Run initgroups() in qemudOpenAsUID()

qemudOpenAsUID is intended to open a file with the credentials of a
specified uid. Current implementation fails if the file is accessible to
one of uid's groups but not owned by uid.

This patch replaces the supplementary group list that the child process
inherited from libvirtd with the default group list of uid.

src/qemu/qemu_driver.c

@@ -41,6 +41,7 @@
 #include <signal.h>
 #include <paths.h>
 #include <pwd.h>
+#include <grp.h>
 #include <stdio.h>
 #include <sys/wait.h>
 #include <sys/ioctl.h>
@@ -6353,6 +6354,7 @@ parent_cleanup:
     char *buf = NULL;
     size_t bufsize = 1024 * 1024;
     int bytesread;
+    struct passwd pwd, *pwd_result;
 
     /* child doesn't need the read side of the pipe */
     close(pipefd[0]);
@@ -6365,6 +6367,26 @@ parent_cleanup:
         goto child_cleanup;
     }
 
+    if (VIR_ALLOC_N(buf, bufsize) < 0) {
+        exit_code = ENOMEM;
+        virReportOOMError();
+        goto child_cleanup;
+    }
+
+    exit_code = getpwuid_r(uid, &pwd, buf, bufsize, &pwd_result);
+    if (pwd_result == NULL) {
+        virReportSystemError(errno,
+                             _("cannot getpwuid_r(%d) to read '%s'"),
+                             uid, path);
+        goto child_cleanup;
+    }
+    if (initgroups(pwd.pw_name, pwd.pw_gid) != 0) {
+        exit_code = errno;
+        virReportSystemError(errno,
+                             _("cannot initgroups(\"%s\", %d) to read '%s'"),
+                             pwd.pw_name, pwd.pw_gid, path);
+        goto child_cleanup;
+    }
     if (setuid(uid) != 0) {
         exit_code = errno;
         virReportSystemError(errno,
@@ -6379,11 +6401,6 @@ parent_cleanup:
                              path, uid);
         goto child_cleanup;
     }
-    if (VIR_ALLOC_N(buf, bufsize) < 0) {
-        exit_code = ENOMEM;
-        virReportOOMError();
-        goto child_cleanup;
-    }
 
     /* read from fd and write to pipefd[1] until EOF */
     do {