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Date: Wed, 7 Sep 2016 09:38:52 -0700
From: Andi Kleen <andi@firstfloor.org>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Kees Cook <keescook@chromium.org>, Andi Kleen <andi@firstfloor.org>,
        Jiri Olsa <jolsa@kernel.org>, lkml <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] fs/proc/kcore.c: Omit kernel text area for hardened
 usercopy feature
Message-ID: <20160907163852.GQ5871@two.firstfloor.org>
References: <1472819145-27260-1-git-send-email-jolsa@kernel.org>
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 <20160905084722.GA3134@krava>
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> ---
> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> index a939f5ed7f89..de07c273f725 100644
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
> @@ -515,8 +515,20 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  		} else {
>  			if (kern_addr_valid(start)) {
>  				unsigned long n;
> +				char *buf;
>  
> -				n = copy_to_user(buffer, (char *)start, tsz);
> +				buf = kzalloc(tsz, GFP_KERNEL);

You have to add some limit and a loop, otherwise a user can eat all kernel memory,
or copies > KMALLOC_MAX wouldn't work. Probably only get a single page.

Also don't need kzalloc, kmalloc is sufficient.

-Andi