[lxc-devel] [PATCH] mount_proc_if_needed: only safe mount when rootfs is defined

[Bogdan Purcareata]

The safe_mount function was introduced in order to address CVE-2015-1335,
one of the vulnerabilities being a mount with a symlink for the
destination path. In scenarios such as lxc-execute with no rootfs, the
destination path is the host /proc, which is previously mounted by the
host, and is unmounted and mounted again in a new set of namespaces,
therefore eliminating the need to check for it being a symlink.

Mount the rootfs normally if the rootfs is NULL, keep the safe mount
only for scenarios where a different rootfs is defined.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata at nxp.com>
---
 src/lxc/conf.c  |  1 +
 src/lxc/utils.c | 10 +++++++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/lxc/conf.c b/src/lxc/conf.c
index 632dde3..1e30c0c 100644
--- a/src/lxc/conf.c
+++ b/src/lxc/conf.c
@@ -3509,6 +3509,7 @@ int ttys_shift_ids(struct lxc_conf *c)
 	return 0;
 }
 
+/* NOTE: not to be called from inside the container namespace! */
 int tmp_proc_mount(struct lxc_conf *lxc_conf)
 {
 	int mounted;
diff --git a/src/lxc/utils.c b/src/lxc/utils.c
index ed8c4c4..e31228d 100644
--- a/src/lxc/utils.c
+++ b/src/lxc/utils.c
@@ -1704,6 +1704,8 @@ int safe_mount(const char *src, const char *dest, const char *fstype,
  *
  * Returns < 0 on failure, 0 if the correct proc was already mounted
  * and 1 if a new proc was mounted.
+ *
+ * NOTE: not to be called from inside the container namespace!
  */
 int mount_proc_if_needed(const char *rootfs)
 {
@@ -1737,8 +1739,14 @@ int mount_proc_if_needed(const char *rootfs)
 	return 0;
 
 domount:
-	if (safe_mount("proc", path, "proc", 0, NULL, rootfs) < 0)
+	if (!strcmp(rootfs,"")) /* rootfs is NULL */
+		ret = mount("proc", path, "proc", 0, NULL);
+	else
+		ret = safe_mount("proc", path, "proc", 0, NULL, rootfs);
+
+	if (ret < 0)
 		return -1;
+
 	INFO("Mounted /proc in container for security transition");
 	return 1;
 }
-- 
1.9.1