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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 14 Dec 2017 14:52:58 -0800
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Subject: Re: [PATCH v2 11/17] selftests/x86/ldt_gdt: Prepare for access bit forced
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>,
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On Thu, Dec 14, 2017 at 2:24 PM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> get_user_pages_fast() (both of them) do indeed test access_ok(), but the
> regular get_user_pages() does not, I suspect because it can operate on a
> foreign mm.

That sounds wrong.

We actually had some very serious reasons why get_user_pages_fast()
needed to check access_ok().

I happen to forget what those reasons were, though.

My mind may be going.

But I think it was something like "you could walk off the page tables
because the undefined address range generates nonsensical values for
the pgd_offset() functions" etc.

But maybe the regular get_user_pages() has some other way to protect
against that.

          Linus