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Date:   Fri, 1 May 2020 05:14:44 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH] vfs: allow unprivileged whiteout creation
Message-ID: <20200501041444.GJ23230@ZenIV.linux.org.uk>
References: <20200409212859.GH28467@miu.piliscsaba.redhat.com>
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On Thu, Apr 09, 2020 at 11:28:59PM +0200, Miklos Szeredi wrote:
> From: Miklos Szeredi <mszeredi@redhat.com>
> 
> Whiteouts, unlike real device node should not require privileges to create.
> 
> The general concern with device nodes is that opening them can have side
> effects.  The kernel already avoids zero major (see
> Documentation/admin-guide/devices.txt).  To be on the safe side the patch
> explicitly forbids registering a char device with 0/0 number (see
> cdev_add()).
> 
> This guarantees that a non-O_PATH open on a whiteout will fail with ENODEV;
> i.e. it won't have any side effect.

>  int vfs_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t dev)
>  {
> +	bool is_whiteout = S_ISCHR(mode) && dev == WHITEOUT_DEV;
>  	int error = may_create(dir, dentry);
>  
>  	if (error)
>  		return error;
>  
> -	if ((S_ISCHR(mode) || S_ISBLK(mode)) && !capable(CAP_MKNOD))
> +	if ((S_ISCHR(mode) || S_ISBLK(mode)) && !capable(CAP_MKNOD) &&
> +	    !is_whiteout)
>  		return -EPERM;

Hmm...  That exposes vfs_whiteout() to LSM; are you sure that you won't
end up with regressions for overlayfs on sufficiently weird setups?