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References: <20171214112726.742649793@infradead.org> <20171214113851.398563731@infradead.org>
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From: Andy Lutomirski <luto@kernel.org>
Date: Thu, 14 Dec 2017 13:27:58 -0800
Message-ID: <CALCETrXroE4jv_gj0Jb8TVbcXoQP6PQ9yfydrBFR5QoYVXpFKQ@mail.gmail.com>
Subject: Re: [PATCH v2 06/17] x86/ldt: Do not install LDT for kernel threads
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To: Peter Zijlstra <peterz@infradead.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>, Linus Torvalds <torvalds@linux-foundation.org>, Andy Lutomirsky <luto@kernel.org>, Dave Hansen <dave.hansen@intel.com>, Borislav Petkov <bpetkov@suse.de>, Greg KH <gregkh@linuxfoundation.org>, Kees Cook <keescook@google.com>, Hugh Dickins <hughd@google.com>, Brian Gerst <brgerst@gmail.com>, Josh Poimboeuf <jpoimboe@redhat.com>, Denys Vlasenko <dvlasenk@redhat.com>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, Juergen Gross <jgross@suse.com>, David Laight <David.Laight@aculab.com>, Eduardo Valentin <eduval@amazon.com>, "Liguori, Anthony" <aliguori@amazon.com>, Will Deacon <will.deacon@arm.com>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Dan Williams <dan.j.williams@intel.com>

On Thu, Dec 14, 2017 at 11:43 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, Dec 14, 2017 at 12:27:32PM +0100, Peter Zijlstra wrote:
>> From: Thomas Gleixner <tglx@linutronix.de>
>>
>> Kernel threads can use the mm of a user process temporarily via use_mm(),
>> but there is no point in installing the LDT which is associated to that mm
>> for the kernel thread.
>
> So thinking about this a bit more; I fear its not correct.
>
> Suppose a kthread does use_mm() and we then schedule to a task of that
> process, we'll not pass through switch_mm() and we'll not install the
> LDT and bad things happen.
>
> Or am I missing something?
>

Nah, you're probably right.

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