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Date: Fri, 12 May 2017 07:28:02 +0200
From: Martin Schwidefsky <schwidefsky@...ibm.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Thomas Garnier <thgarnie@...gle.com>, Greg KH <greg@...ah.com>,
        Ingo
 Molnar <mingo@...nel.org>, Kees Cook <keescook@...omium.org>,
        Daniel Micay
 <danielmicay@...il.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Dave
 Hansen <dave.hansen@...el.com>, Arnd Bergmann <arnd@...db.de>,
        Thomas
 Gleixner <tglx@...utronix.de>,
        David Howells <dhowells@...hat.com>,
        René Nyffenegger <mail@...enyffenegger.ch>,
        Andrew Morton
 <akpm@...ux-foundation.org>,
        "Paul E . McKenney"
 <paulmck@...ux.vnet.ibm.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Tikhomirov
 <ptikhomirov@...tuozzo.com>,
        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 <linux-s390@...r.kernel.org>,
        LKML
 <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        "the
 arch/x86 maintainers" <x86@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org"
 <linux-arm-kernel@...ts.infradead.org>,
        Kernel Hardening
 <kernel-hardening@...ts.openwall.com>,
        Peter Zijlstra
 <a.p.zijlstra@...llo.nl>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: Re: [PATCH v9 1/4] syscalls: Verify address
 limit before returning to user-mode

On Thu, 11 May 2017 16:44:07 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Thu, May 11, 2017 at 4:17 PM, Thomas Garnier <thgarnie@...gle.com> wrote:
> >
> > Ingo: Do you want the change as-is? Would you like it to be optional?
> > What do you think?  
> 
> I'm not ingo, but I don't like that patch. It's in the wrong place -
> that system call return code is too timing-critical to add address
> limit checks.
> 
> Now what I think you *could* do is:
> 
>  - make "set_fs()" actually set a work flag in the current thread flags
> 
>  - do the test in the slow-path (syscall_return_slowpath).
> 
> Yes, yes, that ends up being architecture-specific, but it's fairly simple.
> 
> And it only slows down the system calls that actually use "set_fs()".
> Sure, it will slow those down a fair amount, but they are hopefully a
> small subset of all cases.
> 
> How does that sound to people?  Thats' where we currently do that
> 
>         if (IS_ENABLED(CONFIG_PROVE_LOCKING) &&
>             WARN(irqs_disabled(), "syscall %ld left IRQs disabled",
> regs->orig_ax))
>                 local_irq_enable();
> 
> check too, which is a fairly similar issue.

This is exactly what Heiko did for the s390 backend as a result of this
discussion. See the _CIF_ASCE_SECONDARY bit in arch/s390/kernel/entry.S,
for the hot patch the check for the bit is included in the general
_CIF_WORK test. Only the slow patch gets a bit slower.

git commit b5a882fcf146c87cb6b67c6df353e1c042b8773d
"s390: restore address space when returning to user space".

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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