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Date: Wed, 10 May 2017 09:08:41 +0100
From: Al Viro <viro@...IV.linux.org.uk>
To: Arnd Bergmann <arnd@...db.de>
Cc: Andy Lutomirski <luto@...nel.org>,
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>,
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>, Brian Gerst <brgerst@...il.com>,
"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 Wed, May 10, 2017 at 09:37:04AM +0200, Arnd Bergmann wrote:
> > 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.
BTW, not all get_user() under KERNEL_DS are plain loads. There is an
exception - probe_kernel_read().
> > 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() has some problems too, it adds
> complexity to a rarely-tested code path and can add some noticeable
> overhead in cases where user space access is slow because of
> extra checks.
>
> It's clearly better than set_fs(), but the way I prefer to convert the
> code is to avoid both and instead move compat handlers next to
> the native code, and splitting out the common code between native
> and compat mode into a helper that takes a regular kernel pointer.
>
> I think that's what both Al has done in the past on compat_ioctl()
> and select() and what Christoph does in his latest series, but
> it seems worth pointing out for others that decide to help out here.
Folks, reducing the amount of places where we play with set_fs() is certainly
a good thing. Getting rid of them completely is something entirely different;
I have tried to plot out patch series in this direction many times during the
last 5 years or so, but it's not going to be easy. Tomorrow I can start
posting my notes in that direction (and there are tons of those, unfortunately
mixed with git grep results, highly unprintable personal comments, etc.);
just let me grab some sleep first...
BTW, slow userland access is not just due to extra checks; access_ok(),
in particular, is pretty much noise. The real PITA comes from the things
like STAC/CLAC on recent x86. Or hardware overhead of cross-address-space
block copy insn (e.g. on s390, where it's optimized for multi-cacheline
blocks). Or things like uml, where it's a matter of walking the page
tables for each sodding __get_user(). It's not always just a matter of
address space limit...
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