linux-kernel - Re: [PATCH] autofs: drop dentry reference only when it is never used

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Message-ID: <9f79a8203d29f207cfb71c7aefb3c98f28a23aaf.camel@themaw.net>
Date:   Mon, 26 Nov 2018 18:36:39 +0800
From:   Ian Kent <raven@...maw.net>
To:     Pan Bian <bianpan2016@....com>
Cc:     autofs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] autofs: drop dentry reference only when it is never used

On Mon, 2018-11-26 at 09:58 +0800, Pan Bian wrote:
> The function autofs_expire_run calls dput(dentry) to drop the reference
> count of dentry. However, dentry is read via autofs_dentry_ino(dentry)
> after that. This may result in a use-free-bug. The patch drops the
> reference count of dentry only when it is never used.

Yes, I agree this is a bug and it should be fixed.

The autofs_expire_run() function is used for autofs v3 which is
very old now so it's not likely to be called.

But I think you are correct, if it is called the copy to user
space should trigger a umount and (likley) remove the mount
point directory, maybe I broke this at some point without
realising it ...

So thanks, I'll have a closer look but even if the ref counting
isn't quite what either of us expect this is probably still worth
while.

If I don't see any reason to not do this I'll forward the patch
to Andrew.

Ian
> 
> Signed-off-by: Pan Bian <bianpan2016@....com>
> ---
>  fs/autofs/expire.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/autofs/expire.c b/fs/autofs/expire.c
> index d441244..28d9c2b 100644
> --- a/fs/autofs/expire.c
> +++ b/fs/autofs/expire.c
> @@ -596,7 +596,6 @@ int autofs_expire_run(struct super_block *sb,
>  	pkt.len = dentry->d_name.len;
>  	memcpy(pkt.name, dentry->d_name.name, pkt.len);
>  	pkt.name[pkt.len] = '\0';
> -	dput(dentry);
>  
>  	if (copy_to_user(pkt_p, &pkt, sizeof(struct autofs_packet_expire)))
>  		ret = -EFAULT;
> @@ -609,6 +608,8 @@ int autofs_expire_run(struct super_block *sb,
>  	complete_all(&ino->expire_complete);
>  	spin_unlock(&sbi->fs_lock);
>  
> +	dput(dentry);
> +
>  	return ret;
>  }
>