[PATCH v2 3/9] 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.

Fixes: 479e58549b0f ("crash_dump: store dm crypt keys in kdump reserved memory")
Signed-off-by: Coiby Xu <coiby.xu@gmail.com>
---
 kernel/crash_dump_dm_crypt.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/crash_dump_dm_crypt.c b/kernel/crash_dump_dm_crypt.c
index 4d8a3331bbe7..6377ee86ec50 100644
--- a/kernel/crash_dump_dm_crypt.c
+++ b/kernel/crash_dump_dm_crypt.c
@@ -429,6 +429,7 @@ int crash_load_dm_crypt_keys(struct kimage *image)
 	};
 	int r = 0;
 
+	mutex_lock(&config_keys_subsys.su_mutex);
 
 	if (key_count <= 0) {
 		kexec_dprintk("No dm-crypt keys\n");
@@ -479,6 +480,9 @@ void kexec_file_post_load_cleanup_dm_crypt(struct kimage *image)
 		kfree_sensitive(keys_header);
 		keys_header = NULL;
 	}
+
+	if (mutex_is_locked(&config_keys_subsys.su_mutex))
+		mutex_unlock(&config_keys_subsys.su_mutex);
 }
 
 static int __init configfs_dmcrypt_keys_init(void)
-- 
2.54.0