[PATCH v3] keys: Protect the update method check against type removal

key_update() reads key->type->update, ->def_datalen and ->preparse
before it takes key->sem. A reference on the key does not pin the
module that provides its type, so KEYCTL_UPDATE can race with the
unloading of that module, for example dns_resolver:

  KEYCTL_UPDATE task                  Module unload and key GC
  ------------------                  ------------------------
  keyctl_update_key()
    key_update()
      key_permission()
      load key->type
                                      exit_dns_resolver()
                                        unregister_key_type()
                                          GC: down_write(&key->sem)
                                          GC: retype to key_type_dead
                                          GC: up_write(&key->sem)
                                        unregister_key_type() returns
                                      free_module()
      read saved type's update member

The final access reads freed module memory. With the updating task
held after the key->type load, it faults:

  BUG: unable to handle page fault for address: fffffbfff804107f
  #PF: supervisor read access in kernel mode
  RIP: 0010:key_update+0x12d/0x550
  Call Trace:
   keyctl_update_key+0x6b/0x110
   __do_sys_keyctl+0x21d/0x4f0

This is the same problem that commit 925c84e3cc21 ("keys: Protect the
read method check against type removal") fixed for KEYCTL_READ.

For dns_resolver, which has no update method, key_update() returns at
the check, so doing the check with key->sem held would be enough. A
key type that is built as a module and has an update method, like
encrypted or trusted, gets past the check. If its module is unloaded
after the check and before key_update() takes key->sem, the GC has
already moved the key to key_type_dead, and key_update() calls the
update method of key_type_dead, which is NULL. So key->sem has to stay
held from the check until the update is done.

Take key->sem before the first access to key->type and hold it until
the preparsed payload has been freed. Once a key exists, only the GC
changes key->type, and it does so with key->sem held for writing.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Cen Zhang <redacted>
---
Changes in v3:
- Rename the error label to out_free_preparse and add an empty line before
  the out label.

Changes in v2:
- Refine the commit message.
- Remove the code comment in key_update().
- Drop the Assisted-by tag.

Link to v1: https://lore.kernel.org/r/pm-keys-update-type-removal-20261029-v6-3d92b2f9693854230a02@gmail.com (local)
Link to v2: https://lore.kernel.org/r/pm-keys-update-type-removal-20261029-v7-e79d113bcff91e140d04@gmail.com (local)

 security/keys/key.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/security/keys/key.c b/security/keys/key.c
index e88548f1a704..3f1f0fc13710 100644
--- a/security/keys/key.c
+++ b/security/keys/key.c

@@ -1101,9 +1101,12 @@ int key_update(key_ref_t key_ref, const void *payload, size_t plen)
 	if (ret < 0)
 		return ret;
 
+	down_write(&key->sem);
+
 	/* attempt to update it if supported */
+	ret = -EOPNOTSUPP;
 	if (!key->type->update)
-		return -EOPNOTSUPP;
+		goto out;
 
 	memset(&prep, 0, sizeof(prep));
 	prep.data = payload;

@@ -1113,11 +1116,9 @@ int key_update(key_ref_t key_ref, const void *payload, size_t plen)
 	if (key->type->preparse) {
 		ret = key->type->preparse(&prep);
 		if (ret < 0)
-			goto error;
+			goto out_free_preparse;
 	}
 
-	down_write(&key->sem);
-
 	ret = key->type->update(key, &prep);
 	if (ret == 0) {
 		/* Updating a negative key positively instantiates it */

@@ -1125,11 +1126,12 @@ int key_update(key_ref_t key_ref, const void *payload, size_t plen)
 		notify_key(key, NOTIFY_KEY_UPDATED, 0);
 	}
 
-	up_write(&key->sem);
-
-error:
+out_free_preparse:
 	if (key->type->preparse)
 		key->type->free_preparse(&prep);
+
+out:
+	up_write(&key->sem);
 	return ret;
 }
 EXPORT_SYMBOL(key_update);

base-commit: 9a2a4c51dccc3168c3dc949fcbaa360a478fa228
-- 
2.43.0