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