seccomp: add spawn argument to command line

This patch adds [,spawn=deny] argument to `-sandbox on' option. It
blacklists fork and execve system calls, avoiding Qemu to spawn new
threads or processes.
Signed-off-by: NEduardo Otubo <otubo@redhat.com>

include/sysemu/seccomp.h

@@ -18,6 +18,7 @@
 #define QEMU_SECCOMP_SET_DEFAULT     (1 << 0)
 #define QEMU_SECCOMP_SET_OBSOLETE    (1 << 1)
 #define QEMU_SECCOMP_SET_PRIVILEGED  (1 << 2)
+#define QEMU_SECCOMP_SET_SPAWN       (1 << 3)
 
 #include <seccomp.h>
 

qemu-options.hx

@@ -4018,6 +4018,7 @@ ETEXI
 
 DEF("sandbox", HAS_ARG, QEMU_OPTION_sandbox, \
     "-sandbox on[,obsolete=allow|deny][,elevateprivileges=allow|deny|children]\n" \
+    "          [,spawn=allow|deny]\n" \
     "                Enable seccomp mode 2 system call filter (default 'off').\n" \
     "                use 'obsolete' to allow obsolete system calls that are provided\n" \
     "                    by the kernel, but typically no longer used by modern\n" \
@@ -4025,10 +4026,12 @@ DEF("sandbox", HAS_ARG, QEMU_OPTION_sandbox, \
     "                use 'elevateprivileges' to allow or deny QEMU process to elevate\n" \
     "                    its privileges by blacklisting all set*uid|gid system calls.\n" \
     "                    The value 'children' will deny set*uid|gid system calls for\n" \
-    "                    main QEMU process but will allow forks and execves to run unprivileged\n",
+    "                    main QEMU process but will allow forks and execves to run unprivileged\n" \
+    "                use 'spawn' to avoid QEMU to spawn new threads or processes by\n" \
+    "                     blacklisting *fork and execve\n",
     QEMU_ARCH_ALL)
 STEXI
-@item -sandbox @var{arg}[,obsolete=@var{string}][,elevateprivileges=@var{string}]
+@item -sandbox @var{arg}[,obsolete=@var{string}][,elevateprivileges=@var{string}][,spawn=@var{string}]
 @findex -sandbox
 Enable Seccomp mode 2 system call filter. 'on' will enable syscall filtering and 'off' will
 disable it.  The default is 'off'.
@@ -4037,6 +4040,8 @@ disable it.  The default is 'off'.
 Enable Obsolete system calls
 @item elevateprivileges=@var{string}
 Disable set*uid|gid system calls
+@item spawn=@var{string}
+Disable *fork and execve
 @end table
 ETEXI
 

qemu-seccomp.c

@@ -78,6 +78,10 @@ static const struct QemuSeccompSyscall blacklist[] = {
     { SCMP_SYS(setresgid),              QEMU_SECCOMP_SET_PRIVILEGED },
     { SCMP_SYS(setfsuid),               QEMU_SECCOMP_SET_PRIVILEGED },
     { SCMP_SYS(setfsgid),               QEMU_SECCOMP_SET_PRIVILEGED },
+    /* spawn */
+    { SCMP_SYS(fork),                   QEMU_SECCOMP_SET_SPAWN },
+    { SCMP_SYS(vfork),                  QEMU_SECCOMP_SET_SPAWN },
+    { SCMP_SYS(execve),                 QEMU_SECCOMP_SET_SPAWN },
 };
 
 

vl.c

@@ -280,6 +280,10 @@ static QemuOptsList qemu_sandbox_opts = {
             .name = "elevateprivileges",
             .type = QEMU_OPT_STRING,
         },
+        {
+            .name = "spawn",
+            .type = QEMU_OPT_STRING,
+        },
         { /* end of list */ }
     },
 };
@@ -1083,6 +1087,18 @@ static int parse_sandbox(void *opaque, QemuOpts *opts, Error **errp)
             }
         }
 
+        value = qemu_opt_get(opts, "spawn");
+        if (value) {
+            if (g_str_equal(value, "deny")) {
+                seccomp_opts |= QEMU_SECCOMP_SET_SPAWN;
+            } else if (g_str_equal(value, "allow")) {
+                /* default value */
+            } else {
+                error_report("invalid argument for spawn");
+                return -1;
+            }
+        }
+
         if (seccomp_start(seccomp_opts) < 0) {
             error_report("failed to install seccomp syscall filter "
                          "in the kernel");