[lxc-devel] [PATCH v2] safe_mount: Handle mounting proc and refactor

[Bogdan Purcareata]

On 14.01.2016 01:09, Serge Hallyn wrote:
> Quoting Bogdan Purcareata (bogdan.purcareata at nxp.com):
>> On 11.01.2016 20:59, Serge Hallyn wrote:
>>> Quoting Bogdan Purcareata (bogdan.purcareata at nxp.com):
>>>> The safe_mount primitive will mount the fs in the new container
>>>> environment by using file descriptors referred in /proc/self/fd.
>>>> However, when the mounted filesystem is proc itself, it will have
>>>> been previously unmounted, therefore resulting in an error when
>>>> searching for these file descriptors. This only happens when there's
>>>> no container rootfs prefix (commonly with lxc-execute).
>>>>
>>>> Implement the support for this use case as well, by doing the mount
>>>> based on the full path.
>>>>
>>>> Refactor the whole function in order to remove duplicated code checks
>>>> and improve readability.
>>>>
>>>> Changes since v1:
>>>> - In order to address CVE-2015-1335, still check if the destination is
>>>> not a symlink. Do the mount only if the destination file descriptor
>>>> exists.
>>>>
>>>> Signed-off-by: Bogdan Purcareata <bogdan.purcareata at nxp.com>
>>>> ---
>>>>    src/lxc/utils.c | 49 ++++++++++++++++++++++++++++---------------------
>>>>    1 file changed, 28 insertions(+), 21 deletions(-)
>>>>
>>>> diff --git a/src/lxc/utils.c b/src/lxc/utils.c
>>>> index d9e769d..c53711a 100644
>>>> --- a/src/lxc/utils.c
>>>> +++ b/src/lxc/utils.c
>>>> @@ -1644,9 +1644,9 @@ out:
>>>>    int safe_mount(const char *src, const char *dest, const char *fstype,
>>>>    		unsigned long flags, const void *data, const char *rootfs)
>>>>    {
>>>> -	int srcfd = -1, destfd, ret, saved_errno;
>>>> +	int srcfd = -1, destfd = -1, ret = 0;
>>>>    	char srcbuf[50], destbuf[50]; // only needs enough for /proc/self/fd/<fd>
>>>> -	const char *mntsrc = src;
>>>> +	const char *mntsrc = src, *mntdest = dest;
>>>>
>>>>    	if (!rootfs)
>>>>    		rootfs = "";
>>>> @@ -1655,45 +1655,52 @@ int safe_mount(const char *src, const char *dest, const char *fstype,
>>>>    	if (flags & MS_BIND && src && src[0] != '/') {
>>>>    		INFO("this is a relative bind mount");
>>>>    		srcfd = open_without_symlink(src, NULL);
>>>> -		if (srcfd < 0)
>>>> -			return srcfd;
>>>> +		if (srcfd < 0) {
>>>> +			ret = srcfd;
>>>> +			goto out;
>>>> +		}
>>>>    		ret = snprintf(srcbuf, 50, "/proc/self/fd/%d", srcfd);
>>>>    		if (ret < 0 || ret > 50) {
>>>> -			close(srcfd);
>>>>    			ERROR("Out of memory");
>>>> -			return -EINVAL;
>>>> +			ret = -EINVAL;
>>>> +			goto out_src;
>>>>    		}
>>>>    		mntsrc = srcbuf;
>>>>    	}
>>>>
>>>>    	destfd = open_without_symlink(dest, rootfs);
>>>>    	if (destfd < 0) {
>>>> -		if (srcfd != -1)
>>>> -			close(srcfd);
>>>> -		return destfd;
>>>> +		ret = destfd;
>>>> +		goto out_src;
>>>>    	}
>>>>
>>>>    	ret = snprintf(destbuf, 50, "/proc/self/fd/%d", destfd);
>>>>    	if (ret < 0 || ret > 50) {
>>>> -		if (srcfd != -1)
>>>> -			close(srcfd);
>>>> -		close(destfd);
>>>>    		ERROR("Out of memory");
>>>> -		return -EINVAL;
>>>> +		ret = -EINVAL;
>>>> +		goto out_dest;
>>>>    	}
>>>>
>>>> -	ret = mount(mntsrc, destbuf, fstype, flags, data);
>>>> -	saved_errno = errno;
>>>> -	if (srcfd != -1)
>>>> -		close(srcfd);
>>>> -	close(destfd);
>>>> +	/* make sure the destination descriptor exists */
>>>> +	if (access(destbuf, F_OK) == 0)
>>>> +		mntdest = destbuf;
>>> First, if we're going to shortcut I'd prefer to say "if /proc/self
>>> does not exist then skip this check" fo rnow.
>>> But can we think of any way to still do this check?
>>>
>>> What exactly are the cases?
>>>
>>> 1. lxc-execute, lxc-init tries to mount /proc.  We should be able
>>> to simply have lxc always mount /proc before the pivot_root, so
>>> we can properly do this check.
>>>
>>> what use-cases will break if we demand /proc to exist in the
>>> container?  (We can add an option to umount /proc in lxc-init,
>>> but the directory would have to exist)
>> That's my use case - the failing function is tmp_proc_mount, thus happening
>
> Wait, is your rootfs NULL or not?

Yep, it's NULL - plain "lxc-execute -n foo -f empty.conf -- bash".

> We can handle that case specially.
>
> mount_proc_if_needed() is only called from tmp_proc_mount(), which is only
> called from lxc_setup().  If rootfs is NULL, then all bets are off anyway
> and we can just call mount().  If rootfs is not NULL, then we're mounting
> in a separate container rootfs and we should use safe_mount(), but we
> expect the real /proc to then exist.

In that case, would the following fix be more appropriate?

@@ -1747,8 +1747,14 @@ int mount_proc_if_needed(const char *rootfs)
         return 0;

  domount:
-       if (safe_mount("proc", path, "proc", 0, NULL, rootfs) < 0)
+       if (!strcmp(rootfs,"")) /* rootfs is NULL */
+               ret = mount("proc", path, "proc", 0, NULL);
+       else
+               ret = safe_mount("proc", path, "proc", 0, NULL, rootfs);
+
+       if (ret < 0)
                 return -1;
+
         INFO("Mounted /proc in container for security transition");
         return 1;
  }

Btw checking for the rootfs to be NULL is done this way since 
mount_proc_if_needed is called like:

mount_proc_if_needed(lxc_conf->rootfs.path ? lxc_conf->rootfs.mount : "");

Thanks!
Bogdan P.