linux-kernel - Re: [PATCH 1/2] kernfs: Add option to enable user xattrs

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Date:   Tue, 3 Mar 2020 08:19:21 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Daniel Xu <dxu@...uu.xyz>
Cc:     cgroups@...r.kernel.org, lizefan@...wei.com, hannes@...xchg.org,
        linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
        kernel-team@...com
Subject: Re: [PATCH 1/2] kernfs: Add option to enable user xattrs

Hello,

On Mon, Mar 02, 2020 at 05:39:00PM -0800, Daniel Xu wrote:
> +static int kernfs_vfs_user_xattr_set(const struct xattr_handler *handler,
> +				     struct dentry *unused, struct inode *inode,
> +				     const char *suffix, const void *value,
> +				     size_t size, int flags)
> +{
...
> +	if (value && atomic_inc_return(nr) > KERNFS_MAX_USER_XATTRS) {
> +		ret = -ENOSPC;
> +		goto dec_out;
> +	}

So, we limit the number of user xattrs here but

> +	ret = kernfs_vfs_xattr_set(handler, unused, inode, suffix, value,
> +				   size, flags);

This will call into simple_xattr_set() which doesn't put any further
restriction on size and just calls GFP_KERNEL kmalloc on it allowing
users incur high-order allocations. Maybe it'd make sense to limit
both the number and size?

Thanks.

-- 
tejun