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block: prevent snapshot mode $TMPDIR symlink attack

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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

Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
This commit is contained in:
Jim Meyering 2012-05-28 09:27:54 +02:00 • committed by Anthony Liguori
parent e78bd5ab07
commit eba25057b9
3 changed files with 31 additions and 15 deletions
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37
block.c
View file

@ -409,28 +409,36 @@ int bdrv_create_file(const char* filename, QEMUOptionParameter *options)
return bdrv_create(drv, filename, options);
}
/*
* 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
void get_tmp_filename(char *filename, int size)
{
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
@ -753,7 +761,10 @@ 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)