linux-kernel - Re: [PATCH v2 0/2] x86: Relocate the compat vdso per process

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Message-ID: <CALCETrVyR4GmLZ-yzPCL8xGZOZkRtPkQUcqQ=nzcRqG_Yp60wg@mail.gmail.com>
Date:	Tue, 11 Mar 2014 23:09:11 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>, X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	Martin Runge <Martin.Runge@...de-schwarz.com>,
	Andi Kleen <andi@...stfloor.org>,
	Stefani Seibold <stefani@...bold.net>,
	Andreas Brief <Andreas.Brief@...de-schwarz.com>
Subject: Re: [PATCH v2 0/2] x86: Relocate the compat vdso per process

On Mar 11, 2014 10:02 PM, "H. Peter Anvin" <hpa@...or.com> wrote:
>
> On 03/11/2014 03:15 PM, Andy Lutomirski wrote:
>>
>> The meat of this patch series is in patch 1.  Patch 2 is split out for
>> improved bisectability.
>>
>> Changes from v1: Split into two patches and fixed a comment.
>>
>> Andy Lutomirski (2):
>>    x86: Dynamically relocate the compat vdso
>>    x86_32: Remove user bit from identity map PDE
>>
>>   Documentation/kernel-parameters.txt  |  18 +++-
>>   arch/x86/Kconfig                     |  24 +++--
>>   arch/x86/include/asm/elf.h           |   4 -
>>   arch/x86/include/asm/fixmap.h        |   8 --
>>   arch/x86/include/asm/pgtable_types.h |   7 +-
>>   arch/x86/include/asm/vdso.h          |   5 +-
>>   arch/x86/vdso/vdso-layout.lds.S      |   2 +-
>>   arch/x86/vdso/vdso32-setup.c         | 173 ++++++++++++++++-------------------
>>   arch/x86/vdso/vdso32/vdso32.lds.S    |   2 -
>>   9 files changed, 111 insertions(+), 132 deletions(-)
>>
>
> Why per process?  *If* we have compat vdso turned on, can't we just put it in one place system-wide?

My machine has ASLR turned on, and the vma really does end up
somewhere different each time.  I suspect that most (?) ASLR users
would happily accept the 4k hit to keep ASLR working fully.

Another downside of trying to have a common address for all processes:
someone needs to figure out what that address is.  What happens if an
ELF binary (or interpreter) wants to load something at the chosen
address?

--Andy
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