[Qemu-devel] [PATCH] qga: set umask 0077 when daemonizing (CVE-2013-2007)

[Anthony Liguori]

From: Laszlo Ersek <address@hidden>

The qemu guest agent creates a bunch of files with insecure permissions
when started in daemon mode. For example:

  -rw-rw-rw- 1 root root /var/log/qemu-ga.log
  -rw-rw-rw- 1 root root /var/run/qga.state
  -rw-rw-rw- 1 root root /var/log/qga-fsfreeze-hook.log

In addition, at least all files created with the "guest-file-open" QMP
command, and all files created with shell output redirection (or
otherwise) by utilities invoked by the fsfreeze hook script are affected.

For now mask all file mode bits for "group" and "others" in
become_daemon().

Temporarily, for compatibility reasons, stick with the 0666 file-mode in
case of files newly created by the "guest-file-open" QMP call. Do so
without changing the umask temporarily.

Signed-off-by: Laszlo Ersek <address@hidden>
Signed-off-by: Anthony Liguori <address@hidden>
---
 qga/commands-posix.c | 123 +++++++++++++++++++++++++++++++++++++++++++++++++--
 qga/main.c           |   2 +-
 2 files changed, 120 insertions(+), 5 deletions(-)

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;
-- 
1.8.0