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Thu, 19 Mar 2026 19:59:44 -0700 (PDT) Received: from pve-server ([49.205.216.49]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-35bc610c535sm3976321a91.16.2026.03.19.19.59.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Mar 2026 19:59:44 -0700 (PDT) From: Ritesh Harjani (IBM) To: Jiayuan Chen , linux-ext4@vger.kernel.org, jack@suse.cz Cc: Jiayuan Chen , Jiayuan Chen , Theodore Ts'o , Andreas Dilger , Ritesh Harjani , Ye Bin , linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] ext4: fix use-after-free in update_super_work when racing with umount In-Reply-To: <20260319120336.157873-1-jiayuan.chen@linux.dev> Date: Fri, 20 Mar 2026 08:23:16 +0530 Message-ID: References: <20260319120336.157873-1-jiayuan.chen@linux.dev> Precedence: bulk X-Mailing-List: linux-ext4@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Jiayuan Chen writes: > 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) Yes, the race between kobject_del() and sysfs_notify() -> kernfs_get() is real. and sysfs_remove_dir() explicitely says: * In general, kobject owner is responsible for ensuring removal * doesn't race with other operations and sysfs doesn't provide any * protection Hence this patch which adds a new mutex lock to prevent the above race, looks good to me. So please feel free to add: Reviewed-by: Ritesh Harjani (IBM)