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Subject: Re: [PATCH v2] x86/nmi: Fix some races in NMI uaccess
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <alpine.DEB.2.21.1808301535110.1210@nanos.tec.linutronix.de>
Date:   Thu, 30 Aug 2018 07:21:18 -0700
Cc:     Nadav Amit <nadav.amit@gmail.com>,
        Andy Lutomirski <luto@kernel.org>, x86@kernel.org,
        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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Message-Id: <A71C1AF9-5C7D-4A04-84CB-4C5FE6D0DA58@amacapital.net>
References: <dd956eba16646fd0b15c3c0741269dfd84452dac.1535557289.git.luto@kernel.org> <7202F39D-B4C2-4FA0-868E-2D03BD313BD7@gmail.com> <alpine.DEB.2.21.1808301535110.1210@nanos.tec.linutronix.de>
To:     Thomas Gleixner <tglx@linutronix.de>
Sender: linux-kernel-owner@vger.kernel.org
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> On Aug 30, 2018, at 6:36 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>=20
>> On Wed, 29 Aug 2018, Nadav Amit wrote:
>> at 8:47 AM, Andy Lutomirski <luto@kernel.org> wrote:
>>=20
>>> 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.
>>>=20
>>> 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.
>>>=20
>>> 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>
>>> ---
>>>=20
>>> Nadav, this is intended for your series.  Want to add it right
>>> before the use_temporary_mm() stuff?
>>=20
>> Sure. Thanks! I will apply the following small fix:
>>=20
>>> +
>>> +#ifdef CONFIG_DEBUG_VM
>>> +    WARN_ON_ONCE(!loaded_mm);
>>> +#endif
>>=20
>> Will be changed to VM_WARN_ON_ONCE() in the two instances.
>=20
> Unless I'm completely lost, this can just be applied to tip right
> away. It's not depending on anything else.
>=20

Fine with me. Do you want to do the VM_WARN_ON cleanup yourself or should I s=
end a v3?=