[PATCH] livepatch: Fix ftrace module text permissions race

From: Josh Poimboeuf
Date: Wed May 29 2019 - 15:07:48 EST

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@xxxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
kernel/livepatch/core.c | 6 ++++++
kernel/module.c | 21 ++++++++++++++++++---
kernel/trace/ftrace.c | 10 +++++++++-
3 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 2398832947c6..c4ce08f43bd6 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -18,6 +18,7 @@
#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"
@@ -718,16 +719,21 @@ 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..3c056b56aefa 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;

@@ -1953,7 +1956,7 @@ void module_disable_ro(const struct module *mod)
frob_rodata(&mod->init_layout, set_memory_rw);
}

-void module_enable_ro(const struct module *mod, bool after_init)
+static void __module_enable_ro(const struct module *mod, bool after_init)
{
if (!rodata_enabled)
return;
@@ -1974,6 +1977,13 @@ void module_enable_ro(const struct module *mod, bool after_init)
frob_ro_after_init(&mod->core_layout, set_memory_ro);
}

+void module_enable_ro(const struct module *mod, bool after_init)
+{
+ lockdep_assert_held(&text_mutex);
+
+ __module_enable_ro(mod, after_init);
+}
+
static void module_enable_nx(const struct module *mod)
{
frob_rodata(&mod->core_layout, set_memory_nx);
@@ -1988,6 +1998,8 @@ void set_all_modules_text_rw(void)
{
struct module *mod;

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

@@ -2007,6 +2019,8 @@ void set_all_modules_text_ro(void)
{
struct module *mod;

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

@@ -2027,6 +2041,7 @@ void set_all_modules_text_ro(void)
mutex_unlock(&module_mutex);
}
#else
+static void __module_enable_ro(const struct module *mod, bool after_init) { }
static void module_enable_nx(const struct module *mod) { }
#endif

@@ -3519,7 +3534,7 @@ static noinline int do_init_module(struct module *mod)
/* Switch to core kallsyms now init is done: kallsyms may be walking! */
rcu_assign_pointer(mod->kallsyms, &mod->core_kallsyms);
#endif
- module_enable_ro(mod, true);
+ __module_enable_ro(mod, true);
mod_tree_remove_init(mod);
module_arch_freeing_init(mod);
mod->init_layout.base = NULL;
@@ -3626,7 +3641,7 @@ static int complete_formation(struct module *mod, struct load_info *info)
/* This relies on module_mutex for list integrity. */
module_bug_finalize(info->hdr, info->sechdrs, mod);

- module_enable_ro(mod, false);
+ __module_enable_ro(mod, false);
module_enable_nx(mod);

/* Mark state as coming so strong_try_module_get() ignores us,
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