[Qemu-devel] [PATCHv3 1/3] seccomp: adding blacklist support

[Eduardo Otubo <otubo@linux.vnet.ibm.com>]

v3: The "-netdev tap" option is checked in the vl.c file during the
process of the command line argument list. It sets tap_enabled to true
or false according to the configuration found. Later at the seccomp
filter installation, this value is checked wheter to install or not this
feature.

Adding a system call blacklist right before the vcpus starts. This
filter is composed by the system calls that can't be executed after the
guests are up. This list should be refined as whitelist is, with as much
testing as we can do using virt-test.

Signed-off-by: Eduardo Otubo <otubo@linux.vnet.ibm.com>
---
 include/sysemu/seccomp.h |  6 ++++-
 qemu-seccomp.c           | 64 +++++++++++++++++++++++++++++++++++++++---------
 vl.c                     | 21 +++++++++++++++-
 3 files changed, 77 insertions(+), 14 deletions(-)

diff --git a/include/sysemu/seccomp.h b/include/sysemu/seccomp.h
index 1189fa2..9dc7e52 100644
--- a/include/sysemu/seccomp.h
+++ b/include/sysemu/seccomp.h
@@ -15,8 +15,12 @@
 #ifndef QEMU_SECCOMP_H
 #define QEMU_SECCOMP_H
 
+#define WHITELIST 0
+#define BLACKLIST 1
+
 #include <seccomp.h>
 #include "qemu/osdep.h"
 
-int seccomp_start(void);
+int seccomp_start(int list_type);
+
 #endif
diff --git a/qemu-seccomp.c b/qemu-seccomp.c
index 37d38f8..84a42bc 100644
--- a/qemu-seccomp.c
+++ b/qemu-seccomp.c
@@ -21,7 +21,7 @@ struct QemuSeccompSyscall {
     uint8_t priority;
 };
 
-static const struct QemuSeccompSyscall seccomp_whitelist[] = {
+static const struct QemuSeccompSyscall whitelist[] = {
     { SCMP_SYS(timer_settime), 255 },
     { SCMP_SYS(timer_gettime), 254 },
     { SCMP_SYS(futex), 253 },
@@ -221,32 +221,72 @@ static const struct QemuSeccompSyscall seccomp_whitelist[] = {
     { SCMP_SYS(arch_prctl), 240 }
 };
 
-int seccomp_start(void)
+/*
+ * The second list, called blacklist, basically reduces previously installed
+ * whitelist. All the syscalls configured by the previous whitelist are still
+ * allowed, except for the ones in the blacklist.
+ * */
+
+static const struct QemuSeccompSyscall blacklist[] = {
+    { SCMP_SYS(execve), 255 }
+};
+
+static int process_list(scmp_filter_ctx *ctx,
+                        const struct QemuSeccompSyscall *list,
+                        unsigned int list_size, uint32_t action)
 {
     int rc = 0;
     unsigned int i = 0;
-    scmp_filter_ctx ctx;
 
-    ctx = seccomp_init(SCMP_ACT_KILL);
-    if (ctx == NULL) {
-        goto seccomp_return;
-    }
+    for (i = 0; i < list_size; i++) {
+        rc = seccomp_rule_add(ctx, action, list[i].num, 0);
+        if (rc < 0) {
+            goto seccomp_return;
+        }
 
-    for (i = 0; i < ARRAY_SIZE(seccomp_whitelist); i++) {
-        rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, seccomp_whitelist[i].num, 0);
+        rc = seccomp_syscall_priority(ctx, list[i].num,
+                                      list[i].priority);
         if (rc < 0) {
             goto seccomp_return;
         }
-        rc = seccomp_syscall_priority(ctx, seccomp_whitelist[i].num,
-                                      seccomp_whitelist[i].priority);
+    }
+
+seccomp_return:
+    return rc;
+}
+
+int seccomp_start(int list_type)
+{
+    int rc = 0;
+    scmp_filter_ctx ctx;
+
+    switch (list_type) {
+    case WHITELIST:
+        ctx = seccomp_init(SCMP_ACT_KILL);
+        if (ctx == NULL) {
+            goto seccomp_return;
+        }
+        rc = process_list(ctx, whitelist, ARRAY_SIZE(whitelist), SCMP_ACT_ALLOW);
         if (rc < 0) {
             goto seccomp_return;
         }
+        break;
+    case BLACKLIST:
+        ctx = seccomp_init(SCMP_ACT_ALLOW);
+        if (ctx == NULL) {
+            goto seccomp_return;
+        }
+        rc = process_list(ctx, blacklist, ARRAY_SIZE(blacklist), SCMP_ACT_KILL);
+        break;
+    default:
+        rc = -1;
+        goto seccomp_return;
     }
 
     rc = seccomp_load(ctx);
 
   seccomp_return:
-    seccomp_release(ctx);
+    if (ctx)
+        seccomp_release(ctx);
     return rc;
 }
diff --git a/vl.c b/vl.c
index b4b119a..ee95674 100644
--- a/vl.c
+++ b/vl.c
@@ -179,6 +179,8 @@ int main(int argc, char **argv)
 #define MAX_VIRTIO_CONSOLES 1
 #define MAX_SCLP_CONSOLES 1
 
+static bool enable_blacklist = false;
+static bool tap_enabled = false;
 static const char *data_dir[16];
 static int data_dir_idx;
 const char *bios_name = NULL;
@@ -1033,7 +1035,7 @@ static int parse_sandbox(QemuOpts *opts, void *opaque)
     /* FIXME: change this to true for 1.3 */
     if (qemu_opt_get_bool(opts, "enable", false)) {
 #ifdef CONFIG_SECCOMP
-        if (seccomp_start() < 0) {
+        if (seccomp_start(WHITELIST) < 0) {
             qerror_report(ERROR_CLASS_GENERIC_ERROR,
                           "failed to install seccomp syscall filter in the kernel");
             return -1;
@@ -1765,12 +1767,24 @@ void vm_state_notify(int running, RunState state)
     }
 }
 
+static void install_seccomp_blacklist(void)
+{
+    if (enable_blacklist && !tap_enabled) {
+        if (seccomp_start(BLACKLIST) < 0) {
+            qerror_report(ERROR_CLASS_GENERIC_ERROR,
+                          "failed to install seccomp syscall second level filter in the kernel");
+            exit(1);
+        }
+    }
+}
+
 void vm_start(void)
 {
     if (!runstate_is_running()) {
         cpu_enable_ticks();
         runstate_set(RUN_STATE_RUNNING);
         vm_state_notify(1, RUN_STATE_RUNNING);
+        install_seccomp_blacklist();
         resume_all_vcpus();
         monitor_protocol_event(QEVENT_RESUME, NULL);
     }
@@ -3208,6 +3222,11 @@ int main(int argc, char **argv, char **envp)
                 if (net_client_parse(qemu_find_opts("netdev"), optarg) == -1) {
                     exit(1);
                 }
+
+                if(strcmp(optarg, "tap")){
+                    tap_enabled = true;
+                }
+
                 break;
             case QEMU_OPTION_net:
                 if (net_client_parse(qemu_find_opts("net"), optarg) == -1) {
-- 
1.8.3.1