Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

From: Andy Lutomirski <luto@amacapital.net>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
	Borislav Petkov <bp@alien8.de>, Nadav Amit <nadav.amit@gmail.com>,
	Kees Cook <keescook@chromium.org>,
	Brian Gerst <brgerst@gmail.com>,
	"kernel-hardening@lists.openwall.com" 
	<kernel-hardening@lists.openwall.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>, Jann Horn <jann@thejh.net>,
	Heiko Carstens <heiko.carstens@de.ibm.com>
Subject: Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)
Date: Thu, 23 Jun 2016 12:08:59 -0700	[thread overview]
Message-ID: <CALCETrUkMKLZfCfk7GF_EX4xrxwPfGMoWKR2FGMmjYYqH_yk_A@mail.gmail.com> (raw)
In-Reply-To: <CA+55aFyYruc_7Ax-hjh96zXPWmJJpCqm0yqq-fYpT094owSO_Q@mail.gmail.com>

On Thu, Jun 23, 2016 at 11:46 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, Jun 23, 2016 at 10:52 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> Ugh. Looking around at this, it turns out that a great example of this
>> kind of legacy issue is the debug_mutex stuff.
>
> Interestingly, the *only* other user of ti->task for a full
> allmodconfig build of x86-64 seems to be
>
>   arch/x86/kernel/dumpstack.c
>
> with the print_context_stack() -> print_ftrace_graph_addr() -> task =
> tinfo->task chain.
>
> And that doesn't really seem to want thread_info either. The callers
> all have 'task', and have to generate thread_info from that anyway.
>
> So this attached patch (which includes the previous one) seems to
> build. I didn't actually boot it, but there should be no users left
> unless there is some asm code that has hardcoded offsets..

I think you'll break some architectures when you remove the
initialization of ti->task.  That either needs to be pushed down into
arch code in unicore32, openrisc, microblaze, powerpx, xtensa, sparc,
parisc, arm, mips, s390, and whatever I missed, or you should leave
the field initialized and existing and wait for my patch to
conditionally remove/embed thread_info to get rid of the
initialization part.

On the C side, there's:

arm's contextidr_notifier (easily fixable)

sh's irqctx->tinfo.task = curctx->task; (probably useless) and
print_ftrace_graph_addr

cris's ugdb_trap_user (probably easily fixable)

sparc's arch_trigger_all_cpu_backtrace (possibly quite hard to fix)

sparc's flush_thread (trivial)

sparc's __save_stack_trace (not sure)

unicore's __die (probably easy)

metag's do_softirq_own_stack (not sure if it's useful)

I found these with this coccinelle script:

@@
struct thread_info *ti;
@@

* ti->task

$ spatch --sp-file titask.cocci --dir .