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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 14 Dec 2017 14:52:58 -0800
Message-ID: <CA+55aFwgAn3aWRY9ZJS22pc0atHBzapiGBQ9bSWpPCk2hiGWDg@mail.gmail.com>
Subject: Re: [PATCH v2 11/17] selftests/x86/ldt_gdt: Prepare for access bit forced
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@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 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

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