Subject: Re: [PATCH v4 1/4] syscalls: Restore address limit after a syscall
To: Andy Lutomirski <luto@amacapital.net>,
        Thomas Garnier <thgarnie@google.com>
References: <20170322203834.67556-1-thgarnie@google.com>
 <CALCETrV6K8iSfAwojMFyLrbb3CG2aF4k5hPWc7EDV4ErPDq64w@mail.gmail.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Dave Hansen <dave.hansen@intel.com>,
        David Howells <dhowells@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>,
        Arnd Bergmann <arnd@arndb.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>, Thomas Gleixner <tglx@linutronix.de>,
        Oleg Nesterov <oleg@redhat.com>,
        Pavel Tikhomirov <ptikhomirov@virtuozzo.com>,
        Stephen Smalley <sds@tycho.nsa.gov>, Ingo Molnar <mingo@redhat.com>,
        Andy Lutomirski <luto@kernel.org>, Paolo Bonzini <pbonzini@redhat.com>,
        Rik van Riel <riel@redhat.com>, Kees Cook <keescook@chromium.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Brian Gerst <brgerst@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Russell King <linux@armlinux.org.uk>,
        Vladimir Murzin <vladimir.murzin@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Mark Rutland <mark.rutland@arm.com>, James Morse <james.morse@arm.com>,
        "linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>,
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        <kernel-hardening@lists.openwall.com>
From: "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <5c59808f-6518-f51b-ee71-891b71404fc8@zytor.com>
Date: Wed, 22 Mar 2017 13:54:10 -0700
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On 03/22/17 13:44, Andy Lutomirski wrote:
> 
> While it's possible that silently papering over the bug is slightly
> faster than BUGging, it will allow bugs to continue to exist
> undetected.
> 

It would also allow the test to be inlined (at least on architectures
which have a one-site implementation) and have only the failure case out
of line, with a __noreturn annotation (which allows it to be jumped to
rather than called, which is usually available as a conditional
operation whereas call often isn't.)

That is...

extern void __noreturn __pre_usermode_state_invalid(void);

static void verify_pre_usermode_state(void)
{
	if (unlikely(!segment_eq(get_fs(), USER_DS))
		__pre_usermode_state_invalid();
}

	-hpa