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

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Message-ID: <CAMzpN2jmW3FmKXZmHNaK537oTdcrDvqatEYZk=aT2B7Qx=fEuA@mail.gmail.com>
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: [kernel-hardening] 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