linux-hardening - Re: [PATCH -next] mm: usercopy: add a debugfs interface to bypass the vmalloc check.

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Message-ID: <D99B0795-152B-4D63-94C4-5953192A5E53@kernel.org>
Date: Wed, 04 Dec 2024 14:43:54 +1000
From: Kees Cook <kees@...nel.org>
To: Matthew Wilcox <willy@...radead.org>, Uladzislau Rezki <urezki@...il.com>
CC: Kefeng Wang <wangkefeng.wang@...wei.com>, zuoze <zuoze1@...wei.com>,
 gustavoars@...nel.org, akpm@...ux-foundation.org,
 linux-hardening@...r.kernel.org, linux-mm@...ck.org, keescook@...omium.org
Subject: Re: [PATCH -next] mm: usercopy: add a debugfs interface to bypass the vmalloc check.

On December 4, 2024 5:56:34 AM GMT+10:00, Matthew Wilcox <willy@...radead.org> wrote:
>On Tue, Dec 03, 2024 at 08:02:26PM +0100, Uladzislau Rezki wrote:
>
>I think there are a few other things we can try here.
>
>First, if the copy is small (and I still don't have an answer to that
>...), we can skip the vmalloc lookup if the copy doesn't cross a page
>boundary.

Yeah, this seems a reasonable optimization.

>Anyway, NACK to the original patch; that's just a horrible idea.

Right, please no debugfs knob. The point is to make sure the check cannot be disabled at runtime and to make the check a no-op. Please use the boot param to disable this check. 

-Kees

-- 
Kees Cook