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Subject: Re: [PATCH v2] x86/nmi: Fix some races in NMI uaccess
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <dd956eba16646fd0b15c3c0741269dfd84452dac.1535557289.git.luto@kernel.org>
Date:   Wed, 29 Aug 2018 11:56:08 -0700
Cc:     Borislav Petkov <bp@alien8.de>, Rik van Riel <riel@surriel.com>,
        Jann Horn <jannh@google.com>,
        LKML <linux-kernel@vger.kernel.org>, stable@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>
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References: <dd956eba16646fd0b15c3c0741269dfd84452dac.1535557289.git.luto@kernel.org>
To:     Andy Lutomirski <luto@kernel.org>, x86@kernel.org
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at 8:47 AM, Andy Lutomirski <luto@kernel.org> wrote:

> In NMI context, we might be in the middle of context switching or in
> the middle of switch_mm_irqs_off().  In either case, CR3 might not
> match current->mm, which could cause copy_from_user_nmi() and
> friends to read the wrong memory.
> 
> Fix it by adding a new nmi_uaccess_okay() helper and checking it in
> copy_from_user_nmi() and in __copy_from_user_nmi()'s callers.
> 
> Cc: stable@vger.kernel.org
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Nadav Amit <nadav.amit@gmail.com>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
> 
> Nadav, this is intended for your series.  Want to add it right
> before the use_temporary_mm() stuff?

Sure. Thanks! I will apply the following small fix:

> +
> +#ifdef CONFIG_DEBUG_VM
> +	WARN_ON_ONCE(!loaded_mm);
> +#endif

Will be changed to VM_WARN_ON_ONCE() in the two instances.

Regards,
Nadav