linux-kernel - Re: [PATCH v4 1/4] syscalls: Restore address limit after a syscall

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Date:   Wed, 22 Mar 2017 13:54:10 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Andy Lutomirski <luto@...capital.net>,
        Thomas Garnier <thgarnie@...gle.com>
Cc:     Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Dave Hansen <dave.hansen@...el.com>,
        David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        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>,
        Vladimir Murzin <vladimir.murzin@....com>,
        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>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>, X86 ML <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 v4 1/4] syscalls: Restore address limit after a syscall

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