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Date: Fri, 24 Mar 2017 09:14:50 +0100
From: Heiko Carstens <heiko.carstens@...ibm.com>
To: Kees Cook <keescook@...omium.org>
Cc: Thomas Garnier <thgarnie@...gle.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>,
        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 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.

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