[OE-core] [thud][PATCH v2 2/4] dbus: fix CVE-2019-12749
Kevin Weng
t-keweng at microsoft.com
Tue Aug 13 23:25:56 UTC 2019
Signed-off-by: Kevin Weng <t-keweng at microsoft.com>
---
.../dbus/dbus/CVE-2019-12749.patch | 127 ++++++++++++++++++
meta/recipes-core/dbus/dbus_1.12.10.bb | 1 +
2 files changed, 128 insertions(+)
create mode 100644 meta/recipes-core/dbus/dbus/CVE-2019-12749.patch
diff --git a/meta/recipes-core/dbus/dbus/CVE-2019-12749.patch b/meta/recipes-core/dbus/dbus/CVE-2019-12749.patch
new file mode 100644
index 0000000000..393c70ca21
--- /dev/null
+++ b/meta/recipes-core/dbus/dbus/CVE-2019-12749.patch
@@ -0,0 +1,127 @@
+From f0120c5d97a4cc1b659e86d38f2b1f646ca20ea3 Mon Sep 17 00:00:00 2001
+From: Simon McVittie <smcv at collabora.com>
+Date: Thu, 30 May 2019 12:53:03 +0100
+Subject: [PATCH] auth: Reject DBUS_COOKIE_SHA1 for users other than the server
+ owner
+
+The DBUS_COOKIE_SHA1 authentication mechanism aims to prove ownership
+of a shared home directory by having the server write a secret "cookie"
+into a .dbus-keyrings subdirectory of the desired identity's home
+directory with 0700 permissions, and having the client prove that it can
+read the cookie. This never actually worked for non-malicious clients in
+the case where server uid != client uid (unless the server and client
+both have privileges, such as Linux CAP_DAC_OVERRIDE or traditional
+Unix uid 0) because an unprivileged server would fail to write out the
+cookie, and an unprivileged client would be unable to read the resulting
+file owned by the server.
+
+Additionally, since dbus 1.7.10 we have checked that ~/.dbus-keyrings
+is owned by the uid of the server (a side-effect of a check added to
+harden our use of XDG_RUNTIME_DIR), further ruling out successful use
+by a non-malicious client with a uid differing from the server's.
+
+Joe Vennix of Apple Information Security discovered that the
+implementation of DBUS_COOKIE_SHA1 was susceptible to a symbolic link
+attack: a malicious client with write access to its own home directory
+could manipulate a ~/.dbus-keyrings symlink to cause the DBusServer to
+read and write in unintended locations. In the worst case this could
+result in the DBusServer reusing a cookie that is known to the
+malicious client, and treating that cookie as evidence that a subsequent
+client connection came from an attacker-chosen uid, allowing
+authentication bypass.
+
+This is mitigated by the fact that by default, the well-known system
+dbus-daemon (since 2003) and the well-known session dbus-daemon (in
+stable releases since dbus 1.10.0 in 2015) only accept the EXTERNAL
+authentication mechanism, and as a result will reject DBUS_COOKIE_SHA1
+at an early stage, before manipulating cookies. As a result, this
+vulnerability only applies to:
+
+* system or session dbus-daemons with non-standard configuration
+* third-party dbus-daemon invocations such as at-spi2-core (although
+ in practice at-spi2-core also only accepts EXTERNAL by default)
+* third-party uses of DBusServer such as the one in Upstart
+
+Avoiding symlink attacks in a portable way is difficult, because APIs
+like openat() and Linux /proc/self/fd are not universally available.
+However, because DBUS_COOKIE_SHA1 already doesn't work in practice for
+a non-matching uid, we can solve this vulnerability in an easier way
+without regressions, by rejecting it early (before looking at
+~/.dbus-keyrings) whenever the requested identity doesn't match the
+identity of the process hosting the DBusServer.
+
+Signed-off-by: Simon McVittie <smcv at collabora.com>
+Closes: https://gitlab.freedesktop.org/dbus/dbus/issues/269
+Closes: CVE-2019-12749
+
+Upstream-Status: Backport
+[https://gitlab.freedesktop.org/dbus/dbus/commit
+/47b1a4c41004bf494b87370987b222c934b19016]
+
+CVE: CVE-2019-12749
+
+Signed-off-by: Kevin Weng <t-keweng at microsoft.com>
+---
+ dbus/dbus-auth.c | 32 ++++++++++++++++++++++++++++++++
+ 1 file changed, 32 insertions(+)
+
+diff --git a/dbus/dbus-auth.c b/dbus/dbus-auth.c
+index 37d8d4c9..7390a9d5 100644
+--- a/dbus/dbus-auth.c
++++ b/dbus/dbus-auth.c
+@@ -529,6 +529,7 @@ sha1_handle_first_client_response (DBusAuth *auth,
+ DBusString tmp2;
+ dbus_bool_t retval = FALSE;
+ DBusError error = DBUS_ERROR_INIT;
++ DBusCredentials *myself = NULL;
+
+ _dbus_string_set_length (&auth->challenge, 0);
+
+@@ -565,6 +566,34 @@ sha1_handle_first_client_response (DBusAuth *auth,
+ return FALSE;
+ }
+
++ myself = _dbus_credentials_new_from_current_process ();
++
++ if (myself == NULL)
++ goto out;
++
++ if (!_dbus_credentials_same_user (myself, auth->desired_identity))
++ {
++ /*
++ * DBUS_COOKIE_SHA1 is not suitable for authenticating that the
++ * client is anyone other than the user owning the process
++ * containing the DBusServer: we probably aren't allowed to write
++ * to other users' home directories. Even if we can (for example
++ * uid 0 on traditional Unix or CAP_DAC_OVERRIDE on Linux), we
++ * must not, because the other user controls their home directory,
++ * and could carry out symlink attacks to make us read from or
++ * write to unintended locations. It's difficult to avoid symlink
++ * attacks in a portable way, so we just don't try. This isn't a
++ * regression, because DBUS_COOKIE_SHA1 never worked for other
++ * users anyway.
++ */
++ _dbus_verbose ("%s: client tried to authenticate as \"%s\", "
++ "but that doesn't match this process",
++ DBUS_AUTH_NAME (auth),
++ _dbus_string_get_const_data (data));
++ retval = send_rejected (auth);
++ goto out;
++ }
++
+ /* we cache the keyring for speed, so here we drop it if it's the
+ * wrong one. FIXME caching the keyring here is useless since we use
+ * a different DBusAuth for every connection.
+@@ -679,6 +708,9 @@ sha1_handle_first_client_response (DBusAuth *auth,
+ _dbus_string_zero (&tmp2);
+ _dbus_string_free (&tmp2);
+
++ if (myself != NULL)
++ _dbus_credentials_unref (myself);
++
+ return retval;
+ }
+
+--
+2.22.0
+
diff --git a/meta/recipes-core/dbus/dbus_1.12.10.bb b/meta/recipes-core/dbus/dbus_1.12.10.bb
index d71f7f7042..d7ad1d8be4 100644
--- a/meta/recipes-core/dbus/dbus_1.12.10.bb
+++ b/meta/recipes-core/dbus/dbus_1.12.10.bb
@@ -16,6 +16,7 @@ SRC_URI = "http://dbus.freedesktop.org/releases/dbus/dbus-${PV}.tar.gz \
file://tmpdir.patch \
file://dbus-1.init \
file://clear-guid_from_server-if-send_negotiate_unix_f.patch \
+ file://CVE-2019-12749.patch \
"
SRC_URI[md5sum] = "c3e12b4206e2a7da39d7cc42567790ef"
--
2.22.0
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