Re: [PATCH 5/5] sysctl: pass kernel pointers to ->proc_handler

From: Andrey Ignatov <rdna@fb.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Kees Cook <keescook@chromium.org>,
	Iurii Zaikin <yzaikin@google.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	<linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
	<linux-fsdevel@vger.kernel.org>, <netdev@vger.kernel.org>,
	<bpf@vger.kernel.org>
Subject: Re: [PATCH 5/5] sysctl: pass kernel pointers to ->proc_handler
Date: Tue, 21 Apr 2020 12:23:30 -0700	[thread overview]
Message-ID: <20200421192330.GA60879@rdna-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <20200421171539.288622-6-hch@lst.de>

Christoph Hellwig <hch@lst.de> [Tue, 2020-04-21 10:17 -0700]:
> Instead of having all the sysctl handlers deal with user pointers, which
> is rather hairy in terms of the BPF interaction, copy the input to and
> from  userspace in common code.  This also means that the strings are
> always NUL-terminated by the common code, making the API a little bit
> safer.
> 
> As most handler just pass through the data to one of the common handlers
> a lot of the changes are mechnical.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

...

> @@ -1172,36 +1168,28 @@ int __cgroup_bpf_run_filter_sysctl(struct ctl_table_header *head,
>  		.new_updated = 0,
>  	};
>  	struct cgroup *cgrp;
> +	loff_t pos = 0;
>  	int ret;
>  
>  	ctx.cur_val = kmalloc_track_caller(ctx.cur_len, GFP_KERNEL);
> -	if (ctx.cur_val) {
> -		mm_segment_t old_fs;
> -		loff_t pos = 0;
> -
> -		old_fs = get_fs();
> -		set_fs(KERNEL_DS);
> -		if (table->proc_handler(table, 0, (void __user *)ctx.cur_val,
> -					&ctx.cur_len, &pos)) {
> -			/* Let BPF program decide how to proceed. */
> -			ctx.cur_len = 0;
> -		}
> -		set_fs(old_fs);
> -	} else {
> +	if (!ctx.cur_val ||
> +	    table->proc_handler(table, 0, ctx.cur_val, &ctx.cur_len, &pos)) {
>  		/* Let BPF program decide how to proceed. */
>  		ctx.cur_len = 0;
>  	}
>  
> -	if (write && buf && *pcount) {
> +	if (write && *buf && *pcount) {
>  		/* BPF program should be able to override new value with a
>  		 * buffer bigger than provided by user.
>  		 */
>  		ctx.new_val = kmalloc_track_caller(PAGE_SIZE, GFP_KERNEL);
>  		ctx.new_len = min_t(size_t, PAGE_SIZE, *pcount);
> -		if (!ctx.new_val ||
> -		    copy_from_user(ctx.new_val, buf, ctx.new_len))
> +		if (ctx.new_val) {
> +			memcpy(ctx.new_val, *buf, ctx.new_len);
> +		} else {
>  			/* Let BPF program decide how to proceed. */
>  			ctx.new_len = 0;
> +		}
>  	}
>  
>  	rcu_read_lock();
> @@ -1212,7 +1200,7 @@ int __cgroup_bpf_run_filter_sysctl(struct ctl_table_header *head,
>  	kfree(ctx.cur_val);
>  
>  	if (ret == 1 && ctx.new_updated) {
> -		*new_buf = ctx.new_val;
> +		*buf = ctx.new_val;

Original value of *buf should be freed before overriding it here
otherwise it's lost/leaked unless I missed something.

Other than this BPF part of this patch looks good to me. Feel free to
add my Ack on the next iteration with this fix.

>  		*pcount = ctx.new_len;
>  	} else {
>  		kfree(ctx.new_val);

-- 
Andrey Ignatov