From patchwork Wed Mar 13 12:36:58 2013 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Luis Henriques X-Patchwork-Id: 227265 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Received: from huckleberry.canonical.com (huckleberry.canonical.com [91.189.94.19]) by ozlabs.org (Postfix) with ESMTP id 14CBF2C009E for ; Wed, 13 Mar 2013 23:37:14 +1100 (EST) Received: from localhost ([127.0.0.1] helo=huckleberry.canonical.com) by huckleberry.canonical.com with esmtp (Exim 4.76) (envelope-from ) id 1UFkvX-0005Vw-64; Wed, 13 Mar 2013 12:37:03 +0000 Received: from youngberry.canonical.com ([91.189.89.112]) by huckleberry.canonical.com with esmtp (Exim 4.76) (envelope-from ) id 1UFkvU-0005R0-Pr for kernel-team@lists.ubuntu.com; Wed, 13 Mar 2013 12:37:00 +0000 Received: from bl15-242-192.dsl.telepac.pt ([188.80.242.192] helo=localhost) by youngberry.canonical.com with esmtpsa (TLS1.0:DHE_RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1UFkvU-0004m5-Gr for kernel-team@lists.ubuntu.com; Wed, 13 Mar 2013 12:37:00 +0000 From: Luis Henriques To: kernel-team@lists.ubuntu.com Subject: [Lucid Oneiric Precise Quantal CVE-2013-1792] keys: fix race with concurrent install_user_keyrings() Date: Wed, 13 Mar 2013 12:36:58 +0000 Message-Id: <1363178218-12818-1-git-send-email-luis.henriques@canonical.com> X-Mailer: git-send-email 1.8.1.2 X-BeenThere: kernel-team@lists.ubuntu.com X-Mailman-Version: 2.1.14 Precedence: list List-Id: Kernel team discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , MIME-Version: 1.0 Errors-To: kernel-team-bounces@lists.ubuntu.com Sender: kernel-team-bounces@lists.ubuntu.com From: David Howells CVE-2013-1792 BugLink: http://bugs.launchpad.net/bugs/1152788 This fixes CVE-2013-1792. There is a race in install_user_keyrings() that can cause a NULL pointer dereference when called concurrently for the same user if the uid and uid-session keyrings are not yet created. It might be possible for an unprivileged user to trigger this by calling keyctl() from userspace in parallel immediately after logging in. Assume that we have two threads both executing lookup_user_key(), both looking for KEY_SPEC_USER_SESSION_KEYRING. THREAD A THREAD B =============================== =============================== ==>call install_user_keyrings(); if (!cred->user->session_keyring) ==>call install_user_keyrings() ... user->uid_keyring = uid_keyring; if (user->uid_keyring) return 0; <== key = cred->user->session_keyring [== NULL] user->session_keyring = session_keyring; atomic_inc(&key->usage); [oops] At the point thread A dereferences cred->user->session_keyring, thread B hasn't updated user->session_keyring yet, but thread A assumes it is populated because install_user_keyrings() returned ok. The race window is really small but can be exploited if, for example, thread B is interrupted or preempted after initializing uid_keyring, but before doing setting session_keyring. This couldn't be reproduced on a stock kernel. However, after placing systemtap probe on 'user->session_keyring = session_keyring;' that introduced some delay, the kernel could be crashed reliably. Fix this by checking both pointers before deciding whether to return. Alternatively, the test could be done away with entirely as it is checked inside the mutex - but since the mutex is global, that may not be the best way. Signed-off-by: David Howells Reported-by: Mateusz Guzik Cc: Signed-off-by: Andrew Morton Signed-off-by: James Morris (cherry picked from commit 0da9dfdd2cd9889201bc6f6f43580c99165cd087) Signed-off-by: Luis Henriques --- security/keys/process_keys.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c index 931cfda..75fb18c 100644 --- a/security/keys/process_keys.c +++ b/security/keys/process_keys.c @@ -56,7 +56,7 @@ int install_user_keyrings(void) kenter("%p{%u}", user, user->uid); - if (user->uid_keyring) { + if (user->uid_keyring && user->session_keyring) { kleave(" = 0 [exist]"); return 0; }