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

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Date:   Fri, 24 Mar 2017 08:17:11 -0700
From:   Thomas Garnier <thgarnie@...gle.com>
To:     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>,
        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@...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 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.


-- 
Thomas