linux-kernel - Re: KASAN: use-after-free Read in debugfs

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Date:   Thu, 3 May 2018 17:33:51 +0200
From:   Jan Kara <jack@...e.cz>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     syzbot <syzbot+db48ce201b524603190a@...kaller.appspotmail.com>,
        syzkaller-bugs@...glegroups.com,
        weiping zhang <zhangweiping@...ichuxing.com>,
        Jan Kara <jack@...e.cz>, Jens Axboe <axboe@...nel.dk>,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org
Subject: Re: KASAN: use-after-free Read in debugfs_remove (2)

On Mon 23-04-18 19:34:45, Tetsuo Handa wrote:
> From be88e559ec13f49b1c3aec2457c14c70f6b1926a Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Date: Mon, 23 Apr 2018 11:21:03 +0900
> Subject: [PATCH] bdi: Fix use after free bug in debugfs_remove()
> 
> syzbot is reporting use after free bug in debugfs_remove() [1].
> 
> This is because fault injection made memory allocation for
> debugfs_create_file() from bdi_debug_register() from bdi_register_va()
> fail and continued with setting WB_registered. But when debugfs_remove()
> is called from debugfs_remove(bdi->debug_dir) from bdi_debug_unregister()
>  from bdi_unregister() from release_bdi() because WB_registered was set
> by bdi_register_va(), IS_ERR_OR_NULL(bdi->debug_dir) == false despite
> debugfs_remove(bdi->debug_dir) was already called from bdi_register_va().
> 
> Fix this by making IS_ERR_OR_NULL(bdi->debug_dir) == true.
> 
> [1] https://syzkaller.appspot.com/bug?id=5ab4efd91a96dcea9b68104f159adf4af2a6dfc1
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Reported-by: syzbot <syzbot+049cb4ae097049dac137@...kaller.appspotmail.com>
> Fixes: 97f07697932e6faf ("bdi: convert bdi_debug_register to int")
> Cc: weiping zhang <zhangweiping@...ichuxing.com>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Jens Axboe <axboe@...nel.dk>

Looks good to me. You can add:

Reviewed-by: Jan Kara <jack@...e.cz>

Jens, can you please merge this fix? Thanks!

								Honza

> ---
>  mm/backing-dev.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 023190c..7fe73fa 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -115,6 +115,7 @@ static int bdi_debug_register(struct backing_dev_info *bdi, const char *name)
>  					       bdi, &bdi_debug_stats_fops);
>  	if (!bdi->debug_stats) {
>  		debugfs_remove(bdi->debug_dir);
> +		bdi->debug_dir = NULL;
>  		return -ENOMEM;
>  	}
>  
> -- 
> 1.8.3.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR