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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: 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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