diff --git a/qga/commands-posix.c b/qga/commands-posix.c
index 3b5c536..04c6951 100644
--- a/qga/commands-posix.c
+++ b/qga/commands-posix.c
@@ -18,6 +18,9 @@
 #include <unistd.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/stat.h>
 #include <inttypes.h>
 #include "qga/guest-agent-core.h"
 #include "qga-qmp-commands.h"
@@ -237,9 +240,122 @@ static GuestFileHandle *guest_file_handle_find(int64_t id, Error **err)
     return NULL;
 }
 
+typedef const char * const ccpc;
+
+/* http://pubs.opengroup.org/onlinepubs/9699919799/functions/fopen.html */
+static const struct {
+    ccpc *forms;
+    int oflag_base;
+} guest_file_open_modes[] = {
+    { (ccpc[]){ "r",  "rb",         NULL }, O_RDONLY                      },
+    { (ccpc[]){ "w",  "wb",         NULL }, O_WRONLY | O_CREAT | O_TRUNC  },
+    { (ccpc[]){ "a",  "ab",         NULL }, O_WRONLY | O_CREAT | O_APPEND },
+    { (ccpc[]){ "r+", "rb+", "r+b", NULL }, O_RDWR                        },
+    { (ccpc[]){ "w+", "wb+", "w+b", NULL }, O_RDWR   | O_CREAT | O_TRUNC  },
+    { (ccpc[]){ "a+", "ab+", "a+b", NULL }, O_RDWR   | O_CREAT | O_APPEND }
+};
+
+static int
+find_open_flag(const char *mode_str, Error **err)
+{
+    unsigned mode;
+
+    for (mode = 0; mode < ARRAY_SIZE(guest_file_open_modes); ++mode) {
+        ccpc *form;
+
+        form = guest_file_open_modes[mode].forms;
+        while (*form != NULL && strcmp(*form, mode_str) != 0) {
+            ++form;
+        }
+        if (*form != NULL) {
+            break;
+        }
+    }
+
+    if (mode == ARRAY_SIZE(guest_file_open_modes)) {
+        error_setg(err, "invalid file open mode '%s'", mode_str);
+        return -1;
+    }
+    return guest_file_open_modes[mode].oflag_base | O_NOCTTY | O_NONBLOCK;
+}
+
+#define DEFAULT_NEW_FILE_MODE (S_IRUSR | S_IWUSR | \
+                               S_IRGRP | S_IWGRP | \
+                               S_IROTH | S_IWOTH)
+
+static FILE *
+safe_open_or_create(const char *path, const char *mode, Error **err)
+{
+    Error *local_err = NULL;
+    int oflag;
+
+    oflag = find_open_flag(mode, &local_err);
+    if (local_err == NULL) {
+        int fd;
+
+        /* If the caller wants / allows creation of a new file, we implement it
+         * with a two step process: open() + (open() / fchmod()).
+         *
+         * First we insist on creating the file exclusively as a new file. If
+         * that succeeds, we're free to set any file-mode bits on it. (The
+         * motivation is that we want to set those file-mode bits independently
+         * of the current umask.)
+         *
+         * If the exclusive creation fails because the file already exists
+         * (EEXIST is not possible for any other reason), we just attempt to
+         * open the file, but in this case we won't be allowed to change the
+         * file-mode bits on the preexistent file.
+         *
+         * The pathname should never disappear between the two open()s in
+         * practice. If it happens, then someone very likely tried to race us.
+         * In this case just go ahead and report the ENOENT from the second
+         * open() to the caller.
+         *
+         * If the caller wants to open a preexistent file, then the first
+         * open() is decisive and its third argument is ignored, and the second
+         * open() and the fchmod() are never called.
+         */
+        fd = open(path, oflag | ((oflag & O_CREAT) ? O_EXCL : 0), 0);
+        if (fd == -1 && errno == EEXIST) {
+            oflag &= ~(unsigned)O_CREAT;
+            fd = open(path, oflag);
+        }
+
+        if (fd == -1) {
+            error_setg_errno(&local_err, errno, "failed to open file '%s' "
+                             "(mode: '%s')", path, mode);
+        } else {
+            qemu_set_cloexec(fd);
+
+            if ((oflag & O_CREAT) && fchmod(fd, DEFAULT_NEW_FILE_MODE) == -1) {
+                error_setg_errno(&local_err, errno, "failed to set permission "
+                                 "0%03o on new file '%s' (mode: '%s')",
+                                 (unsigned)DEFAULT_NEW_FILE_MODE, path, mode);
+            } else {
+                FILE *f;
+
+                f = fdopen(fd, mode);
+                if (f == NULL) {
+                    error_setg_errno(&local_err, errno, "failed to associate "
+                                     "stdio stream with file descriptor %d, "
+                                     "file '%s' (mode: '%s')", fd, path, mode);
+                } else {
+                    return f;
+                }
+            }
+
+            close(fd);
+        }
+    }
+
+    error_propagate(err, local_err);
+    return NULL;
+}
+
 int64_t qmp_guest_file_open(const char *path, bool has_mode, const char *mode, Error **err)
 {
     FILE *fh;
+    Error *local_err = NULL;
     int fd;
     int64_t ret = -1, handle;
 
@@ -247,10 +363,9 @@ int64_t qmp_guest_file_open(const char *path, bool has_mode, const char *mode, E
         mode = "r";
     }
     slog("guest-file-open called, filepath: %s, mode: %s", path, mode);
-    fh = fopen(path, mode);
-    if (!fh) {
-        error_setg_errno(err, errno, "failed to open file '%s' (mode: '%s')",
-                         path, mode);
+    fh = safe_open_or_create(path, mode, &local_err);
+    if (local_err != NULL) {
+        error_propagate(err, local_err);
         return -1;
     }
 
diff --git a/qga/main.c b/qga/main.c
index 1841759..44a2836 100644
--- a/qga/main.c
+++ b/qga/main.c
@@ -478,7 +478,7 @@ static void become_daemon(const char *pidfile)
         }
     }
 
-    umask(0);
+    umask(S_IRWXG | S_IRWXO);
     sid = setsid();
     if (sid < 0) {
         goto fail;
