linux-kernel - Re: [PATCH] x86/uaccess: instrument copy_from_user

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Message-ID: <Y1o72704bVK0FgCr@hirez.programming.kicks-ass.net>
Date:   Thu, 27 Oct 2022 10:05:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     Marco Elver <elver@...gle.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>, x86@...nel.org
Subject: Re: [PATCH] x86/uaccess: instrument copy_from_user_nmi()

On Wed, Oct 26, 2022 at 11:38:53AM -0700, Alexander Potapenko wrote:
> A bigger issue from the NMI perspective is probably
> having __msan_poison_alloca() inserted in every non-noinstr kernel
> function, because that hook may acquire the stackdepot lock.

*urgghhh* that's broken, that must not be. There is a *TON* of NMI
functions that are non-noinstr.

What's worse, it seems to do a memory allocation as well, and that's out
the window with PREEMPT_RT where you can't do even GFP_ATOMIC from
regular IRQ context.

That function is wholly unacceptable to be added to every kernel
function.