[PATCH v4] kprobes, x86/alternatives: use text_mutex to protect smp_alt_modules

[Zhou Chengming]

Changes:
- Add a comment about text_mutex protecting this on x86.
- Add a lockdep_assert_held() check.

Fixes: 2cfa197 "ftrace/alternatives: Introducing *_text_reserved
functions"

We use alternatives_text_reserved() to check if the address is in
the fixed pieces of alternative reserved, but the problem is that
we don't hold the smp_alt mutex when call this function. So the list
traversal may encounter a deleted list_head if another path is doing
alternatives_smp_module_del().

One solution is that we can hold smp_alt mutex before call this
function, but the difficult point is that the callers of this
functions, arch_prepare_kprobe() and arch_prepare_optimized_kprobe(),
are called inside the text_mutex. So we must hold smp_alt mutex
before we go into these arch dependent code. But we can't now,
the smp_alt mutex is the arch dependent part, only x86 has it.
Maybe we can export another arch dependent callback to solve this.

But there is a simpler way to handle this problem. We can reuse the
text_mutex to protect smp_alt_modules instead of using another mutex.
And all the arch dependent checks of kprobes are inside the text_mutex,
so it's safe now.

Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Acked-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Zhou Chengming <zhouchengming1@xxxxxxxxxx>
---
 arch/x86/kernel/alternative.c | 26 +++++++++++++-------------
 kernel/extable.c              |  2 ++
 2 files changed, 15 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 3344d33..3ad92fe 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -442,7 +442,6 @@ static void alternatives_smp_lock(const s32 *start, const s32 *end,
 {
 	const s32 *poff;
 
-	mutex_lock(&text_mutex);
 	for (poff = start; poff < end; poff++) {
 		u8 *ptr = (u8 *)poff + *poff;
 
@@ -452,7 +451,6 @@ static void alternatives_smp_lock(const s32 *start, const s32 *end,
 		if (*ptr == 0x3e)
 			text_poke(ptr, ((unsigned char []){0xf0}), 1);
 	}
-	mutex_unlock(&text_mutex);
 }
 
 static void alternatives_smp_unlock(const s32 *start, const s32 *end,
@@ -460,7 +458,6 @@ static void alternatives_smp_unlock(const s32 *start, const s32 *end,
 {
 	const s32 *poff;
 
-	mutex_lock(&text_mutex);
 	for (poff = start; poff < end; poff++) {
 		u8 *ptr = (u8 *)poff + *poff;
 
@@ -470,7 +467,6 @@ static void alternatives_smp_unlock(const s32 *start, const s32 *end,
 		if (*ptr == 0xf0)
 			text_poke(ptr, ((unsigned char []){0x3E}), 1);
 	}
-	mutex_unlock(&text_mutex);
 }
 
 struct smp_alt_module {
@@ -489,8 +485,7 @@ struct smp_alt_module {
 	struct list_head next;
 };
 static LIST_HEAD(smp_alt_modules);
-static DEFINE_MUTEX(smp_alt);
-static bool uniproc_patched = false;	/* protected by smp_alt */
+static bool uniproc_patched = false;	/* protected by text_mutex */
 
 void __init_or_module alternatives_smp_module_add(struct module *mod,
 						  char *name,
@@ -499,7 +494,7 @@ void __init_or_module alternatives_smp_module_add(struct module *mod,
 {
 	struct smp_alt_module *smp;
 
-	mutex_lock(&smp_alt);
+	mutex_lock(&text_mutex);
 	if (!uniproc_patched)
 		goto unlock;
 
@@ -526,14 +521,14 @@ void __init_or_module alternatives_smp_module_add(struct module *mod,
 smp_unlock:
 	alternatives_smp_unlock(locks, locks_end, text, text_end);
 unlock:
-	mutex_unlock(&smp_alt);
+	mutex_unlock(&text_mutex);
 }
 
 void __init_or_module alternatives_smp_module_del(struct module *mod)
 {
 	struct smp_alt_module *item;
 
-	mutex_lock(&smp_alt);
+	mutex_lock(&text_mutex);
 	list_for_each_entry(item, &smp_alt_modules, next) {
 		if (mod != item->mod)
 			continue;
@@ -541,7 +536,7 @@ void __init_or_module alternatives_smp_module_del(struct module *mod)
 		kfree(item);
 		break;
 	}
-	mutex_unlock(&smp_alt);
+	mutex_unlock(&text_mutex);
 }
 
 void alternatives_enable_smp(void)
@@ -551,7 +546,7 @@ void alternatives_enable_smp(void)
 	/* Why bother if there are no other CPUs? */
 	BUG_ON(num_possible_cpus() == 1);
 
-	mutex_lock(&smp_alt);
+	mutex_lock(&text_mutex);
 
 	if (uniproc_patched) {
 		pr_info("switching to SMP code\n");
@@ -563,10 +558,13 @@ void alternatives_enable_smp(void)
 					      mod->text, mod->text_end);
 		uniproc_patched = false;
 	}
-	mutex_unlock(&smp_alt);
+	mutex_unlock(&text_mutex);
 }
 
-/* Return 1 if the address range is reserved for smp-alternatives */
+/*
+ * Return 1 if the address range is reserved for smp-alternatives.
+ * Must hold text_mutex.
+ */
 int alternatives_text_reserved(void *start, void *end)
 {
 	struct smp_alt_module *mod;
@@ -574,6 +572,8 @@ int alternatives_text_reserved(void *start, void *end)
 	u8 *text_start = start;
 	u8 *text_end = end;
 
+	lockdep_assert_held(&text_mutex);
+
 	list_for_each_entry(mod, &smp_alt_modules, next) {
 		if (mod->text > text_end || mod->text_end < text_start)
 			continue;
diff --git a/kernel/extable.c b/kernel/extable.c
index 9aa1cc4..97c4406 100644
--- a/kernel/extable.c
+++ b/kernel/extable.c
@@ -31,6 +31,8 @@
  * mutex protecting text section modification (dynamic code patching).
  * some users need to sleep (allocating memory...) while they hold this lock.
  *
+ * Note: Also protects smp alternatives modification on x86.
+ *
  * NOT exported to modules - patching kernel text is a really delicate matter.
  */
 DEFINE_MUTEX(text_mutex);
-- 
1.8.3.1