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Date: Thu, 11 May 2017 22:34:31 -0700
From: Kees Cook <keescook@...omium.org>
To: Martin Schwidefsky <schwidefsky@...ibm.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Thomas Garnier <thgarnie@...gle.com>,
Greg KH <greg@...ah.com>, Ingo Molnar <mingo@...nel.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, May 11, 2017 at 10:28 PM, Martin Schwidefsky
<schwidefsky@...ibm.com> wrote:
> 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".
If I'm understanding this, it won't catch corruption of addr_limit
during fast-path syscalls, though (i.e. addr_limit changed without a
call to set_fs()). :( This addr_limit corruption is mostly only a risk
archs without THREAD_INFO_IN_TASK, but it would still be nice to catch
unbalanced set_fs() code, so I like the idea. I like getting rid of
addr_limit entirely even more, but that'll take some time. :)
-Kees
--
Kees Cook
Pixel Security
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