Date:   Wed, 29 May 2019 20:34:44 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Johannes Erdfelt <johannes@...felt.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Ingo Molnar <mingo@...hat.com>, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Oops caused by race between livepatch and ftrace

+++ Josh Poimboeuf [29/05/19 12:39 -0500]:
>On Wed, May 29, 2019 at 07:29:04PM +0200, Jessica Yu wrote:
>> +++ Josh Poimboeuf [21/05/19 11:42 -0500]:
>> > On Tue, May 21, 2019 at 10:42:04AM -0400, Steven Rostedt wrote:
>> > > On Tue, 21 May 2019 09:16:29 -0500
>> > > Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>> > >
>> > > > > Hmm, this may blow up with lockdep, as I believe we already have a
>> > > > > locking dependency of:
>> > > > >
>> > > > >  text_mutex -> ftrace_lock
>> > > > >
>> > > > > And this will reverses it. (kprobes appears to take the locks in this
>> > > > > order).
>> > > > >
>> > > > > Perhaps have live kernel patching grab ftrace_lock?
>> > > >
>> > > > Where does kprobes call into ftrace with the text_mutex?  I couldn't
>> > > > find it.
>> > >
>> > > Hmm, maybe it doesn't. I was looking at the arm_kprobe_ftrace() but
>> > > it doesn't call it with text_mutex().
>> > >
>> > > Maybe it is fine, but we had better perform a lot of testing with
>> > > lockdep on to make sure.
>> >
>> > Hm.  I suppose using ftrace_lock might be less risky since that lock is
>> > only used internally by ftrace (up until now).  But I think it would
>> > also make less sense because the text_mutex is supposed to protect code
>> > patching.  And presumably ftrace_lock is supposed to be ftrace-specific.
>> >
>> > Here's the latest patch, still using text_mutex.  I added some lockdep
>> > assertions to ensure the permissions toggling functions are always
>> > called with text_mutex.  It's running through 0-day right now.  I can
>> > try to run it through various tests with CONFIG_LOCKDEP.
>> >
>> >
>> > From: Josh Poimboeuf <jpoimboe@...hat.com>
>> > Subject: [PATCH] livepatch: Fix ftrace module text permissions race
>> >
>> > It's possible for livepatch and ftrace to be toggling a module's text
>> > permissions at the same time, resulting in the following panic:
>> >
>> >  BUG: unable to handle page fault for address: ffffffffc005b1d9
>> >  #PF: supervisor write access in kernel mode
>> >  #PF: error_code(0x0003) - permissions violation
>> >  PGD 3ea0c067 P4D 3ea0c067 PUD 3ea0e067 PMD 3cc13067 PTE 3b8a1061
>> >  Oops: 0003 [#1] PREEMPT SMP PTI
>> >  CPU: 1 PID: 453 Comm: insmod Tainted: G           O  K   5.2.0-rc1-a188339ca5 #1
>> >  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-20181126_142135-anatol 04/01/2014
>> >  RIP: 0010:apply_relocate_add+0xbe/0x14c
>> >  Code: fa 0b 74 21 48 83 fa 18 74 38 48 83 fa 0a 75 40 eb 08 48 83 38 00 74 33 eb 53 83 38 00 75 4e 89 08 89 c8 eb 0a 83 38 00 75 43 <89> 08 48 63 c1 48 39 c8 74 2e eb 48 83 38 00 75 32 48 29 c1 89 08
>> >  RSP: 0018:ffffb223c00dbb10 EFLAGS: 00010246
>> >  RAX: ffffffffc005b1d9 RBX: 0000000000000000 RCX: ffffffff8b200060
>> >  RDX: 000000000000000b RSI: 0000004b0000000b RDI: ffff96bdfcd33000
>> >  RBP: ffffb223c00dbb38 R08: ffffffffc005d040 R09: ffffffffc005c1f0
>> >  R10: ffff96bdfcd33c40 R11: ffff96bdfcd33b80 R12: 0000000000000018
>> >  R13: ffffffffc005c1f0 R14: ffffffffc005e708 R15: ffffffff8b2fbc74
>> >  FS:  00007f5f447beba8(0000) GS:ffff96bdff900000(0000) knlGS:0000000000000000
>> >  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> >  CR2: ffffffffc005b1d9 CR3: 000000003cedc002 CR4: 0000000000360ea0
>> >  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> >  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> >  Call Trace:
>> >   klp_init_object_loaded+0x10f/0x219
>> >   ? preempt_latency_start+0x21/0x57
>> >   klp_enable_patch+0x662/0x809
>> >   ? virt_to_head_page+0x3a/0x3c
>> >   ? kfree+0x8c/0x126
>> >   patch_init+0x2ed/0x1000 [livepatch_test02]
>> >   ? 0xffffffffc0060000
>> >   do_one_initcall+0x9f/0x1c5
>> >   ? kmem_cache_alloc_trace+0xc4/0xd4
>> >   ? do_init_module+0x27/0x210
>> >   do_init_module+0x5f/0x210
>> >   load_module+0x1c41/0x2290
>> >   ? fsnotify_path+0x3b/0x42
>> >   ? strstarts+0x2b/0x2b
>> >   ? kernel_read+0x58/0x65
>> >   __do_sys_finit_module+0x9f/0xc3
>> >   ? __do_sys_finit_module+0x9f/0xc3
>> >   __x64_sys_finit_module+0x1a/0x1c
>> >   do_syscall_64+0x52/0x61
>> >   entry_SYSCALL_64_after_hwframe+0x44/0xa9
>> >
>> > The above panic occurs when loading two modules at the same time with
>> > ftrace enabled, where at least one of the modules is a livepatch module:
>> >
>> > CPU0					CPU1
>> > klp_enable_patch()
>> >  klp_init_object_loaded()
>> >    module_disable_ro()
>> >    					ftrace_module_enable()
>> > 					  ftrace_arch_code_modify_post_process()
>> > 				    	    set_all_modules_text_ro()
>> >      klp_write_object_relocations()
>> >        apply_relocate_add()
>> > 	  *patches read-only code* - BOOM
>> >
>> > A similar race exists when toggling ftrace while loading a livepatch
>> > module.
>> >
>> > Fix it by ensuring that the livepatch and ftrace code patching
>> > operations -- and their respective permissions changes -- are protected
>> > by the text_mutex.
>> >
>> > Reported-by: Johannes Erdfelt <johannes@...felt.com>
>> > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
>> > ---
>> > kernel/livepatch/core.c |  6 ++++++
>> > kernel/module.c         |  9 +++++++++
>> > kernel/trace/ftrace.c   | 10 +++++++++-
>> > 3 files changed, 24 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
>> > index 91cd519756d3..2d17e6e364b5 100644
>> > --- a/kernel/livepatch/core.c
>> > +++ b/kernel/livepatch/core.c
>> > @@ -30,6 +30,7 @@
>> > #include <linux/elf.h>
>> > #include <linux/moduleloader.h>
>> > #include <linux/completion.h>
>> > +#include <linux/memory.h>
>> > #include <asm/cacheflush.h>
>> > #include "core.h"
>> > #include "patch.h"
>> > @@ -730,16 +731,21 @@ static int klp_init_object_loaded(struct klp_patch *patch,
>> > 	struct klp_func *func;
>> > 	int ret;
>> >
>> > +	mutex_lock(&text_mutex);
>> > +
>> > 	module_disable_ro(patch->mod);
>> > 	ret = klp_write_object_relocations(patch->mod, obj);
>> > 	if (ret) {
>> > 		module_enable_ro(patch->mod, true);
>> > +		mutex_unlock(&text_mutex);
>> > 		return ret;
>> > 	}
>> >
>> > 	arch_klp_init_object_loaded(patch, obj);
>> > 	module_enable_ro(patch->mod, true);
>> >
>> > +	mutex_unlock(&text_mutex);
>> > +
>> > 	klp_for_each_func(obj, func) {
>> > 		ret = klp_find_object_symbol(obj->name, func->old_name,
>> > 					     func->old_sympos,
>> > diff --git a/kernel/module.c b/kernel/module.c
>> > index 6e6712b3aaf5..4d9f3281c0c5 100644
>> > --- a/kernel/module.c
>> > +++ b/kernel/module.c
>> > @@ -64,6 +64,7 @@
>> > #include <linux/bsearch.h>
>> > #include <linux/dynamic_debug.h>
>> > #include <linux/audit.h>
>> > +#include <linux/memory.h>
>> > #include <uapi/linux/module.h>
>> > #include "module-internal.h"
>> >
>> > @@ -1943,6 +1944,8 @@ static void frob_writable_data(const struct module_layout *layout,
>> > /* livepatching wants to disable read-only so it can frob module. */
>> > void module_disable_ro(const struct module *mod)
>> > {
>> > +	lockdep_assert_held(&text_mutex);
>> > +
>> > 	if (!rodata_enabled)
>> > 		return;
>> >
>> > @@ -1955,6 +1958,8 @@ void module_disable_ro(const struct module *mod)
>> >
>> > void module_enable_ro(const struct module *mod, bool after_init)
>> > {
>> > +	lockdep_assert_held(&text_mutex);
>>
>> Hi Josh!
>>
>> The lockdep WARN_ON triggers when loading a module under a lockdep enabled kernel:
>>
>> [    6.139583] WARNING: CPU: 0 PID: 102 at /home/ppyu/jeyu-linux/kernel/module.c:1961 module_enable_ro+0x121/0x130
>> [    6.143641] Modules linked in:
>> [    6.144879] CPU: 0 PID: 102 Comm: insmod Not tainted 5.2.0-rc2+ #1
>> [    6.147325] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.0-0-g63451fc-prebuilt.qemu-project.org 04/01/2014
>> [    6.152046] RIP: 0010:module_enable_ro+0x121/0x130
>> [    6.153986] Code: 48 c7 c6 70 97 05 81 5d 41 5c 41 5d e9 b8 e2 ff ff be ff ff ff ff 48 c7 c7 40 b2 65 82 e8 a7 c7 fa ff 85 c0 0f 85 f8 fe ff ff <0f> 0b e9 f1 fe ff ff 0f 1f 84 00 00 00 00 00 8b 05 9e a5 66 01 55
>> [    6.161277] RSP: 0018:ffffc9000041fd18 EFLAGS: 00010246
>> [    6.162919] RAX: 0000000000000000 RBX: ffffffffc0002000 RCX: 000000000000000b
>> [    6.164783] RDX: ffff88803d793fc0 RSI: ffffffff8265b240 RDI: ffff88803d7947e8
>> [    6.166820] RBP: 0000000000000000 R08: 000000000007ade8 R09: ffffffff823989a6
>> [    6.168579] R10: ffffc9000049c8e8 R11: 0000000000000002 R12: ffffffffc00021c0
>> [    6.169994] R13: 0000000000000000 R14: 0000000000000030 R15: ffffc9000041fe78
>> [    6.171390] FS:  00007fdc49708700(0000) GS:ffff88803d800000(0000) knlGS:0000000000000000
>> [    6.173007] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [    6.174047] CR2: 00007fdc496c0000 CR3: 000000003fb84000 CR4: 00000000000006f0
>> [    6.175168] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> [    6.176326] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> [    6.177384] Call Trace:
>> [    6.177730]  load_module+0x1b96/0x2620
>> [    6.178231]  ? rcu_read_lock_sched_held+0x53/0x60
>> [    6.178865]  ? __alloc_pages_nodemask+0x2fa/0x350
>> [    6.179497]  ? __do_sys_init_module+0x135/0x170
>> [    6.180097]  ? _cond_resched+0x10/0x40
>> [    6.180594]  __do_sys_init_module+0x135/0x170
>> [    6.181177]  do_syscall_64+0x4b/0x1c0
>> [    6.181681]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> It triggers since we call module_enable_ro() along the load_module()
>> path to prepare for a coming module. We don't take the text_mutex
>> there since we're not modifying any text, just memory protections.
>> Leaving the lockdep assert in module_disable_ro() and
>> set_all_modules_text_*() should be fine though, since I think
>> livepatch and ftrace are the only users of those functions.
>
>Yeah, I discovered that already:
>
>  https://lkml.kernel.org/r/20190522130014.yvkbio62meatqvwf@treble

Whoops, I missed that in the thread /o\

>The new patch (which I will hopefully be able to post soon) will fix
>that up.

Sounds good, thanks!

Jessica

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