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Date: Fri, 10 Feb 2017 21:49:54 +0000
From: Russell King - ARM Linux <linux@...linux.org.uk>
To: Kees Cook <keescook@...omium.org>
Cc: Mark Rutland <mark.rutland@....com>,
Stephen Bates <stephen.bates@...s.com>,
"linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
Arnd Bergmann <arnd@...db.de>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Linux API <linux-api@...r.kernel.org>,
Will Deacon <will.deacon@....com>,
LKML <linux-kernel@...r.kernel.org>,
Andy Lutomirski <luto@...capital.net>,
Dave Hansen <dave.hansen@...el.com>, Jeff Moyer <jmoyer@...hat.com>,
René Nyffenegger <mail@...enyffenegger.ch>,
Milosz Tanski <milosz@...in.com>,
Thomas Garnier <thgarnie@...gle.com>,
"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC] syscalls: Restore address limit after a syscall
On Fri, Feb 10, 2017 at 12:49:34PM -0800, Kees Cook wrote:
> On Fri, Feb 10, 2017 at 11:22 AM, Russell King - ARM Linux
> <linux@...linux.org.uk> wrote:
> > On Thu, Feb 09, 2017 at 06:42:34PM -0800, Andy Lutomirski wrote:
> >> On Thu, Feb 9, 2017 at 3:41 PM, Thomas Garnier <thgarnie@...gle.com> wrote:
> >> > So by default it is in the wrapper. If selected, an architecture can
> >> > disable the wrapper put it in the best places. Understood correctly?
> >>
> >> Sounds good to me.
> >>
> >> Presumably the result should go through -mm. Want to cc: akpm and
> >> linux-arch@ on the next version?
> >>
> >> I've also cc'd arm and s390 folks -- those are the other arches that
> >> try to be on top of hardening.
> >
> > The best place for this on ARM is in the assembly code, rather than in
> > the hundreds of system calls - having it in one place is surely better
> > for reducing the cache impact.
> >
> > This (untested) patch should be sufficient for ARM - there's two choices
> > which I think make sense to do this:
> > 1. Immediately after returning the syscall
> > 2. Immediately before any returning to userspace
> >
> > (1) has the advantage that the address limit will be forced for the
> > exit-path works that we do, preventing those making accesses to kernel
> > space.
> >
> > (2) has the advantage that we'd guarantee that the address limit will
> > be forced while userspace is running for the next entry into kernel
> > space.
> >
> > There's actually a third option as well:
> >
> > (3) forcing the address limit on entry to the kernel from userspace.
> >
> > This patch implements option 1.
> >
> > arch/arm/kernel/entry-common.S | 6 ++++++
> > 1 files changed, 6 insertions(+)
> >
> > diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
> > index eb5cd77bf1d8..6a717a2ccb88 100644
> > --- a/arch/arm/kernel/entry-common.S
> > +++ b/arch/arm/kernel/entry-common.S
> > @@ -39,6 +39,8 @@
> > ret_fast_syscall:
> > UNWIND(.fnstart )
> > UNWIND(.cantunwind )
> > + mov r1, #TASK_SIZE
> > + str r1, [tsk, #TI_ADDR_LIMIT]
> > disable_irq_notrace @ disable interrupts
> > ldr r1, [tsk, #TI_FLAGS] @ re-check for syscall tracing
> > tst r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
> > @@ -64,6 +66,8 @@ ENDPROC(ret_fast_syscall)
> > ret_fast_syscall:
> > UNWIND(.fnstart )
> > UNWIND(.cantunwind )
> > + mov r1, #TASK_SIZE
> > + str r1, [tsk, #TI_ADDR_LIMIT]
> > str r0, [sp, #S_R0 + S_OFF]! @ save returned r0
> > disable_irq_notrace @ disable interrupts
> > ldr r1, [tsk, #TI_FLAGS] @ re-check for syscall tracing
> > @@ -262,6 +266,8 @@ ENDPROC(vector_swi)
> > b ret_slow_syscall
> >
> > __sys_trace_return:
> > + mov r1, #TASK_SIZE
> > + str r1, [tsk, #TI_ADDR_LIMIT]
> > str r0, [sp, #S_R0 + S_OFF]! @ save returned r0
> > mov r0, sp
> > bl syscall_trace_exit
> >
>
> That looks pretty great! If I'm reading the macros correctly, this'll
> only happen on _actual_ syscall exit, right? So all the crazy OABI
> stuff won't suddenly break? e.g.:
Correct.
> Is there a similarly good place to do this for arm64?
I'd imagine similar places exist in arm64:
ret_fast_syscall
ret_to_user
maybe?
--
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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