[Qemu-devel] [PATCH 7/8] os-posix: Provide new -runasid option

[Ian Jackson <ian.jackson@eu.citrix.com>]

This allows the caller to specify a uid and gid to use, even if there
is no corresponding password entry.  This will be useful in certain
Xen configurations.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
---
v3: Error messages fixed.  Thanks to Peter Maydell and Ross Lagerwall.
v2: Coding style fixes.
---
 os-posix.c      | 49 ++++++++++++++++++++++++++++++++++++++++---------
 qemu-options.hx | 12 ++++++++++++
 2 files changed, 52 insertions(+), 9 deletions(-)

diff --git a/os-posix.c b/os-posix.c
index 92e9d85..6cc5868 100644
--- a/os-posix.c
+++ b/os-posix.c
@@ -43,6 +43,8 @@
 #endif
 
 static struct passwd *user_pwd;
+static uid_t user_uid = (uid_t)-1;
+static gid_t user_gid = (gid_t)-1;
 static const char *chroot_dir;
 static int daemonize;
 static int daemon_pipe;
@@ -134,6 +136,9 @@ void os_set_proc_name(const char *s)
  */
 void os_parse_cmd_args(int index, const char *optarg)
 {
+    unsigned long lv;
+    char *ep;
+    int rc;
     switch (index) {
 #ifdef CONFIG_SLIRP
     case QEMU_OPTION_smb:
@@ -150,6 +155,22 @@ void os_parse_cmd_args(int index, const char *optarg)
             exit(1);
         }
         break;
+    case QEMU_OPTION_runasid:
+        errno = 0;
+        lv = strtoul(optarg, &ep, 0); /* can't qemu_strtoul, want *ep=='.' */
+        user_uid = lv; /* overflow here is ID in C99 */
+        if (errno || *ep != '.' || user_uid != lv || user_uid == (uid_t)-1) {
+            fprintf(stderr, "Could not obtain uid from \"%s\"", optarg);
+            exit(1);
+        }
+        lv = 0;
+        rc = qemu_strtoul(ep + 1, 0, 0, &lv);
+        user_gid = lv; /* overflow here is ID in C99 */
+        if (rc || user_gid != lv || user_gid == (gid_t)-1) {
+            fprintf(stderr, "Could not obtain gid from \"%s\"", optarg);
+            exit(1);
+        }
+        break;
     case QEMU_OPTION_chroot:
         chroot_dir = optarg;
         break;
@@ -166,18 +187,28 @@ void os_parse_cmd_args(int index, const char *optarg)
 
 static void change_process_uid(void)
 {
-    if (user_pwd) {
-        if (setgid(user_pwd->pw_gid) < 0) {
-            fprintf(stderr, "Failed to setgid(%d)\n", user_pwd->pw_gid);
+    if (user_pwd || user_uid != (uid_t)-1) {
+        gid_t intended_gid = user_pwd ? user_pwd->pw_gid : user_gid;
+        uid_t intended_uid = user_pwd ? user_pwd->pw_uid : user_uid;
+        if (setgid(intended_gid) < 0) {
+            fprintf(stderr, "Failed to setgid(%d)\n", intended_gid);
             exit(1);
         }
-        if (initgroups(user_pwd->pw_name, user_pwd->pw_gid) < 0) {
-            fprintf(stderr, "Failed to initgroups(\"%s\", %d)\n",
-                    user_pwd->pw_name, user_pwd->pw_gid);
-            exit(1);
+        if (user_pwd) {
+            if (initgroups(user_pwd->pw_name, user_pwd->pw_gid) < 0) {
+                fprintf(stderr, "Failed to initgroups(\"%s\", %d)\n",
+                        user_pwd->pw_name, user_pwd->pw_gid);
+                exit(1);
+            }
+        } else {
+            if (setgroups(1, &user_gid) < 0) {
+                fprintf(stderr, "Failed to setgroups(1, [%d])",
+                        user_gid);
+                exit(1);
+            }
         }
-        if (setuid(user_pwd->pw_uid) < 0) {
-            fprintf(stderr, "Failed to setuid(%d)\n", user_pwd->pw_uid);
+        if (setuid(intended_uid) < 0) {
+            fprintf(stderr, "Failed to setuid(%d)\n", intended_uid);
             exit(1);
         }
         if (setuid(0) != -1) {
diff --git a/qemu-options.hx b/qemu-options.hx
index 9f6e2ad..34a5329 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -3968,6 +3968,18 @@ Immediately before starting guest execution, drop root privileges, switching
 to the specified user.
 ETEXI
 
+#ifndef _WIN32
+DEF("runasid", HAS_ARG, QEMU_OPTION_runasid, \
+    "-runasid uid.gid     change to numeric uid and gid just before starting the VM\n",
+    QEMU_ARCH_ALL)
+#endif
+STEXI
+@item -runasid @var{uid}.@var{gid}
+@findex -runasid
+Immediately before starting guest execution, drop root privileges, switching
+to the specified uid and gid.
+ETEXI
+
 DEF("prom-env", HAS_ARG, QEMU_OPTION_prom_env,
     "-prom-env variable=value\n"
     "                set OpenBIOS nvram variables\n",
-- 
2.1.4