kprobes, x86/alternatives: Use text_mutex to protect smp_alt_modules

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.

Signed-off-by: Zhou Chengming <zhouchengming1@huawei.com>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bp@suse.de
Fixes: 2cfa197 "ftrace/alternatives: Introducing *_text_reserved functions"
Link: http://lkml.kernel.org/r/1509585501-79466-1-git-send-email-zhouchengming1@huawei.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 3344d3382e91393fa7dd16ee336db3ca11a3a433..dbaf14d69ebd510b86a4a0e9e83bbd1862d99d14 100644 (file)
--- 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 9aa1cc41ecf79c8c2cc5fb9dbd3fbd418025c0dd..a17fdb63dc3e470955dcfd5e5861920b9db2749f 100644 (file)
--- 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);