[PATCH v3 03/10] crash_dump: Disallow writing to dm-crypt configfs during kexec_file_load syscall

[Coiby Xu <coiby.xu@gmail.com>]

If writing to the configfs group happens concurrently during
kexec_file_load syscall, it may lead to the following issues,
  - buffer overflow if dm-crypt keys are added after allocation
  - stale total_keys if dm-crypt keys are removed during iteration
  - keys_header will not be freed if config/crash_dm_crypt_key/reuse is
    set true

So hold config_keys_subsys.su_mutex for the entire sequence during the
kexec_file_load syscall to ensure a consistent snapshot.

Fixes: 479e58549b0f ("crash_dump: store dm crypt keys in kdump reserved memory")
Suggested-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Coiby Xu <coiby.xu@gmail.com>
---
 kernel/crash_dump_dm_crypt.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/kernel/crash_dump_dm_crypt.c b/kernel/crash_dump_dm_crypt.c
index 4335b6cb1fc4..d2e66c6fe6f3 100644
--- a/kernel/crash_dump_dm_crypt.c
+++ b/kernel/crash_dump_dm_crypt.c
@@ -293,6 +293,7 @@ static ssize_t config_keys_reuse_show(struct config_item *item, char *page)
 static ssize_t config_keys_reuse_store(struct config_item *item,
 					   const char *page, size_t count)
 {
+	struct mutex *lock;
 	bool val;
 	int r;
 
@@ -302,8 +303,12 @@ static ssize_t config_keys_reuse_store(struct config_item *item,
 		return -EINVAL;
 	}
 
+	lock = &to_config_group(item)->cg_subsys->su_mutex;
+	mutex_lock(lock);
+
+	r = -EINVAL;
 	if (kstrtobool(page, &val) || !val)
-		return -EINVAL;
+		goto unlock;
 
 	if (is_dm_key_reused) {
 		pr_info("Already got dm-crypt keys, please continue with kexec_file_load syscall\n");
@@ -311,11 +316,15 @@ static ssize_t config_keys_reuse_store(struct config_item *item,
 		r = get_keys_from_kdump_reserved_memory();
 		if (r) {
 			pr_warn("Failed to get dm-crypt keys from reserved memory\n");
-			return r;
+			goto unlock;
 		}
 		is_dm_key_reused = true;
 	}
 
+	r = count;
+
+unlock:
+	mutex_unlock(lock);
 	return count;
 }
 
@@ -421,6 +430,8 @@ static int build_keys_header(void)
 	return 0;
 }
 
+static bool mutex_acquired;
+
 int crash_load_dm_crypt_keys(struct kimage *image)
 {
 	struct kexec_buf kbuf = {
@@ -432,6 +443,9 @@ int crash_load_dm_crypt_keys(struct kimage *image)
 	};
 	int r = 0;
 
+	mutex_lock(&config_keys_subsys.su_mutex);
+	mutex_acquired = true;
+
 	if (key_count <= 0) {
 		kexec_dprintk("No dm-crypt keys\n");
 		return 0;
@@ -481,6 +495,11 @@ void kexec_file_post_load_cleanup_dm_crypt(struct kimage *image)
 		kfree_sensitive(keys_header);
 		keys_header = NULL;
 	}
+
+	if (mutex_acquired) {
+		mutex_unlock(&config_keys_subsys.su_mutex);
+		mutex_acquired = false;
+	}
 }
 
 static int __init configfs_dmcrypt_keys_init(void)
-- 
2.55.0