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SubjectRe: Oops caused by race between livepatch and ftrace
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);
+
if (!rodata_enabled)
return;

@@ -1988,6 +1993,8 @@ void set_all_modules_text_rw(void)
{
struct module *mod;

+ lockdep_assert_held(&text_mutex);
+
if (!rodata_enabled)
return;

@@ -2007,6 +2014,8 @@ void set_all_modules_text_ro(void)
{
struct module *mod;

+ lockdep_assert_held(&text_mutex);
+
if (!rodata_enabled)
return;

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index a12aff849c04..8259d4ba8b00 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -34,6 +34,7 @@
#include <linux/hash.h>
#include <linux/rcupdate.h>
#include <linux/kprobes.h>
+#include <linux/memory.h>

#include <trace/events/sched.h>

@@ -2610,10 +2611,12 @@ static void ftrace_run_update_code(int command)
{
int ret;

+ mutex_lock(&text_mutex);
+
ret = ftrace_arch_code_modify_prepare();
FTRACE_WARN_ON(ret);
if (ret)
- return;
+ goto out_unlock;

/*
* By default we use stop_machine() to modify the code.
@@ -2625,6 +2628,9 @@ static void ftrace_run_update_code(int command)

ret = ftrace_arch_code_modify_post_process();
FTRACE_WARN_ON(ret);
+
+out_unlock:
+ mutex_unlock(&text_mutex);
}

static void ftrace_run_modify_code(struct ftrace_ops *ops, int command,
@@ -5776,6 +5782,7 @@ void ftrace_module_enable(struct module *mod)
struct ftrace_page *pg;

mutex_lock(&ftrace_lock);
+ mutex_lock(&text_mutex);

if (ftrace_disabled)
goto out_unlock;
@@ -5837,6 +5844,7 @@ void ftrace_module_enable(struct module *mod)
ftrace_arch_code_modify_post_process();

out_unlock:
+ mutex_unlock(&text_mutex);
mutex_unlock(&ftrace_lock);

process_cached_mods(mod->name);
--
2.20.1
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