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Date: Tue, 9 May 2017 12:05:12 -0400
From: Brian Gerst <brgerst@...il.com>
To: Andy Lutomirski <luto@...nel.org>
Cc: Christoph Hellwig <hch@...radead.org>, Ingo Molnar <mingo@...nel.org>, Greg KH <greg@...ah.com>, 
	Thomas Garnier <thgarnie@...gle.com>, Martin Schwidefsky <schwidefsky@...ibm.com>, 
	Heiko Carstens <heiko.carstens@...ibm.com>, Dave Hansen <dave.hansen@...el.com>, 
	Arnd Bergmann <arnd@...db.de>, Thomas Gleixner <tglx@...utronix.de>, David Howells <dhowells@...hat.com>, 
	René Nyffenegger <mail@...enyffenegger.ch>, 
	Andrew Morton <akpm@...ux-foundation.org>, 
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>, "Eric W . Biederman" <ebiederm@...ssion.com>, 
	Oleg Nesterov <oleg@...hat.com>, Pavel Tikhomirov <ptikhomirov@...tuozzo.com>, 
	Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Rik van Riel <riel@...hat.com>, Kees Cook <keescook@...omium.org>, 
	Josh Poimboeuf <jpoimboe@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, 
	Christian Borntraeger <borntraeger@...ibm.com>, Russell King <linux@...linux.org.uk>, 
	Will Deacon <will.deacon@....com>, Catalin Marinas <catalin.marinas@....com>, 
	Mark Rutland <mark.rutland@....com>, James Morse <james.morse@....com>, 
	linux-s390 <linux-s390@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Linux API <linux-api@...r.kernel.org>, "the arch/x86 maintainers" <x86@...nel.org>, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: Re: [PATCH v9 1/4] syscalls: Verify address
 limit before returning to user-mode

On Tue, May 9, 2017 at 9:00 AM, Andy Lutomirski <luto@...nel.org> wrote:
> On Tue, May 9, 2017 at 1:56 AM, Christoph Hellwig <hch@...radead.org> wrote:
>> On Tue, May 09, 2017 at 08:45:22AM +0200, Ingo Molnar wrote:
>>> We only have ~115 code blocks in the kernel that set/restore KERNEL_DS, it would
>>> be a pity to add a runtime check to every system call ...
>>
>> I think we should simply strive to remove all of them that aren't
>> in core scheduler / arch code.  Basically evetyytime we do the
>>
>>         oldfs = get_fs();
>>         set_fs(KERNEL_DS);
>>         ..
>>         set_fs(oldfs);
>>
>> trick we're doing something wrong, and there should always be better
>> ways to archive it.  E.g. using iov_iter with a ITER_KVEC type
>> consistently would already remove most of them.
>
> How about trying to remove all of them?  If we could actually get rid
> of all of them, we could drop the arch support, and we'd get faster,
> simpler, shorter uaccess code throughout the kernel.
>
> The ones in kernel/compat.c are generally garbage.  They should be
> using compat_alloc_user_space().  Ditto for kernel/power/user.c.

compat_alloc_user_space() is a hack that should go away too.  It ends
up copying the data three times.

The more efficient solution to this is to have a core syscall function
that only accesses kernel memory, and then have two front-end
functions (native and compat) that do the actual reads and writes to
userspace, with conversion in the compat case.

--
Brian Gerst

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