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Date: Fri, 24 Mar 2017 10:46:41 -0700
From: Kees Cook <keescook@...omium.org>
To: Christian Borntraeger <borntraeger@...ibm.com>
Cc: Thomas Garnier <thgarnie@...gle.com>, Heiko Carstens <heiko.carstens@...ibm.com>, 
	Martin Schwidefsky <schwidefsky@...ibm.com>, David Howells <dhowells@...hat.com>, 
	Arnd Bergmann <arnd@...db.de>, Dave Hansen <dave.hansen@...el.com>, Al Viro <viro@...iv.linux.org.uk>, 
	Thomas Gleixner <tglx@...utronix.de>, René Nyffenegger <mail@...enyffenegger.ch>, 
	Andrew Morton <akpm@...ux-foundation.org>, 
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>, Ingo Molnar <mingo@...nel.org>, 
	Oleg Nesterov <oleg@...hat.com>, Pavel Tikhomirov <ptikhomirov@...tuozzo.com>, 
	Stephen Smalley <sds@...ho.nsa.gov>, Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>, 
	Andy Lutomirski <luto@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>, Rik van Riel <riel@...hat.com>, 
	Josh Poimboeuf <jpoimboe@...hat.com>, Borislav Petkov <bp@...en8.de>, Brian Gerst <brgerst@...il.com>, 
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.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@...r.kernel.org" <linux-s390@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Linux API <linux-api@...r.kernel.org>, "x86@...nel.org" <x86@...nel.org>, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH] lkdtm: add bad USER_DS test

On Fri, Mar 24, 2017 at 8:24 AM, Christian Borntraeger
<borntraeger@...ibm.com> wrote:
> On 03/24/2017 04:17 PM, Thomas Garnier wrote:
>> On Fri, Mar 24, 2017 at 1:14 AM, Heiko Carstens
>> <heiko.carstens@...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@...omium.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.

Yeah, but it seems like getting this right for all architectures isn't
sane. I'm going to change it to use KERNEL_DS but also post a SIGKILL
to the process. That way it'll still trigger the syscall return
checking, but will be unable to continue running with a potentially
uncaught KERNEL_DS.

-Kees

-- 
Kees Cook
Pixel Security

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