From: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: x86: PIE support and option to extend KASLR randomization
Date: Thu, 24 Aug 2017 14:42:44 -0700
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Cc: Ingo Molnar <mingo@kernel.org>, Herbert Xu <herbert@gondor.apana.org.au>,
	"David S . Miller" <davem@davemloft.net>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
	"H . Peter Anvin" <hpa@zytor.com>, Peter Zijlstra <peterz@infradead.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
	Matthias Kaehlcke <mka@chromium.org>, Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Juergen Gross <jgross@suse.com>, Paolo Bonzini <pbonzini@redhat.com>,
	=?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
	Joerg Roedel <joro@8bytes.org>, Tom Lendacky <thomas.lendacky@amd.com>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@suse.de>, Brian Gerst <brgerst@gmail.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>, "Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Len Brown <len.brown@intel.com>, Pavel Machek <pavel@ucw.cz>, Tejun H
To: Thomas Garnier <thgarnie@google.com>
Sender: linus971@gmail.com
In-Reply-To: <CAJcbSZGhvwt=5ERtBHLJnwS=6AXBZLTMfrafzeUCqYy=-MKWDg@mail.gmail.com>

On Thu, Aug 24, 2017 at 2:13 PM, Thomas Garnier <thgarnie@google.com> wrote:
>
> My original performance testing was done with an Ubuntu generic
> configuration. This configuration has the CONFIG_FUNCTION_TRACER
> option which was incompatible with PIE. The tracer failed to replace
> the __fentry__ call by a nop slide on each traceable function because
> the instruction was not the one expected. If PIE is enabled, gcc
> generates a difference call instruction based on the GOT without
> checking the visibility options (basically call *__fentry__@GOTPCREL).

Gah.

Don't we actually have *more* address bits for randomization at the
low end, rather than getting rid of -mcmodel=kernel?

Has anybody looked at just moving kernel text by smaller values than
the page size? Yeah, yeah, the kernel has several sections that need
page alignment, but I think we could relocate normal text by just the
cacheline size, and that sounds like it would give several bits of
randomness with little downside.

Or has somebody already looked at it and I just missed it?

               Linus