148 lines
5 KiB
Diff
148 lines
5 KiB
Diff
From d3a9befc86113228f77c89030336faa84a5557c0 Mon Sep 17 00:00:00 2001
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From: Christian Brauner <christian.brauner@ubuntu.com>
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Date: Tue, 12 Feb 2019 17:31:14 +0100
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Subject: [PATCH] rexec: make rexecution opt-in for library callers
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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We cannot rexecute the liblxc shared library unconditionally as this would
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break most of our downstreams. Here are some scenarios:
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- anyone performing a dlopen() on the shared library (e.g. users of the LXC
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Python bindings)
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- LXD as it needs to know the absolute path to its own executable based on
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/proc/self/exe etc.
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This commit makes the rexecution of liblxc conditional on whether the
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LXC_MEMFD_REXEC environment variable is set or not. If it is then liblxc is
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unconditionally rexecuted.
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The only relevant attack vector exists for lxc-attach which we simply reexecute
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unconditionally.
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Reported-by: Stéphane Graber <stgraber@ubuntu.com>
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Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
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---
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src/lxc/Makefile.am | 4 +++-
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src/lxc/rexec.c | 4 ++--
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src/lxc/rexec.h | 26 ++++++++++++++++++++++++++
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src/lxc/tools/lxc_attach.c | 18 ++++++++++++++++++
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4 files changed, 49 insertions(+), 3 deletions(-)
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create mode 100644 src/lxc/rexec.h
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diff --git a/src/lxc/Makefile.am b/src/lxc/Makefile.am
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index e1499a7eb..ef19df9e0 100644
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--- src/lxc/Makefile.am
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+++ src/lxc/Makefile.am
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@@ -25,6 +25,7 @@ noinst_HEADERS = api_extensions.h \
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monitor.h \
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namespace.h \
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raw_syscalls.h \
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+ rexec.h \
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start.h \
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state.h \
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storage/btrfs.h \
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@@ -180,7 +181,7 @@ liblxc_la_SOURCES += ../include/strlcat.c ../include/strlcat.h
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endif
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if ENFORCE_MEMFD_REXEC
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-liblxc_la_SOURCES += rexec.c
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+liblxc_la_SOURCES += rexec.c rexec.h
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endif
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AM_CFLAGS = -DLXCROOTFSMOUNT=\"$(LXCROOTFSMOUNT)\" \
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@@ -307,6 +308,7 @@ LDADD = liblxc.la \
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if ENABLE_TOOLS
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lxc_attach_SOURCES = tools/lxc_attach.c \
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+ rexec.c rexec.h \
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tools/arguments.c tools/arguments.h
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lxc_autostart_SOURCES = tools/lxc_autostart.c \
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tools/arguments.c tools/arguments.h
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diff --git a/src/lxc/rexec.c b/src/lxc/rexec.c
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index 3ce499b1e..024728d85 100644
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--- src/lxc/rexec.c
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+++ src/lxc/rexec.c
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@@ -142,7 +142,7 @@ static void lxc_rexec_as_memfd(char **argv, char **envp, const char *memfd_name)
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errno = saved_errno;
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}
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-static int lxc_rexec(const char *memfd_name)
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+int lxc_rexec(const char *memfd_name)
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{
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int ret;
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char **argv = NULL, **envp = NULL;
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@@ -179,7 +179,7 @@ static int lxc_rexec(const char *memfd_name)
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*/
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__attribute__((constructor)) static void liblxc_rexec(void)
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{
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- if (lxc_rexec("liblxc")) {
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+ if (getenv("LXC_MEMFD_REXEC") && lxc_rexec("liblxc")) {
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fprintf(stderr, "Failed to re-execute liblxc via memory file descriptor\n");
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_exit(EXIT_FAILURE);
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}
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diff --git a/src/lxc/rexec.h b/src/lxc/rexec.h
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new file mode 100644
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index 000000000..088ded932
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--- /dev/null
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+++ src/lxc/rexec.h
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@@ -0,0 +1,26 @@
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+/* liblxcapi
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+ *
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+ * Copyright © 2019 Christian Brauner <christian.brauner@ubuntu.com>.
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+ * Copyright © 2019 Canonical Ltd.
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+ *
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+ * This library is free software; you can redistribute it and/or
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+ * modify it under the terms of the GNU Lesser General Public
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+ * License as published by the Free Software Foundation; either
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+ * version 2.1 of the License, or (at your option) any later version.
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+
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+ * This library is distributed in the hope that it will be useful,
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+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
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+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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+ * Lesser General Public License for more details.
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+
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+ * You should have received a copy of the GNU Lesser General Public License
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+ * along with this library; if not, write to the Free Software Foundation,
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+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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+ */
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+
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+#ifndef __LXC_REXEC_H
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+#define __LXC_REXEC_H
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+
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+extern int lxc_rexec(const char *memfd_name);
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+
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+#endif /* __LXC_REXEC_H */
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diff --git a/src/lxc/tools/lxc_attach.c b/src/lxc/tools/lxc_attach.c
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index 3de0d7747..d10b6ecc2 100644
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--- src/lxc/tools/lxc_attach.c
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+++ src/lxc/tools/lxc_attach.c
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@@ -44,10 +44,28 @@
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#include "config.h"
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#include "confile.h"
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#include "log.h"
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+#include "rexec.h"
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#include "utils.h"
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lxc_log_define(lxc_attach, lxc);
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+/**
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+ * This function will copy any binary that calls liblxc into a memory file and
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+ * will use the memfd to rexecute the binary. This is done to prevent attacks
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+ * through the /proc/self/exe symlink to corrupt the host binary when host and
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+ * container are in the same user namespace or have set up an identity id
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+ * mapping: CVE-2019-5736.
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+ */
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+#ifdef ENFORCE_MEMFD_REXEC
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+__attribute__((constructor)) static void lxc_attach_rexec(void)
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+{
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+ if (!getenv("LXC_MEMFD_REXEC") && lxc_rexec("lxc-attach")) {
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+ fprintf(stderr, "Failed to re-execute lxc-attach via memory file descriptor\n");
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+ _exit(EXIT_FAILURE);
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+ }
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+}
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+#endif
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+
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static int my_parser(struct lxc_arguments *args, int c, char *arg);
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static int add_to_simple_array(char ***array, ssize_t *capacity, char *value);
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static bool stdfd_is_pty(void);
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