kernel-hardening - Re: Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

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Date: Wed, 10 May 2017 08:27:47 +0100
From: Al Viro <viro@...IV.linux.org.uk>
To: Christoph Hellwig <hch@...radead.org>
Cc: Andy Lutomirski <luto@...nel.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>, 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 Tue, May 09, 2017 at 11:53:01PM -0700, Christoph Hellwig wrote:
> On Wed, May 10, 2017 at 04:12:54AM +0100, Al Viro wrote:
> > What's the point?  What's wrong with having kernel_read()/kernel_readv()/etc.?
> > You still have set_fs() in there; doing that one level up in call chain would
> > be just fine...  IDGI.
> 
> The problem is that they modify the address limit, which the whole
> subthread here wants to get rid of.

And you *still* do the same.  Christoph, this is ridiculous - the worst
part of the area is not a couple of functions in fs/read_write.c, it's
a fucking lot of ->read() and ->write() instances in shitty driver code,
pardon the redundance.  And _that_ is still done under set_fs(KERNEL_DS).

Claiming that set_fs() done one function deeper in callchain (both in
fs/read_write.c) is somehow better because it reduces the amount of code
under that thing...  Get real, please - helpers that encapsulate those
set_fs() pairs (a-la kernel_read(), etc.) absolutely make sense and
converting their open-coded instances to calls of those helpers is clearly
a good thing.  However, we are not
	* getting rid of low-quality code run under KERNEL_DS
	* gettind rid of set_fs() itself
	* getting a generic kernel_read() variant that would really take
an iov_iter.

That's what I'm objecting to.  Centralized kernel_readv() et.al. - sure,
and fs/read_write.c is the right place for those.  No arguments here.
Conversion to those - absolutely; drivers have no fucking business touching
set_fs() at all.  But your primitives are trouble waiting to happen.
Let them take kvec arrays.  And let them, in case when there's no
->read_iter()/->write_iter(), do set_fs().  Statically, without this
if (iter->type & ITER_KVEC) ... stuff.

> > Another delicate place: you can't assume that write() always advances
> > file position by its (positive) return value.  btrfs stuff is sensitive
> > to that.
> 
> If we don't want to assume that we need to pass pointer to pos to
> kernel_read/write.  Which might be a good idea in general.

Yes.

> > ashmem probably _is_ OK with demanding ->read_iter(), but I'm not sure
> > about blind asma->file->f_pos += ret.  That's begging for races.  Actually,
> > scratch that - it *is* racy.
> 
> I think the proper fix is to not even bother to maintain f_pos of the
> backing file, as we don't ever use it - all reads from it pass in
> an explicit position anyway.

vfs_llseek() used by ashmem_llseek()...
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