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Date:	Thu, 16 Jun 2016 14:27:15 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Andy Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 00/13] Virtually mapped stacks with guard pages (x86, core)

On Thu, Jun 16, 2016 at 11:14 AM, Andy Lutomirski <luto@...capital.net> wrote:
> Adding Paul, because RCU blew up.
>
> On Thu, Jun 16, 2016 at 10:50 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Wed, Jun 15, 2016 at 11:05 PM, Heiko Carstens
>> <heiko.carstens@...ibm.com> wrote:
>>> On Wed, Jun 15, 2016 at 05:28:22PM -0700, Andy Lutomirski wrote:
>>>> Since the dawn of time, a kernel stack overflow has been a real PITA
>>>> to debug, has caused nondeterministic crashes some time after the
>>>> actual overflow, and has generally been easy to exploit for root.
>>>>
>>>> With this series, arches can enable HAVE_ARCH_VMAP_STACK.  Arches
>>>> that enable it (just x86 for now) get virtually mapped stacks with
>>>> guard pages.  This causes reliable faults when the stack overflows.
>>>>
>>>> If the arch implements it well, we get a nice OOPS on stack overflow
>>>> (as opposed to panicing directly or otherwise exploding badly).  On
>>>> x86, the OOPS is nice, has a usable call trace, and the overflowing
>>>> task is killed cleanly.
>>>
>>> Do you have numbers which reflect the performance impact of this change?
>>>
>>
>> Hmm.  My attempt to benchmark it caused some of the vmalloc core code
>> to hang.  I'll dig around.
>
> [  488.482010] Call Trace:
> [  488.482389]  <IRQ>  [<ffffffff810da5f6>] sched_show_task+0xb6/0x110
> [  488.483341]  [<ffffffff81108c7a>] rcu_check_callbacks+0x83a/0x840
> [  488.484226]  [<ffffffff810dd48a>] ? account_system_time+0x7a/0x110
> [  488.485157]  [<ffffffff8111c0f0>] ? tick_sched_do_timer+0x30/0x30
> [  488.486133]  [<ffffffff8110d314>] update_process_times+0x34/0x60
> [  488.487050]  [<ffffffff8111bb51>] tick_sched_handle.isra.13+0x31/0x40
> [  488.488018]  [<ffffffff8111c128>] tick_sched_timer+0x38/0x70
> [  488.488853]  [<ffffffff8110db2a>] __hrtimer_run_queues+0xda/0x250
> [  488.489739]  [<ffffffff8110e263>] hrtimer_interrupt+0xa3/0x190
> [  488.490630]  [<ffffffff810952f3>] local_apic_timer_interrupt+0x33/0x50
> [  488.491660]  [<ffffffff81095d58>] smp_apic_timer_interrupt+0x38/0x50
> [  488.492644]  [<ffffffff8194d022>] apic_timer_interrupt+0x82/0x90
> [  488.493502]  [<ffffffff810ee1c0>] ? queued_spin_lock_slowpath+0x20/0x190
> [  488.494550]  [<ffffffff8194c21b>] _raw_spin_lock+0x1b/0x20
> [  488.495321]  [<ffffffff811b7a54>] find_vmap_area+0x14/0x60
> [  488.496197]  [<ffffffff811b8f69>] find_vm_area+0x9/0x20
> [  488.496922]  [<ffffffff810afb19>] account_kernel_stack+0x89/0x100
> [  488.497885]  [<ffffffff810aff76>] free_task+0x16/0x50
> [  488.498599]  [<ffffffff810b0042>] __put_task_struct+0x92/0x120
> [  488.499525]  [<ffffffff810b4a66>] delayed_put_task_struct+0x76/0x80
> [  488.500348]  [<ffffffff81107969>] rcu_process_callbacks+0x1f9/0x5e0
> [  488.501208]  [<ffffffff810b7ca1>] __do_softirq+0xf1/0x280
> [  488.501932]  [<ffffffff810b7f7e>] irq_exit+0x9e/0xa0
> [  488.502955]  [<ffffffff81095d5d>] smp_apic_timer_interrupt+0x3d/0x50
> [  488.503943]  [<ffffffff8194d022>] apic_timer_interrupt+0x82/0x90
> [  488.504886]  <EOI>  [<ffffffff8194c20b>] ? _raw_spin_lock+0xb/0x20
> [  488.505877]  [<ffffffff811b7c33>] ? __get_vm_area_node+0xc3/0x160
> [  488.506812]  [<ffffffff810e013e>] ? task_move_group_fair+0x7e/0x90
> [  488.507730]  [<ffffffff811b9360>] __vmalloc_node_range+0x70/0x280
> [  488.508689]  [<ffffffff810b1ce5>] ? _do_fork+0xc5/0x370
> [  488.509512]  [<ffffffff811ceb9b>] ? kmem_cache_alloc_node+0x7b/0x170
> [  488.510502]  [<ffffffff81327418>] ? current_has_perm+0x38/0x40
> [  488.511430]  [<ffffffff810b0501>] copy_process.part.46+0x141/0x1760
> [  488.512449]  [<ffffffff810b1ce5>] ? _do_fork+0xc5/0x370
> [  488.513285]  [<ffffffff8111faf3>] ? do_futex+0x293/0xad0
> [  488.514093]  [<ffffffff810df14a>] ? check_preempt_wakeup+0x10a/0x240
> [  488.515108]  [<ffffffff810d92c2>] ? wake_up_new_task+0xf2/0x180
> [  488.516043]  [<ffffffff810b1ce5>] _do_fork+0xc5/0x370
> [  488.516786]  [<ffffffff8112039d>] ? SyS_futex+0x6d/0x150
> [  488.517615]  [<ffffffff810b2014>] SyS_clone+0x14/0x20
> [  488.518385]  [<ffffffff81002b72>] do_syscall_64+0x52/0xb0
> [  488.519239]  [<ffffffff8194c4e1>] entry_SYSCALL64_slow_path+0x25/0x25
>
> The bug seems straightforward: vmap_area_lock is held, the RCU softirq
> fires, and vmap_area_lock recurses and deadlocks.  Lockdep agrees with
> my assessment and catches the bug immediately on boot.
>
> What's the right fix?  Change all spin_lock calls on vmap_area_lock to
> spin_lock_bh?  Somehow ask RCU not to call delayed_put_task_struct
> from bh context?  I would naively have expected RCU to only call its
> callbacks from thread context, but I was clearly wrong.

I fixed (worked around?) it by caching the vm_struct * so I can skip
calling find_vm_area.  vfree works in IRQ context.  IMO this is still
a wee bit ugly.

--Andy

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