Return-Path: Received: from mail-pg0-f67.google.com ([74.125.83.67]:38876 "EHLO mail-pg0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752105AbdHPVRR (ORCPT ); Wed, 16 Aug 2017 17:17:17 -0400 From: Eric Biggers To: keyrings@vger.kernel.org Cc: David Howells , linux-api@vger.kernel.org, linux-cifs@vger.kernel.org, linux-fscrypt@vger.kernel.org, linux-nfs@vger.kernel.org, Gwendal Grignou , Jaegeuk Kim , Paul Crowley , Richard Weinberger , Ryo Hashimoto , Tyler Hicks , stable@vger.kernel.org Subject: [PATCH v2] KEYS: make keyctl_invalidate() also require Setattr permission Date: Wed, 16 Aug 2017 14:14:03 -0700 Message-Id: <20170816211403.121920-1-ebiggers3@gmail.com> Sender: linux-nfs-owner@vger.kernel.org List-ID: From: Eric Biggers Currently, a process only needs Search permission on a key to invalidate it with keyctl_invalidate(), which has an effect similar to unlinking it from all keyrings. Unfortunately, this significantly compromises the keys permission system because as a result, there is no way to grant read-only access to keys without also granting de-facto "delete" access. Even worse, processes may invalidate entire _keyrings_, given only permission to "search" them. It is not even possible to block this using SELinux, because SELinux is likewise only asked for Search permission, which needs to be allowed for read-only access. Key invalidation seems to have intended for cases where keyrings are used as caches, and keys can be re-requested at any time by an upcall to /sbin/request-key. However, keyrings are not always used in this way. For example, users of filesystem-level encryption (EXT4, F2FS, and/or UBIFS encryption) usually wish to grant many differently-privileged processes access to the same keys, set up in a shared keyring ahead of time. Permitting everyone to invalidate keys in this scenario enables a trivial denial-of-service. And even users of keyrings as "just caches" may wish to restrict invalidation because it may require significant work or user interaction to regenerate keys. This patch fixes the flaw by requiring both Search and Setattr permission to invalidate a key rather than just Search permission. Requiring Setattr permission is appropriate because Setattr permission also allows revoking (via keyctl_revoke()) or expiring (via keyctl_set_timeout()) the key, which also make the key inaccessible regardless of how many keyrings it is in. Continuing to require Search permission ensures we do not decrease the level of permissions required. Alternatively, the problem could be solved by requiring Write permission. However, that would be less appropriate because Write permission is ostensibly meant to deal with the key payload, not the key itself. A new "Invalidate" permission would also work, but that would require keyctl_setperm() users to start including a new permission which never existed before, which would be much more likely to break users of keyctl_invalidate(). Finally, we could only allow invalidating keys which the kernel has explicitly marked as invalidatible, e.g. the "id_resolver" keys created for NFSv4 ID mapping. However, that likewise would be much more likely to break users. This is a user-visible API change. But we don't seem to have much choice, and any breakage will be limited to users who override the default key permissions using keyctl_setperm() to remove Setattr permission, then later call keyctl_invalidate(). There are probably not many such users, possibly even none at all. In fact, the only users of keyctl_invalidate() I could find are nfsidmap and the Chromium OS cryptohome daemon. nfsidmap will be unaffected because it runs as root and actually relies on the "root is permitted to invalidate certain special keys" exception (KEY_FLAG_ROOT_CAN_INVAL) rather than the actual key permissions. cryptohomed will be unaffected because it already owns the keys and sets KEY_USR_SETATTR permission on them. Cc: linux-api@vger.kernel.org Cc: linux-cifs@vger.kernel.org Cc: linux-fscrypt@vger.kernel.org Cc: linux-nfs@vger.kernel.org Cc: Gwendal Grignou Cc: Jaegeuk Kim Cc: Paul Crowley Cc: Richard Weinberger Cc: Ryo Hashimoto Cc: Tyler Hicks Cc: stable@vger.kernel.org # v3.5+ Signed-off-by: Eric Biggers --- Documentation/security/keys/core.rst | 4 ++-- security/keys/keyctl.c | 4 ++-- 2 files changed, 4 insertions(+), 4 deletions(-) diff --git a/Documentation/security/keys/core.rst b/Documentation/security/keys/core.rst index 1648fa80b3bf..c8030f628810 100644 --- a/Documentation/security/keys/core.rst +++ b/Documentation/security/keys/core.rst @@ -815,8 +815,8 @@ The keyctl syscall functions are: immediately, though they are still visible in /proc/keys until deleted (they're marked with an 'i' flag). - A process must have search permission on the key for this function to be - successful. + A process must have Search and Setattr permission on the key for this + function to be successful. * Compute a Diffie-Hellman shared secret or public key:: diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c index ab0b337c84b4..0739e7934e74 100644 --- a/security/keys/keyctl.c +++ b/security/keys/keyctl.c @@ -400,7 +400,7 @@ long keyctl_revoke_key(key_serial_t id) /* * Invalidate a key. * - * The key must be grant the caller Invalidate permission for this to work. + * The key must grant the caller Search and Setattr permission for this to work. * The key and any links to the key will be automatically garbage collected * immediately. * @@ -416,7 +416,7 @@ long keyctl_invalidate_key(key_serial_t id) kenter("%d", id); - key_ref = lookup_user_key(id, 0, KEY_NEED_SEARCH); + key_ref = lookup_user_key(id, 0, KEY_NEED_SEARCH | KEY_NEED_SETATTR); if (IS_ERR(key_ref)) { ret = PTR_ERR(key_ref); -- 2.14.1.480.gb18f417b89-goog