seccomp: add elevateprivileges argument to command line

This patch introduces the new argument
[,elevateprivileges=allow|deny|children] to the `-sandbox on'. It allows
or denies Qemu process to elevate its privileges by blacklisting all
set*uid|gid system calls. The 'children' option will let forks and
execves run unprivileged.

Signed-off-by: Eduardo Otubo <otubo@redhat.com>

include/sysemu/seccomp.h

@@ -17,6 +17,7 @@
 
 #define QEMU_SECCOMP_SET_DEFAULT     (1 << 0)
 #define QEMU_SECCOMP_SET_OBSOLETE    (1 << 1)
+#define QEMU_SECCOMP_SET_PRIVILEGED  (1 << 2)
 
 #include <seccomp.h>
 

qemu-options.hx

@@ -4017,20 +4017,26 @@ Old param mode (ARM only).
 ETEXI
 
 DEF("sandbox", HAS_ARG, QEMU_OPTION_sandbox, \
-    "-sandbox on[,obsolete=allow|deny]\n" \
+    "-sandbox on[,obsolete=allow|deny][,elevateprivileges=allow|deny|children]\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" \
-    "                    C library implementations.\n",
+    "                    C library implementations.\n" \
+    "                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",
     QEMU_ARCH_ALL)
 STEXI
-@item -sandbox @var{arg}[,obsolete=@var{string}]
+@item -sandbox @var{arg}[,obsolete=@var{string}][,elevateprivileges=@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'.
 @table @option
 @item obsolete=@var{string}
 Enable Obsolete system calls
+@item elevateprivileges=@var{string}
+Disable set*uid|gid system calls
 @end table
 ETEXI
 

qemu-seccomp.c

@@ -67,6 +67,17 @@ static const struct QemuSeccompSyscall blacklist[] = {
     { SCMP_SYS(sysfs),                  QEMU_SECCOMP_SET_OBSOLETE },
     { SCMP_SYS(uselib),                 QEMU_SECCOMP_SET_OBSOLETE },
     { SCMP_SYS(ustat),                  QEMU_SECCOMP_SET_OBSOLETE },
+    /* privileged */
+    { SCMP_SYS(setuid),                 QEMU_SECCOMP_SET_PRIVILEGED },
+    { SCMP_SYS(setgid),                 QEMU_SECCOMP_SET_PRIVILEGED },
+    { SCMP_SYS(setpgid),                QEMU_SECCOMP_SET_PRIVILEGED },
+    { SCMP_SYS(setsid),                 QEMU_SECCOMP_SET_PRIVILEGED },
+    { SCMP_SYS(setreuid),               QEMU_SECCOMP_SET_PRIVILEGED },
+    { SCMP_SYS(setregid),               QEMU_SECCOMP_SET_PRIVILEGED },
+    { SCMP_SYS(setresuid),              QEMU_SECCOMP_SET_PRIVILEGED },
+    { SCMP_SYS(setresgid),              QEMU_SECCOMP_SET_PRIVILEGED },
+    { SCMP_SYS(setfsuid),               QEMU_SECCOMP_SET_PRIVILEGED },
+    { SCMP_SYS(setfsgid),               QEMU_SECCOMP_SET_PRIVILEGED },
 };
 
 

vl.c

@@ -29,6 +29,7 @@
 
 #ifdef CONFIG_SECCOMP
 #include "sysemu/seccomp.h"
+#include "sys/prctl.h"
 #endif
 
 #if defined(CONFIG_VDE)
@@ -275,6 +276,10 @@ static QemuOptsList qemu_sandbox_opts = {
             .name = "obsolete",
             .type = QEMU_OPT_STRING,
         },
+        {
+            .name = "elevateprivileges",
+            .type = QEMU_OPT_STRING,
+        },
         { /* end of list */ }
     },
 };
@@ -1056,6 +1061,28 @@ static int parse_sandbox(void *opaque, QemuOpts *opts, Error **errp)
             }
         }
 
+        value = qemu_opt_get(opts, "elevateprivileges");
+        if (value) {
+            if (g_str_equal(value, "deny")) {
+                seccomp_opts |= QEMU_SECCOMP_SET_PRIVILEGED;
+            } else if (g_str_equal(value, "children")) {
+                seccomp_opts |= QEMU_SECCOMP_SET_PRIVILEGED;
+
+                /* calling prctl directly because we're
+                 * not sure if host has CAP_SYS_ADMIN set*/
+                if (prctl(PR_SET_NO_NEW_PRIVS, 1)) {
+                    error_report("failed to set no_new_privs "
+                                 "aborting");
+                    return -1;
+                }
+            } else if (g_str_equal(value, "allow")) {
+                /* default value */
+            } else {
+                error_report("invalid argument for elevateprivileges");
+                return -1;
+            }
+        }
+
         if (seccomp_start(seccomp_opts) < 0) {
             error_report("failed to install seccomp syscall filter "
                          "in the kernel");