Re: [PATCH 24/30] x86, kaiser: disable native VSYSCALL

From: Andy Lutomirski <luto@kernel.org>
To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	moritz.lipp@iaik.tugraz.at,
	Daniel Gruss <daniel.gruss@iaik.tugraz.at>,
	michael.schwarz@iaik.tugraz.at,
	richard.fellner@student.tugraz.at,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Kees Cook <keescook@google.com>, Hugh Dickins <hughd@google.com>,
	X86 ML <x86@kernel.org>
Subject: Re: [PATCH 24/30] x86, kaiser: disable native VSYSCALL
Date: Mon, 13 Nov 2017 18:15:57 -0800	[thread overview]
Message-ID: <CALCETrXNMYvF8FKzSnvoTR=bQiNBqBz93d+tJq1knhD8MWBYpg@mail.gmail.com> (raw)
In-Reply-To: <8b23c090-f7c3-5110-011b-07e5131eb996@linux.intel.com>

On Mon, Nov 13, 2017 at 1:07 PM, Dave Hansen
<dave.hansen@linux.intel.com> wrote:
> On 11/12/2017 07:52 PM, Andy Lutomirski wrote:
>> On Fri, Nov 10, 2017 at 3:04 PM, Dave Hansen
>> <dave.hansen@linux.intel.com> wrote:
>>> On 11/10/2017 02:06 PM, Andy Lutomirski wrote:
>>>> I have nothing against disabling native.  I object to breaking the
>>>> weird binary tracing behavior in the emulation mode, especially if
>>>> it's tangled up with KAISER.  I got all kinds of flak in an earlier
>>>> version of the vsyscall emulation patches when I broke that use case.
>>>> KAISER may get very widely backported -- let's not make changes that
>>>> are already known to break things.
>>>
>>> Is the thing that broke a "user mode program that actually looks at the
>>> vsyscall page"?  Like Linus is referring to here:
>>>
>> Yes.  But I disagree with Linus.  I think it would be perfectly
>> reasonable to enable KAISER and to use a tool like pin on a legacy
>> binary from some enterprise distribution.  I bet there are lots of
>> enterprise distributions that are still supported that use vsyscalls.
>
> All we need to do in the end here is to re-set _PAGE_USER on the user
> page table PGD that is used by the vsyscall page.  We should be able to
> do that with a line or two of code in kaiser_init().  We can do it
> conditionally on when the VDSO is not compile-time disabled.
>
> I can do this as a follow-on patch, or as the last one in the KAISER
> series and leave it up to our esteemed maintainers to decide whether
> they want to do it or not.  Sound good?
>
> Are there any userspace tests around that I can use for this, or will I
> have to cook something up?

I don't.  This old test might be adaptable:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/misc-tests.git/tree/test_vsyscall.cc

What you'd want to do is to add a variant that allocates some RWX
memory, memcpys the vsyscall page there, and tests that it still works
(but only if the vsyscall page worked in the first place).

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