[Qemu-devel] [PATCH 3/6] seccomp: add elevateprivileges argument to command line

[Eduardo Otubo]

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 <address@hidden>
---
 include/sysemu/seccomp.h |  1 +
 qemu-options.hx          |  8 ++++++--
 qemu-seccomp.c           | 29 +++++++++++++++++++++++++++++
 vl.c                     | 18 ++++++++++++++++++
 4 files changed, 54 insertions(+), 2 deletions(-)

diff --git a/include/sysemu/seccomp.h b/include/sysemu/seccomp.h
index 7a7bde246b..e6e78d85ce 100644
--- a/include/sysemu/seccomp.h
+++ b/include/sysemu/seccomp.h
@@ -16,6 +16,7 @@
 #define QEMU_SECCOMP_H
 
 #define OBSOLETE    0x0001
+#define PRIVILEGED  0x0010
 
 #include <seccomp.h>
 
diff --git a/qemu-options.hx b/qemu-options.hx
index c3200f303b..6dc776aae3 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -4004,8 +4004,10 @@ Old param mode (ARM only).
 ETEXI
 
 DEF("sandbox", HAS_ARG, QEMU_OPTION_sandbox, \
-    "-sandbox on[,obsolete=allow]  Enable seccomp mode 2 system call filter 
(default 'off').\n" \
-    "                              obsolete: Allow obsolete system calls",
+    "-sandbox on[,obsolete=allow][,elevateprivileges=allow|deny|children]\n" \
+    "                               Enable seccomp mode 2 system call filter 
(default 'off').\n" \
+    "                               obsolete: Allow obsolete system calls\n" \
+    "                               elevateprivileges: allows or denies Qemu 
process to elevate its privileges by blacklisting all set*uid|gid system calls. 
'children' will deny set*uid|gid system calls for main Qemu process but will 
allow forks and execves to run unprivileged",
     QEMU_ARCH_ALL)
 STEXI
 @item -sandbox @var{arg}[,address@hidden
@@ -4015,6 +4017,8 @@ disable it.  The default is 'off'.
 @table @option
 @item address@hidden
 Enable Obsolete system calls
address@hidden address@hidden
+Disable set*uid|gid systema calls
 @end table
 ETEXI
 
diff --git a/qemu-seccomp.c b/qemu-seccomp.c
index c6a8b28260..c204cef9ee 100644
--- a/qemu-seccomp.c
+++ b/qemu-seccomp.c
@@ -31,6 +31,19 @@ struct QemuSeccompSyscall {
     uint8_t priority;
 };
 
+static const struct QemuSeccompSyscall privileged_syscalls[] = {
+    { SCMP_SYS(setuid), 255 },
+    { SCMP_SYS(setgid), 255 },
+    { SCMP_SYS(setpgid), 255 },
+    { SCMP_SYS(setsid), 255 },
+    { SCMP_SYS(setreuid), 255 },
+    { SCMP_SYS(setregid), 255 },
+    { SCMP_SYS(setresuid), 255 },
+    { SCMP_SYS(setresgid), 255 },
+    { SCMP_SYS(setfsuid), 255 },
+    { SCMP_SYS(setfsgid), 255 },
+};
+
 static const struct QemuSeccompSyscall obsolete[] = {
     { SCMP_SYS(readdir), 255 },
     { SCMP_SYS(_sysctl), 255 },
@@ -110,6 +123,22 @@ int seccomp_start(uint8_t seccomp_opts)
         }
     }
 
+    if (seccomp_opts & PRIVILEGED) {
+        for (i = 0; i < ARRAY_SIZE(privileged_syscalls); i++) {
+            rc = seccomp_rule_add(ctx, SCMP_ACT_KILL,
+                                 privileged_syscalls[i].num, 0);
+            if (rc < 0) {
+                goto seccomp_return;
+            }
+            rc = seccomp_syscall_priority(ctx, privileged_syscalls[i].num,
+                    privileged_syscalls[i].priority);
+            if (rc < 0) {
+                goto seccomp_return;
+            }
+        }
+    }
+
+
     rc = seccomp_load(ctx);
 
   seccomp_return:
diff --git a/vl.c b/vl.c
index 6a297851aa..0b857083a3 100644
--- a/vl.c
+++ b/vl.c
@@ -275,6 +275,10 @@ static QemuOptsList qemu_sandbox_opts = {
             .name = "obsolete",
             .type = QEMU_OPT_STRING,
         },
+        {
+            .name = "elevateprivileges",
+            .type = QEMU_OPT_STRING,
+        },
         { /* end of list */ }
     },
 };
@@ -1046,6 +1050,20 @@ static int parse_sandbox(void *opaque, QemuOpts *opts, 
Error **errp)
             }
         }
 
+        value = qemu_opt_get(opts,"elevateprivileges");
+        if (value) {
+            if (strcmp(value, "deny") == 0) {
+                seccomp_opts |= PRIVILEGED;
+            }
+            if (strcmp(value, "children") == 0) {
+                seccomp_opts |= PRIVILEGED;
+
+               /* calling prctl directly because we're
+                * not sure if host has CAP_SYS_ADMIN set*/
+               prctl(PR_SET_NO_NEW_PRIVS, 1);
+           }
+        }
+
         if (seccomp_start(seccomp_opts) < 0) {
             error_report("failed to install seccomp syscall filter "
                          "in the kernel");
-- 
2.13.0