[PATCH v2] crash_dump: release keyring reference at the correct time

restore_dm_crypt_keys_to_thread_keyring() gets a reference to the user
keyring before restoring the saved dm-crypt keys.

The same keyring reference is then passed to add_key_to_keyring() for each
saved key, but add_key_to_keyring() drops that reference on every call.
This is only balanced when exactly one key is restored. With multiple
keys, the keyring reference is dropped too many times and may trigger a
refcount underflow or use-after-free.

When more than five keys are restored, a refcount underflow/use-after-free
warning can be triggered.

The early error paths after lookup_user_key() also return without dropping
the keyring reference.

Keep ownership of the keyring reference in
restore_dm_crypt_keys_to_thread_keyring(), drop it once on all exit paths,
and make add_key_to_keyring() only use the reference without consuming it.

Fixes: 62f17d9df692 ("crash_dump: retrieve dm crypt keys in kdump kernel")
Signed-off-by: Guangshuo Li <[email protected]>
---
v2:
  - Mention the refcount underflow/use-after warning seen when restoring
    more than five keys, as suggested by Coiby Xu.
  - Order local variables in reverse Christmas tree style.

 kernel/crash_dump_dm_crypt.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/kernel/crash_dump_dm_crypt.c b/kernel/crash_dump_dm_crypt.c
index cb875ddb6ba6..c685497cd470 100644
--- a/kernel/crash_dump_dm_crypt.c
+++ b/kernel/crash_dump_dm_crypt.c
@@ -81,7 +81,6 @@ static int add_key_to_keyring(struct dm_crypt_key *dm_key,
 		kexec_dprintk("Error when adding key");
 	}
 
-	key_ref_put(keyring_ref);
 	return r;
 }
 
@@ -104,6 +103,7 @@ static int restore_dm_crypt_keys_to_thread_keyring(void)
 	struct dm_crypt_key *key;
 	size_t keys_header_size;
 	key_ref_t keyring_ref;
+	int ret = 0;
 	u64 addr;
 
 	/* find the target keyring (which must be writable) */
@@ -118,7 +118,8 @@ static int restore_dm_crypt_keys_to_thread_keyring(void)
 	dm_crypt_keys_read((char *)&key_count, sizeof(key_count), &addr);
 	if (key_count > KEY_NUM_MAX) {
 		kexec_dprintk("Failed to read the number of dm-crypt keys\n");
-		return -1;
+		ret = -1;
+		goto out;
 	}
 
 	kexec_dprintk("There are %u keys\n", key_count);
@@ -126,8 +127,10 @@ static int restore_dm_crypt_keys_to_thread_keyring(void)
 
 	keys_header_size = get_keys_header_size(key_count);
 	keys_header = kzalloc(keys_header_size, GFP_KERNEL);
-	if (!keys_header)
-		return -ENOMEM;
+	if (!keys_header) {
+		ret = -ENOMEM;
+		goto out;
+	}
 
 	dm_crypt_keys_read((char *)keys_header, keys_header_size, &addr);
 
@@ -137,7 +140,9 @@ static int restore_dm_crypt_keys_to_thread_keyring(void)
 		add_key_to_keyring(key, keyring_ref);
 	}
 
-	return 0;
+out:
+	key_ref_put(keyring_ref);
+	return ret;
 }
 
 static int read_key_from_user_keyring(struct dm_crypt_key *dm_key)
-- 
2.43.0