Re: Oops caused by race between livepatch and ftrace

[Jessica Yu <jeyu@kernel.org>]

+++ 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@redhat.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@redhat.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@erdfelt.com>
>Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.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.

Thanks,

Jessica