[systemd-devel] [PATCH] libudev-monitor: ensure proper string termination

[Lennart Poettering]

On Sun, 25.01.15 07:10, Topi Miettinen (toiwoton at gmail.com) wrote:

> On 01/25/15 03:34, Zbigniew Jędrzejewski-Szmek wrote:
> > On Sat, Jan 24, 2015 at 10:39:56AM +0200, Topi Miettinen wrote:
> >> Leave space for the terminating zero when reading and make sure
> >> that the last byte is zero. This also makes the check for long packets
> >> equivalent to code before 9c89c1ca: reject also packets with size 8192.
> >> ---
> >>  src/libudev/libudev-monitor.c | 4 +++-
> >>  1 file changed, 3 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/src/libudev/libudev-monitor.c b/src/libudev/libudev-monitor.c
> >> index 4cfb2f6..b7fc031 100644
> >> --- a/src/libudev/libudev-monitor.c
> >> +++ b/src/libudev/libudev-monitor.c
> >> @@ -590,7 +590,7 @@ retry:
> >>          if (udev_monitor == NULL)
> >>                  return NULL;
> >>          iov.iov_base = &buf;
> >> -        iov.iov_len = sizeof(buf);
> >> +        iov.iov_len = sizeof(buf) - 1; /* Leave space for terminating zero */
> >>          memzero(&smsg, sizeof(struct msghdr));
> >>          smsg.msg_iov = &iov;
> >>          smsg.msg_iovlen = 1;
> >> @@ -642,6 +642,8 @@ retry:
> >>          if (udev_device == NULL)
> >>                  return NULL;
> >>  
> >> +        buf.raw[sizeof(buf.raw) - 1] = '\0';
> >> +
> >>          if (memcmp(buf.raw, "libudev", 8) == 0) {
> >>                  /* udev message needs proper version magic */
> >>                  if (buf.nlh.magic != htonl(UDEV_MONITOR_MAGIC)) {
> > A buffer only needs to be terminated by a zero in certain cases: usually if it
> > is passed to a function which expectes that. iovecs can contain binary data,
> > and have an explicit size field, so they do not need to be zero-terminated.
> > Is there a reason why the buffer has to be zero-terminated in this case?
> 
> String functions strcmp, strlen and strstr, used a few lines later,
> expect null byte terminated strings. Alternatively they could be changed
> to strncmp and friends where the scope can be limited to only the buffer.

But the data comes from the kernel (and that's verified, securely),
hence I am wondering what kind of vulnerability you have precisely in
mind. If we don't trust the kernel, then we'll have quite a problem...

Lennart

-- 
Lennart Poettering, Red Hat