netdev - Re: [PATCH 6/6] sysctl: pass kernel pointers to ->proc

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Date:   Fri, 17 Apr 2020 11:17:18 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH 6/6] sysctl: pass kernel pointers to ->proc_handler

On Fri, Apr 17, 2020 at 08:41:46AM +0200, Christoph Hellwig wrote:
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index b6f5d459b087..d5c9a9bf4e90 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -539,13 +539,13 @@ static struct dentry *proc_sys_lookup(struct inode *dir, struct dentry *dentry,
>  	return err;
>  }
>  
> -static ssize_t proc_sys_call_handler(struct file *filp, void __user *buf,
> +static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
>  		size_t count, loff_t *ppos, int write)
>  {
>  	struct inode *inode = file_inode(filp);
>  	struct ctl_table_header *head = grab_header(inode);
>  	struct ctl_table *table = PROC_I(inode)->sysctl_entry;
> -	void *new_buf = NULL;
> +	void *kbuf;
>  	ssize_t error;
>  
>  	if (IS_ERR(head))
> @@ -564,27 +564,36 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *buf,
>  	if (!table->proc_handler)
>  		goto out;
>  
> -	error = BPF_CGROUP_RUN_PROG_SYSCTL(head, table, write, buf, &count,
> -					   ppos, &new_buf);
> +	if (write) {
> +		kbuf = memdup_user_nul(ubuf, count);
> +		if (IS_ERR(kbuf)) {
> +			error = PTR_ERR(kbuf);
> +			goto out;
> +		}
> +	} else {
> +		error = -ENOMEM;
> +		kbuf = kzalloc(count, GFP_KERNEL);
> +		if (!kbuf)
> +			goto out;
> +	}
> +
> +	error = BPF_CGROUP_RUN_PROG_SYSCTL(head, table, write, &kbuf, &count,
> +					   ppos);
>  	if (error)
> -		goto out;
> +		goto out_free_buf;
>  
>  	/* careful: calling conventions are nasty here */

I think this comment can go now ;-)

> -	if (new_buf) {
> -		mm_segment_t old_fs;
> -
> -		old_fs = get_fs();
> -		set_fs(KERNEL_DS);
> -		error = table->proc_handler(table, write, (void __user *)new_buf,
> -					    &count, ppos);
> -		set_fs(old_fs);
> -		kfree(new_buf);
> -	} else {
> -		error = table->proc_handler(table, write, buf, &count, ppos);
> -	}
> +	error = table->proc_handler(table, write, kbuf, &count, ppos);
> +	if (error)
> +		goto out_free_buf;
> +
> +	error = -EFAULT;
> +	if (copy_to_user(ubuf, kbuf, count))
> +		goto out_free_buf;

Can we skip this if !write?  Indeed, don't we have to in case the user has
passed a pointer to a read-only memory page?