linux-kernel - Re: [PATCH] lkdtm: add bad USER

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Date:   Fri, 24 Mar 2017 16:24:28 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Thomas Garnier <thgarnie@...gle.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>
Cc:     Kees Cook <keescook@...omium.org>,
        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 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.