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Date: Tue, 9 May 2017 12:05:12 -0400
From: Brian Gerst <>
To: Andy Lutomirski <>
Cc: Christoph Hellwig <>, Ingo Molnar <>, Greg KH <>, 
	Thomas Garnier <>, Martin Schwidefsky <>, 
	Heiko Carstens <>, Dave Hansen <>, 
	Arnd Bergmann <>, Thomas Gleixner <>, David Howells <>, 
	René Nyffenegger <>, 
	Andrew Morton <>, 
	"Paul E . McKenney" <>, "Eric W . Biederman" <>, 
	Oleg Nesterov <>, Pavel Tikhomirov <>, 
	Ingo Molnar <>, "H . Peter Anvin" <>, Paolo Bonzini <>, 
	Rik van Riel <>, Kees Cook <>, 
	Josh Poimboeuf <>, Borislav Petkov <>, 
	"Kirill A . Shutemov" <>, 
	Christian Borntraeger <>, Russell King <>, 
	Will Deacon <>, Catalin Marinas <>, 
	Mark Rutland <>, James Morse <>, 
	linux-s390 <>, LKML <>, 
	Linux API <>, "the arch/x86 maintainers" <>, 
	"" <>, 
	Kernel Hardening <>, 
	Linus Torvalds <>, Peter Zijlstra <>
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 <> wrote:
> On Tue, May 9, 2017 at 1:56 AM, Christoph Hellwig <> 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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