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Date:   Thu, 11 May 2017 23:13:49 -0700
From:   Andy Lutomirski <luto@...nel.org>
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>,
        Martin Schwidefsky <schwidefsky@...ibm.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: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address
 limit before returning to user-mode

[resending because kernel.org seems to have mangled my SMTP
credentials.  I wonder if this is a common problem.]

On Thu, May 11, 2017 at 4:44 PM, 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.
>

I like this.  It wouldn't help the problem that I suspect is a major
part of the motivation for this patch: a stack overflow could
overwrite addr_limit.  But we fixed that for real already.

Slightly off-topic: I would *love* to see syscall_return_slowpath() or
similar moved or at least mostly moved into generic code.  Aside from
the fact that it used to be written in asm, there's nothing
fundamentally arch-specific about it.

>
> 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.

It won't even slow them down that much.  The slow path is reasonably
fast these days.

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