CVE 2018-6556: verify netns fd in lxc-user-nic

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

diff --git a/src/lxc/cmd/lxc_user_nic.c b/src/lxc/cmd/lxc_user_nic.c
index ec9cd97e0738c94df479cb0a167a73fa6ace0198..c5beb6c8da96e45f8077687b02e621695d137041 100644 (file)
--- a/src/lxc/cmd/lxc_user_nic.c
+++ b/src/lxc/cmd/lxc_user_nic.c
@@ -1179,12 +1179,41 @@ int main(int argc, char *argv[])
                        exit(EXIT_FAILURE);
                }
        } else if (request == LXC_USERNIC_DELETE) {
-               netns_fd = open(args.pid, O_RDONLY);
+               char opath[LXC_PROC_PID_FD_LEN];
+
+               /* Open the path with O_PATH which will not trigger an actual
+                * open(). Don't report an errno to the caller to not leak
+                * information whether the path exists or not.
+                * When stracing setuid is stripped so this is not a concern
+                * either.
+                */
+               netns_fd = open(args.pid, O_PATH | O_CLOEXEC);
                if (netns_fd < 0) {
-                       usernic_error("Could not open \"%s\": %s\n", args.pid,
-                                     strerror(errno));
+                       usernic_error("Failed to open \"%s\"\n", args.pid);
+                       exit(EXIT_FAILURE);
+               }
+
+               if (!fhas_fs_type(netns_fd, NSFS_MAGIC)) {
+                       usernic_error("Path \"%s\" does not refer to a network namespace path\n", args.pid);
+                       close(netns_fd);
+                       exit(EXIT_FAILURE);
+               }
+
+               ret = snprintf(opath, sizeof(opath), "/proc/self/fd/%d", netns_fd);
+               if (ret < 0 || (size_t)ret >= sizeof(opath)) {
+                       close(netns_fd);
+                       exit(EXIT_FAILURE);
+               }
+
+               /* Now get an fd that we can use in setns() calls. */
+               ret = open(opath, O_RDONLY | O_CLOEXEC);
+               if (ret < 0) {
+                       usernic_error("Failed to open \"%s\": %s\n", args.pid, strerror(errno));
+                       close(netns_fd);
                        exit(EXIT_FAILURE);
                }
+               close(netns_fd);
+               netns_fd = ret;
        }
 
        if (!create_db_dir(LXC_USERNIC_DB)) {

diff --git a/src/lxc/macro.h b/src/lxc/macro.h
index 733cb33229e762e68473d8f08e61c5465877037d..0a9698e495b47864b3b6026b2683a4e7014f3099 100644 (file)
--- a/src/lxc/macro.h
+++ b/src/lxc/macro.h
@@ -69,6 +69,10 @@
 #define CGROUP2_SUPER_MAGIC 0x63677270
 #endif
 
+#ifndef NSFS_MAGIC
+#define NSFS_MAGIC 0x6e736673
+#endif
+
 /* Useful macros */
 /* Maximum number for 64 bit integer is a string with 21 digits: 2^64 - 1 = 21 */
 #define LXC_NUMSTRLEN64 21

diff --git a/src/lxc/utils.c b/src/lxc/utils.c
index b4d3459ba1236287a1f1eb4f63fb3485d7f53667..22c72272fb61841a98c4cbad04cc20a64e414ab8 100644 (file)
--- a/src/lxc/utils.c
+++ b/src/lxc/utils.c
@@ -2551,6 +2551,18 @@ bool has_fs_type(const char *path, fs_type_magic magic_val)
        return has_type;
 }
 
+bool fhas_fs_type(int fd, fs_type_magic magic_val)
+{
+       int ret;
+       struct statfs sb;
+
+       ret = fstatfs(fd, &sb);
+       if (ret < 0)
+               return false;
+
+       return is_fs_type(&sb, magic_val);
+}
+
 bool lxc_nic_exists(char *nic)
 {
 #define __LXC_SYS_CLASS_NET_LEN 15 + IFNAMSIZ + 1

diff --git a/src/lxc/utils.h b/src/lxc/utils.h
index fa78721be8b27ad203da5e8ed6e6d4173e3a1051..920f82faf826eee04ca9ef681f0121603b1fe6d9 100644 (file)
--- a/src/lxc/utils.h
+++ b/src/lxc/utils.h
@@ -471,6 +471,7 @@ extern void *must_realloc(void *orig, size_t sz);
 /* __typeof__ should be safe to use with all compilers. */
 typedef __typeof__(((struct statfs *)NULL)->f_type) fs_type_magic;
 extern bool has_fs_type(const char *path, fs_type_magic magic_val);
+extern bool fhas_fs_type(int fd, fs_type_magic magic_val);
 extern bool is_fs_type(const struct statfs *fs, fs_type_magic magic_val);
 extern bool lxc_nic_exists(char *nic);
 extern int lxc_make_tmpfile(char *template, bool rm);