[PATCH v4 2/9] 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. To have serial
access to config/crash_dm_crypt_key/reuse, use the kexec lock as we also
need to access kexec_crash_image serially.

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 | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/kernel/crash_dump_dm_crypt.c b/kernel/crash_dump_dm_crypt.c
index 4335b6cb1fc4..026c7de4ad85 100644
--- a/kernel/crash_dump_dm_crypt.c
+++ b/kernel/crash_dump_dm_crypt.c
@@ -7,6 +7,7 @@
 #include <linux/configfs.h>
 #include <linux/module.h>
 #include <linux/sysfs.h>
+#include "kexec_internal.h"
 
 #define KEY_NUM_MAX 128	/* maximum dm crypt keys */
 #define KEY_SIZE_MAX 256	/* maximum dm crypt key size */
@@ -296,14 +297,20 @@ static ssize_t config_keys_reuse_store(struct config_item *item,
 	bool val;
 	int r;
 
+	if (!kexec_trylock()) {
+		r = -EBUSY;
+		goto unlock;
+	}
+
+	r = -EINVAL;
 	if (!kexec_crash_image || !kexec_crash_image->dm_crypt_keys_addr) {
 		kexec_dprintk(
 			"dm-crypt keys haven't be saved to crash-reserved memory\n");
-		return -EINVAL;
+		goto unlock;
 	}
 
 	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");
@@ -311,12 +318,15 @@ 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;
 	}
 
-	return count;
+	r = count;
+unlock:
+	kexec_unlock();
+	return r;
 }
 
 CONFIGFS_ATTR(config_keys_, reuse);
@@ -421,6 +431,8 @@ 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 +444,9 @@ 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 +496,11 @@ 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