kernel-hardening - Re: [PATCH v2 4/4] arm64/syscalls: Specific usage of verify_pre_usermode

Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CAJcbSZFbcGrAEoV8KpYKH-6PgpFDQ-cBt_P7MqX2_hShAPjvvg@mail.gmail.com>
Date: Thu, 9 Mar 2017 08:19:31 -0800
From: Thomas Garnier <thgarnie@...gle.com>
To: Mark Rutland <mark.rutland@....com>
Cc: David Howells <dhowells@...hat.com>, Dave Hansen <dave.hansen@...el.com>, 
	Arnd Bergmann <arnd@...db.de>, Al Viro <viro@...iv.linux.org.uk>, 
	René Nyffenegger <mail@...enyffenegger.ch>, 
	Andrew Morton <akpm@...ux-foundation.org>, Kees Cook <keescook@...omium.org>, 
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>, "David S . Miller" <davem@...emloft.net>, 
	Andy Lutomirski <luto@...nel.org>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, 
	Nicolas Pitre <nicolas.pitre@...aro.org>, Petr Mladek <pmladek@...e.com>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Sergey Senozhatsky <sergey.senozhatsky@...il.com>, 
	Helge Deller <deller@....de>, Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...nel.org>, 
	Oleg Nesterov <oleg@...hat.com>, John Stultz <john.stultz@...aro.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Pavel Tikhomirov <ptikhomirov@...tuozzo.com>, 
	Frederic Weisbecker <fweisbec@...il.com>, Stephen Smalley <sds@...ho.nsa.gov>, 
	Stanislav Kinsburskiy <skinsbursky@...tuozzo.com>, Ingo Molnar <mingo@...hat.com>, 
	"H . Peter Anvin" <hpa@...or.com>, Paolo Bonzini <pbonzini@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Josh Poimboeuf <jpoimboe@...hat.com>, Brian Gerst <brgerst@...il.com>, Jan Beulich <JBeulich@...e.com>, 
	Christian Borntraeger <borntraeger@...ibm.com>, "Luis R . Rodriguez" <mcgrof@...nel.org>, 
	He Chen <he.chen@...ux.intel.com>, Russell King <linux@...linux.org.uk>, 
	Will Deacon <will.deacon@....com>, Catalin Marinas <catalin.marinas@....com>, 
	James Morse <james.morse@....com>, Pratyush Anand <panand@...hat.com>, 
	Vladimir Murzin <vladimir.murzin@....com>, Chris Metcalf <cmetcalf@...lanox.com>, 
	Andre Przywara <andre.przywara@....com>, Linux API <linux-api@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, "the arch/x86 maintainers" <x86@...nel.org>, 
	linux-arm-kernel@...ts.infradead.org, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2 4/4] arm64/syscalls: Specific usage of verify_pre_usermode_state

On Thu, Mar 9, 2017 at 8:05 AM, Mark Rutland <mark.rutland@....com> wrote:
> On Thu, Mar 09, 2017 at 07:56:49AM -0800, Thomas Garnier wrote:
>> On Thu, Mar 9, 2017 at 4:23 AM, Mark Rutland <mark.rutland@....com> wrote:
>> > We generally stick to lower case for the arm64 assembly macros. If we
>> > need this, we should stick to the existing convention.
>> >
>> >> +/* Similar to set_fs(USER_DS) in verify_pre_usermode_state without a warning. */
>> >> +.macro VERIFY_PRE_USERMODE_STATE
>> >> +     mov     x1, #TASK_SIZE_64
>> >> +     str     x1, [tsk, #TSK_TI_ADDR_LIMIT]
>> >> +.endm
>> >
>> > We need arm64's set_fs() to configure UAO, too, so this is much weaker
>> > than set_fs(), and will leave __{get,put}_user and
>> > __copy_{to,from}_user() able to access kernel memory.
>> >
>> > We don't currently have an asm helper to clear UAO, and unconditionally
>> > poking that on exception return is liable to be somewhat expensive.
>> >
>> > Also, given we're only trying to catch this in syscalls, I'm afraid I
>> > don't see what we gain by doing this in the entry assembly.
>>
>> I optimized all architectures from the arm (32-bit) discussion. I will
>> come back to a simple bl to the verify function. Thanks!
>
> What I was trying to ask was do we need to touch the assembly at all
> here?

You don't but he generic solution add code to every single syscall.

> Are we trying to protect the non-syscall cases by doing this in
> assembly? If so, it'd be worth calling out in the commit message.

It is an added benefit but not required.

> If so, we could add the necessary helper to clear UAO.

I can look at set_fs and fix it on the next iteraiton.

> If not, doing this in the entry assembly only saves the small overhead
> of reading and comparing the addr_limit for in-kernel use of the
> syscalls (e.g. in the compat wrappers), and we may as well rely on the
> common !ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE implementation.

You also don't have the code added for each syscall and a call.

>
> Thanks,
> Mark.



-- 
Thomas
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.