From mboxrd@z Thu Jan  1 00:00:00 1970
From: Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
Subject: Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address
 limit before returning to user-mode
Date: Fri, 12 May 2017 14:17:19 -0700
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To: Al Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
Cc: Russell King - ARM Linux <linux-I+IVW8TIWO2tmTQ+vhA3Yw@public.gmane.org>, Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>, Kernel Hardening <kernel-hardening-ZwoEplunGu1jrUoiu81ncdBPR1lH4CV8@public.gmane.org>, Greg KH <greg-U8xfFu+wG4EAvxtiuMwx3w@public.gmane.org>, Heiko Carstens <heiko.carstens-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>, LKML <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, David Howells <dhowells-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Dave Hansen <dave.hansen-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>, "H . Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>, Ingo Molnar <mingo-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Pavel Tikhomirov <ptikhomirov-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>, linux-s390 <linux-s390-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, the arch/x86 maintainers <x86-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>, Christian Borntraeger <borntraeger-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>, =?UTF-8?Q?Ren=C3=A9_Nyffenegger?= <mail-gLCNRsNSrVdVZEhyV+6z5nIPMjoJpjVV@public.gmane.org>, Catalin Marinas <catalin.marinas-5wv7dgnIgG8@public.gmane.org>P
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On Fri, May 12, 2017 at 2:06 PM, Al Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org> wrote:
> On Fri, May 12, 2017 at 09:21:06PM +0100, Russell King - ARM Linux wrote:
>> On Fri, May 12, 2017 at 12:30:02PM -0700, Kees Cook wrote:
>> > I'm clearly not explaining things well enough. I shouldn't say
>> > "corruption", I should say "malicious manipulation". The methodology
>> > of attacks against the stack are quite different from the other kinds
>> > of attacks like use-after-free, heap overflow, etc. Being able to
>> > exhaust the kernel stack (either due to deep recursion or unbounded
>> > alloca())
>>
>> I really hope we don't have alloca() use in the kernel.  Do you have
>> evidence to support that assertion?
>>
>> IMHO alloca() (or similar) should not be present in any kernel code
>> because we have a limited stack - we have kmalloc() etc for that kind
>> of thing.
>
> No alloca(), but there are VLAs.  Said that, the whole "what if they
> can bugger thread_info and/or task_struct and go after set_fs() state"
> is idiocy, of course - in that case the box is fucked, no matter what.

Two things are at risk from stack exhaustion: thread_info (mainly
addr_limit) when on the stack (fixed by THREAD_INFO_IN_TASK), and
overflow into adjacent allocations (fixed by VMAP_STACK). The latter
is fundamentally a heap overflow.

-Kees

-- 
Kees Cook
Pixel Security