843; cv=none; b=Ae+Tq+txOPf//aNIJODjdS32GN5IyolSlCRJRx1iqGlWykzG6W4QMwKVGZW3P5Yb9uSuRAPEoQy+szCzSTBwyyl00sQWZveiQSaPVuLl4sYTaHFxruntCrZAgzCkpq+LmBQ5lYkER92f67Ne//G/PXonsBn4FyFdXATogT4kUC4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773905843; c=relaxed/simple; bh=0WU3ReMLVeVW+owWf9VCsdQCCcnxhOTgqZbsoIsQebM=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=F6URsGTtIcGZqzcw3/9dSccbuL/5Nz2z6tTDubTsBwimOnK4/V+vRYQGzKubtP8Vb5dIH3XV+6fZt5ngOPNmNyux6vlVsA1HRJz+CqkQkvXqh8vridIlIOTuJ2W34tva9bVo9EZcNnHnn6DbJbumv7kQjVzL2X0YKFRxR52ZZiw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=AR629A8y; arc=none smtp.client-ip=95.215.58.183 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="AR629A8y" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1773905837; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=rZThncH9Cr3zKPkN8Mtkjfi2oBzQzRGvK2Ypc9OlV0o=; b=AR629A8yr4hDOAJDFwI3BxGaE3iCQxaXgTCGBPgArjyUazNxG9+Efcv1SXR+vNEWPwwI/8 +PGaAAd5laNuIsRfFrX+eBquWBdRlRvMT0TwPwFDWR1dB4bjxFlNCSOae2hq3Idvwxq69D khAfogZYNGQxpp5m1dklXnUJjRMwxxs= From: Jiayuan Chen To: linux-ext4@vger.kernel.org, jack@suse.cz Cc: Jiayuan Chen , Jiayuan Chen , "Theodore Ts'o" , Andreas Dilger , Ye Bin , Ritesh Harjani , linux-kernel@vger.kernel.org Subject: [PATCH v2] ext4: fix use-after-free in update_super_work when racing with umount Date: Thu, 19 Mar 2026 15:36:50 +0800 Message-ID: <20260319073651.79209-1-jiayuan.chen@linux.dev> Precedence: bulk X-Mailing-List: linux-ext4@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT From: Jiayuan Chen Commit b98535d09179 ("ext4: fix bug_on in start_this_handle during umount filesystem") moved ext4_unregister_sysfs() before flushing s_sb_upd_work to prevent new error work from being queued via /proc/fs/ext4/xx/mb_groups reads during unmount. However, this introduced a use-after-free because update_super_work calls ext4_notify_error_sysfs() -> sysfs_notify() which accesses the kobject's kernfs_node after it has been freed by kobject_del() in ext4_unregister_sysfs(): update_super_work ext4_put_super ----------------- -------------- ext4_unregister_sysfs(sb) kobject_del(&sbi->s_kobj) __kobject_del() sysfs_remove_dir() kobj->sd = NULL sysfs_put(sd) kernfs_put() // RCU free ext4_notify_error_sysfs(sbi) sysfs_notify(&sbi->s_kobj) kn = kobj->sd // stale pointer kernfs_get(kn) // UAF on freed kernfs_node ext4_journal_destroy() flush_work(&sbi->s_sb_upd_work) Instead of reordering the teardown sequence, fix this by making ext4_notify_error_sysfs() detect that sysfs has already been torn down by checking s_kobj.state_in_sysfs, and skipping the sysfs_notify() call in that case. A dedicated mutex (s_error_notify_mutex) serializes ext4_notify_error_sysfs() against kobject_del() in ext4_unregister_sysfs() to prevent TOCTOU races where the kobject could be deleted between the state_in_sysfs check and the sysfs_notify() call. Fixes: b98535d09179 ("ext4: fix bug_on in start_this_handle during umount filesystem") Cc: Jiayuan Chen Suggested-by: Jan Kara Signed-off-by: Jiayuan Chen --- v1 -> v2: Use state_in_sysfs instead of reordering the teardown sequence. https://lore.kernel.org/linux-ext4/20260313065206.152645-1-jiayuan.chen@linux.dev/ --- fs/ext4/ext4.h | 1 + fs/ext4/sysfs.c | 9 ++++++++- 2 files changed, 9 insertions(+), 1 deletion(-) diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h index b76966dc06c0..c012e85d2515 100644 --- a/fs/ext4/ext4.h +++ b/fs/ext4/ext4.h @@ -1574,6 +1574,7 @@ struct ext4_sb_info { struct proc_dir_entry *s_proc; struct kobject s_kobj; struct completion s_kobj_unregister; + struct mutex s_error_notify_mutex; /* protects sysfs_notify vs kobject_del */ struct super_block *s_sb; struct buffer_head *s_mmp_bh; diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c index d2ecc1026c0c..f2505988e010 100644 --- a/fs/ext4/sysfs.c +++ b/fs/ext4/sysfs.c @@ -597,7 +597,10 @@ static const struct kobj_type ext4_feat_ktype = { void ext4_notify_error_sysfs(struct ext4_sb_info *sbi) { - sysfs_notify(&sbi->s_kobj, NULL, "errors_count"); + mutex_lock(&sbi->s_error_notify_mutex); + if (sbi->s_kobj.state_in_sysfs) + sysfs_notify(&sbi->s_kobj, NULL, "errors_count"); + mutex_unlock(&sbi->s_error_notify_mutex); } static struct kobject *ext4_root; @@ -610,6 +613,7 @@ int ext4_register_sysfs(struct super_block *sb) int err; init_completion(&sbi->s_kobj_unregister); + mutex_init(&sbi->s_error_notify_mutex); err = kobject_init_and_add(&sbi->s_kobj, &ext4_sb_ktype, ext4_root, "%s", sb->s_id); if (err) { @@ -644,7 +648,10 @@ void ext4_unregister_sysfs(struct super_block *sb) if (sbi->s_proc) remove_proc_subtree(sb->s_id, ext4_proc_root); + + mutex_lock(&sbi->s_error_notify_mutex); kobject_del(&sbi->s_kobj); + mutex_unlock(&sbi->s_error_notify_mutex); } int __init ext4_init_sysfs(void) -- 2.43.0