Re: [PATCH v2] jffs2: fix use-after-free in jffs2_garbage_collect_thread()

[Zhihao Cheng <chengzhihao1@huawei.com>]

在 2026/4/8 18:31, Dmitriy Chumachenko 写道:
> During fuzz testing, the following issue was discovered.
> 
> BUG: KASAN: use-after-free in __lock_acquire+0x3f22/0x53c0 kernel/locking/lockdep.c:4825
> Read of size 8 at addr ffff888053cfa098 by task jffs2_gcd_mtd0/11093
> 
> CPU: 1 PID: 11093 Comm: jffs2_gcd_mtd0 Not tainted 5.10.232-syzkaller #0
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
> Call Trace:
>   __dump_stack lib/dump_stack.c:77 [inline]
>   dump_stack+0x107/0x167 lib/dump_stack.c:118
>   print_address_description.constprop.0+0x1c/0x220 mm/kasan/report.c:377
>   __kasan_report mm/kasan/report.c:537 [inline]
>   kasan_report.cold+0x1f/0x37 mm/kasan/report.c:554
>   __lock_acquire+0x3f22/0x53c0 kernel/locking/lockdep.c:4825
>   lock_acquire kernel/locking/lockdep.c:5566 [inline]
>   lock_acquire+0x197/0x480 kernel/locking/lockdep.c:5531
>   __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
>   _raw_spin_lock_irqsave+0x36/0x60 kernel/locking/spinlock.c:159
>   complete+0x13/0x60 kernel/sched/completion.c:32
>   complete_and_exit+0x20/0x40 kernel/exit.c:943
>   jffs2_garbage_collect_thread+0x554/0x750 fs/jffs2/background.c:164
>   kthread+0x3a9/0x490 kernel/kthread.c:328
>   ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:298
> 
> Allocated by task 11091:
>   kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
>   kasan_set_track mm/kasan/common.c:56 [inline]
>   __kasan_kmalloc.constprop.0+0xc9/0xd0 mm/kasan/common.c:461
>   kmalloc include/linux/slab.h:552 [inline]
>   kzalloc include/linux/slab.h:664 [inline]
>   jffs2_init_fs_context+0x41/0xd0 fs/jffs2/super.c:314
>   alloc_fs_context+0x4f9/0x840 fs/fs_context.c:267
>   do_new_mount fs/namespace.c:2896 [inline]
>   path_mount+0xb99/0x2140 fs/namespace.c:3247
>   do_mount fs/namespace.c:3260 [inline]
>   __do_sys_mount fs/namespace.c:3468 [inline]
>   __se_sys_mount fs/namespace.c:3445 [inline]
>   __x64_sys_mount+0x283/0x300 fs/namespace.c:3445
>   do_syscall_64+0x30/0x40 arch/x86/entry/common.c:46
>   entry_SYSCALL_64_after_hwframe+0x67/0xd1
> 
> Freed by task 28546:
>   kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
>   kasan_set_track+0x1c/0x30 mm/kasan/common.c:56
>   kasan_set_free_info+0x1b/0x30 mm/kasan/generic.c:355
>   __kasan_slab_free+0x112/0x170 mm/kasan/common.c:422
>   slab_free_hook mm/slub.c:1542 [inline]
>   slab_free_freelist_hook+0xb8/0x1b0 mm/slub.c:1576
>   slab_free mm/slub.c:3149 [inline]
>   kfree+0xd9/0x360 mm/slub.c:4125
>   deactivate_locked_super+0x96/0x170 fs/super.c:335
>   deactivate_super+0xb2/0xd0 fs/super.c:366
>   cleanup_mnt+0x3a3/0x530 fs/namespace.c:1118
>   task_work_run+0xdf/0x1a0 kernel/task_work.c:185
>   tracehook_notify_resume include/linux/tracehook.h:188 [inline]
>   exit_to_user_mode_loop kernel/entry/common.c:172 [inline]
>   exit_to_user_mode_prepare+0x1de/0x1f0 kernel/entry/common.c:199
>   syscall_exit_to_user_mode+0x38/0x1e0 kernel/entry/common.c:274
> 
> In jffs2_garbage_collect_thread() gc_task is set to NULL and then
> kthread_complete_and_exit() calls complete() on gc_thread_exit. These
> operations are not atomic: stop path can see gc_task == NULL, skip
> wait_for_completion(), and the caller frees jffs2_sb_info while the GC
> thread still accesses gc_thread_exit in complete().
> 
> Moreover, spin_unlock() itself accesses c after complete() has woken the
> stop path:
> 
> jffs2_kill_sb                    jffs2_garbage_collect_thread
>      jffs2_stop_garbage_collect_thread
>        spin_lock
>        send_sig(SIGKILL)
>        wait = 1
>        spin_unlock
>                                       goto die
>                                       spin_lock
>                                       c->gc_task = NULL
>                                       spin_unlock
>                                       kthread_complete_and_exit()
>                                         complete(&c->gc_thread_exit)
>        wait_for_completion()
>      kfree(c)
> 
> Fix by adding a gc_thread_started flag that is set when the GC thread is
> successfully started. Use this flag instead of gc_task to decide whether
> to wait. The flag is never cleared by the GC thread, so
> wait_for_completion() is always called when start() succeeded, regardless
> of the current value of gc_task.
>                                     
> Found by Linux Verification Center (linuxtesting.org) with Syzkaller.
> 
> Fixes: e2d48b1a98bb ("[JFFS2] Fix cleanup in case of GC-Task not started")
> Signed-off-by: Dmitriy Chumachenko <Dmitry.Chumachenko@cyberprotect.ru>
> ---
> v1->v2: Use gc_thread_started flag as a reliable indicator instead
>      of gc_task. Drop complete() under erase_completion_lock
>      (thanks, Zhihao Cheng).
> 
>   fs/jffs2/background.c  | 7 ++++---
>   fs/jffs2/jffs2_fs_sb.h | 1 +
>   2 files changed, 5 insertions(+), 3 deletions(-)
> 

Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
> diff --git a/fs/jffs2/background.c b/fs/jffs2/background.c
> index bb0ee1a59e71..5e5ed9053326 100644
> --- a/fs/jffs2/background.c
> +++ b/fs/jffs2/background.c
> @@ -52,6 +52,7 @@ int jffs2_start_garbage_collect_thread(struct jffs2_sb_info *c)
>   		/* Wait for it... */
>   		jffs2_dbg(1, "Garbage collect thread is pid %d\n", tsk->pid);
>   		wait_for_completion(&c->gc_thread_start);
> +		c->gc_thread_started = true;
>   		ret = tsk->pid;
>   	}
>   
> @@ -60,16 +61,16 @@ int jffs2_start_garbage_collect_thread(struct jffs2_sb_info *c)
>   
>   void jffs2_stop_garbage_collect_thread(struct jffs2_sb_info *c)
>   {
> -	int wait = 0;
>   	spin_lock(&c->erase_completion_lock);
>   	if (c->gc_task) {
>   		jffs2_dbg(1, "Killing GC task %d\n", c->gc_task->pid);
>   		send_sig(SIGKILL, c->gc_task, 1);
> -		wait = 1;
>   	}
>   	spin_unlock(&c->erase_completion_lock);
> -	if (wait)
> +	if (c->gc_thread_started) {
>   		wait_for_completion(&c->gc_thread_exit);
> +		c->gc_thread_started = false;
> +	}
>   }
>   
>   static int jffs2_garbage_collect_thread(void *_c)
> diff --git a/fs/jffs2/jffs2_fs_sb.h b/fs/jffs2/jffs2_fs_sb.h
> index 5a7091746f68..4c833e0ff03c 100644
> --- a/fs/jffs2/jffs2_fs_sb.h
> +++ b/fs/jffs2/jffs2_fs_sb.h
> @@ -55,6 +55,7 @@ struct jffs2_sb_info {
>   	unsigned int flags;
>   
>   	struct task_struct *gc_task;	/* GC task struct */
> +	bool gc_thread_started;         /* GC thread was successfully started */
>   	struct completion gc_thread_start; /* GC thread start completion */
>   	struct completion gc_thread_exit; /* GC thread exit completion port */
>   
> 


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