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References: <20181023163157.41441-1-kirill.shutemov@linux.intel.com> <20181023163157.41441-3-kirill.shutemov@linux.intel.com>
In-Reply-To: <20181023163157.41441-3-kirill.shutemov@linux.intel.com>
From: Andy Lutomirski <luto@kernel.org>
Date: Wed, 24 Oct 2018 11:49:17 -0700
Message-ID: <CALCETrUsqCzU6VO0h4EFpsdXOOn-kJY7ogwKQiQScNY9YJ6hWA@mail.gmail.com>
Subject: Re: [PATCH 2/2] x86/ldt: Unmap PTEs for the slow before freeing LDT
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Sender: owner-linux-mm@kvack.org
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To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>, Dave Hansen <dave.hansen@linux.intel.com>, Andrew Lutomirski <luto@kernel.org>, Peter Zijlstra <peterz@infradead.org>, X86 ML <x86@kernel.org>, Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>

On Tue, Oct 23, 2018 at 9:32 AM Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> wrote:
>
> modify_ldt(2) leaves old LDT mapped after we switch over to the new one.
> Memory for the old LDT gets freed and the pages can be re-used.
>
> Leaving the mapping in place can have security implications. The mapping
> is present in userspace copy of page tables and Meltdown-like attack can
> read these freed and possibly reused pages.

Code looks okay.  But:

> -       /*
> -        * Did we already have the top level entry allocated?  We can't
> -        * use pgd_none() for this because it doens't do anything on
> -        * 4-level page table kernels.
> -        */
> -       pgd = pgd_offset(mm, LDT_BASE_ADDR);

This looks like an unrelated cleanup.  Can it be its own patch?