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From: Andy Lutomirski <luto@kernel.org>
Date: Tue, 12 Dec 2017 10:06:51 -0800
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Subject: Re: [patch 05/16] mm: Allow special mappings with user access cleared
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>, Borislav Petkov <bpetkov@suse.de>,
        Greg KH <gregkh@linuxfoundation.org>, Kees Cook <keescook@google.com>,
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On Tue, Dec 12, 2017 at 10:05 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, Dec 12, 2017 at 10:00:08AM -0800, Andy Lutomirski wrote:
>> On Tue, Dec 12, 2017 at 9:32 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>> > From: Peter Zijstra <peterz@infradead.org>
>> >
>> > In order to create VMAs that are not accessible to userspace create a new
>> > VM_NOUSER flag. This can be used in conjunction with
>> > install_special_mapping() to inject 'kernel' data into the userspace map.
>> >
>> > Similar to how arch_vm_get_page_prot() allows adding _PAGE_flags to
>> > pgprot_t, introduce arch_vm_get_page_prot_excl() which masks
>> > _PAGE_flags from pgprot_t and use this to implement VM_NOUSER for x86.
>>
>> How does this interact with get_user_pages(), etc?
>
> gup would find the page. These patches do in fact rely on that through
> the populate things.
>

Blech.  So you can write(2) from the LDT to a file and you can even
sendfile it, perhaps.  What happens if it's get_user_page()'d when
modify_ldt() wants to free it?

This patch series scares the crap out of me.