From 2d96e0d8673003d961c1c7b01c26a2fe60ca5bdd Mon Sep 17 00:00:00 2001
From: Jim Meyering <jim@meyering.net>
Date: Mon, 28 May 2012 09:27:54 +0200
Subject: [PATCH] block: prevent snapshot mode $TMPDIR symlink attack

In snapshot mode, bdrv_open creates an empty temporary file without
checking for mkstemp or close failure, and ignoring the possibility
of a buffer overrun given a surprisingly long $TMPDIR.
Change the get_tmp_filename function to return int (not void),
so that it can inform its two callers of those failures.
Also avoid the risk of buffer overrun and do not ignore mkstemp
or close failure.
Update both callers (in block.c and vvfat.c) to propagate
temp-file-creation failure to their callers.

get_tmp_filename creates and closes an empty file, while its
callers later open that presumed-existing file with O_CREAT.
The problem was that a malicious user could provoke mkstemp failure
and race to create a symlink with the selected temporary file name,
thus causing the qemu process (usually root owned) to open through
the symlink, overwriting an attacker-chosen file.

This addresses CVE-2012-2652.
http://bugzilla.redhat.com/CVE-2012-2652

Signed-off-by: Jim Meyering <meyering@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit c2d76497b6eafcaedc806e07804e7bed55a98a0b)
---
 block.c       | 37 ++++++++++++++++++++++++-------------
 block/vvfat.c |  7 ++++++-
 block_int.h   |  2 +-
 3 files changed, 31 insertions(+), 15 deletions(-)

diff --git a/block.c b/block.c
index d015887..b752e84 100644
--- a/block.c
+++ b/block.c
@@ -272,28 +272,36 @@ int bdrv_create_file(const char* filename, QEMUOptionParameter *options)
     return bdrv_create(drv, filename, options);
 }
 
-#ifdef _WIN32
-void get_tmp_filename(char *filename, int size)
+/*
+ * Create a uniquely-named empty temporary file.
+ * Return 0 upon success, otherwise a negative errno value.
+ */
+int get_tmp_filename(char *filename, int size)
 {
+#ifdef _WIN32
     char temp_dir[MAX_PATH];
-
-    GetTempPath(MAX_PATH, temp_dir);
-    GetTempFileName(temp_dir, "qem", 0, filename);
-}
+    /* GetTempFileName requires that its output buffer (4th param)
+       have length MAX_PATH or greater.  */
+    assert(size >= MAX_PATH);
+    return (GetTempPath(MAX_PATH, temp_dir)
+            && GetTempFileName(temp_dir, "qem", 0, filename)
+            ? 0 : -GetLastError());
 #else
-void get_tmp_filename(char *filename, int size)
-{
     int fd;
     const char *tmpdir;
-    /* XXX: race condition possible */
     tmpdir = getenv("TMPDIR");
     if (!tmpdir)
         tmpdir = "/tmp";
-    snprintf(filename, size, "%s/vl.XXXXXX", tmpdir);
+    if (snprintf(filename, size, "%s/vl.XXXXXX", tmpdir) >= size) {
+        return -EOVERFLOW;
+    }
     fd = mkstemp(filename);
-    close(fd);
-}
+    if (fd < 0 || close(fd)) {
+        return -errno;
+    }
+    return 0;
 #endif
+}
 
 /*
  * Detect host devices. By convention, /dev/cdrom[N] is always
@@ -601,7 +609,10 @@ int bdrv_open(BlockDriverState *bs, const char *filename, int flags,
 
         bdrv_delete(bs1);
 
-        get_tmp_filename(tmp_filename, sizeof(tmp_filename));
+        ret = get_tmp_filename(tmp_filename, sizeof(tmp_filename));
+        if (ret < 0) {
+            return ret;
+        }
 
         /* Real path is meaningless for protocols */
         if (is_protocol)
diff --git a/block/vvfat.c b/block/vvfat.c
index a310ce8..19e3aca 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -2799,7 +2799,12 @@ static int enable_write_target(BDRVVVFATState *s)
     array_init(&(s->commits), sizeof(commit_t));
 
     s->qcow_filename = g_malloc(1024);
-    get_tmp_filename(s->qcow_filename, 1024);
+    ret = get_tmp_filename(s->qcow_filename, 1024);
+    if (ret < 0) {
+        g_free(s->qcow_filename);
+        s->qcow_filename = NULL;
+        return ret;
+    }
 
     bdrv_qcow = bdrv_find_format("qcow");
     options = parse_option_parameters("", bdrv_qcow->create_options, NULL);
diff --git a/block_int.h b/block_int.h
index 77c0187..415a84a 100644
--- a/block_int.h
+++ b/block_int.h
@@ -238,7 +238,7 @@ struct BlockDriverAIOCB {
     BlockDriverAIOCB *next;
 };
 
-void get_tmp_filename(char *filename, int size);
+int get_tmp_filename(char *filename, int size);
 
 void *qemu_aio_get(AIOPool *pool, BlockDriverState *bs,
                    BlockDriverCompletionFunc *cb, void *opaque);