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

[Topi Miettinen]

On 01/27/15 00:19, Lennart Poettering wrote:
> 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...

Right, I didn't look at the context. In that case, this would be just
defensive programming.

-Topi

> 
> Lennart
>