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Date: Fri, 12 May 2017 07:54:58 +0200
From: Martin Schwidefsky <schwidefsky@...ibm.com>
To: Kees Cook <keescook@...omium.org>
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, 11 May 2017 22:34:31 -0700
Kees Cook <keescook@...omium.org> wrote:
> 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. :)
Well for s390 there is no addr_limit as we use two separate address space
for kernel vs. user. The equivalent to the addr_limit corruption on a
fast-path syscall would be changing CR7 outside of set_fs. This boils
down to the question what we are protection against? Bad code with
unbalanced set_fs or evil code that changes addr_limit/CR7 outside of
set_fs
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.
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