From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christian Borntraeger <borntraeger@de.ibm.com>
Subject: Re: [PATCH] lkdtm: add bad USER_DS test
Date: Fri, 24 Mar 2017 16:24:28 +0100
Message-ID: <a1ab8afa-5462-dc24-2106-0a6b43773c5b@de.ibm.com>
References: <20170323203419.GA62859@beast> <20170324081450.GA5891@osiris>
 <CAJcbSZHJ1WFcE7QY6qLM9RMUJbGXxTpp3WsCNJBX4h1U-4HEjA@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Return-path: <kernel-hardening-return-7104-glkh-kernel-hardening=m.gmane.org@lists.openwall.com>
List-Post: <mailto:kernel-hardening@lists.openwall.com>
List-Help: <mailto:kernel-hardening-help@lists.openwall.com>
List-Unsubscribe: <mailto:kernel-hardening-unsubscribe@lists.openwall.com>
List-Subscribe: <mailto:kernel-hardening-subscribe@lists.openwall.com>
In-Reply-To: <CAJcbSZHJ1WFcE7QY6qLM9RMUJbGXxTpp3WsCNJBX4h1U-4HEjA@mail.gmail.com>
To: Thomas Garnier <thgarnie@google.com>, Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Kees Cook <keescook@chromium.org>, Martin Schwidefsky <schwidefsky@de.ibm.com>, David Howells <dhowells@redhat.com>, Arnd Bergmann <arnd@arndb.de>, Dave Hansen <dave.hansen@intel.com>, Al Viro <viro@zeniv.linux.org.uk>, Thomas Gleixner <tglx@linutronix.de>, =?UTF-8?Q?Ren=c3=a9_Nyffenegger?= <mail@renenyffenegger.ch>, Andrew Morton <akpm@linux-foundation.org>, "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>, Ingo Molnar <mingo@kernel.org>, Oleg Nesterov <oleg@redhat.com>, Pavel Tikhomirov <ptikhomirov@virtuozzo.com>, Stephen Smalley <sds@tycho.nsa.gov>, Ingo Molnar <mingo@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>, Andy Lutomirski <luto@kernel.org>, Paolo Bonzini <pbonzini@redhat.com>, Rik van Riel <riel@redhat.com>, Josh Poimboeuf <jpoimboe@redhat.com>, Borislav Petkov <bp@ali>
List-Id: linux-api@vger.kernel.org

On 03/24/2017 04:17 PM, Thomas Garnier wrote:
> On Fri, Mar 24, 2017 at 1:14 AM, Heiko Carstens
> <heiko.carstens@de.ibm.com> wrote:
>> On Thu, Mar 23, 2017 at 01:34:19PM -0700, Kees Cook wrote:
>>> This adds CORRUPT_USER_DS to check that the get_fs() test on syscall return
>>> still sees USER_DS during the new VERIFY_PRE_USERMODE_STATE checks.
>>>
>>> Signed-off-by: Kees Cook <keescook@chromium.org>
>>
>> ...
>>
>>> +void lkdtm_CORRUPT_USER_DS(void)
>>> +{
>>> +     /*
>>> +      * Test that USER_DS has been set correctly on exiting a syscall.
>>> +      * Since setting this higher than USER_DS (TASK_SIZE) would introduce
>>> +      * an exploitable condition, we lower it instead, since that should
>>> +      * not create as large a problem on an unprotected system.
>>> +      */
>>> +     mm_segment_t lowfs;
>>> +#ifdef MAKE_MM_SEG
>>> +     lowfs = MAKE_MM_SEG(TASK_SIZE - PAGE_SIZE);
>>> +#else
>>> +     lowfs = TASK_SIZE - PAGE_SIZE;
>>> +#endif
>>> +
>>> +     pr_info("setting bad task size limit\n");
>>> +     set_fs(lowfs);
>>> +}
>>
>> This won't work on architectures where the set_fs() argument does not
>> contain an address but an address space identifier. This is true e.g. for
>> s390 and as far as I know also for sparc.
>> On s390 we have complete distinct address spaces for kernel and user space
>> that each start at address zero.
>>
> 
> The patch that enforce USER_DS is disabled on s390 anyway. I guess, we
> can just do a set_fs(KERNEL_DS) for the others.

that would enable the test, but it would also mean that lkdtm can be used by
a program to escalate its rights. I think that is the reason why Kees did this
lowfs things.