Re: kdbus: add code for buses, domains and endpoints

[Eric W. Biederman]

Djalal Harouni <tixxdz@xxxxxxxxxx> writes:

> On Wed, Oct 29, 2014 at 08:59:44PM -0700, Eric W. Biederman wrote:
>> Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes:
>> 
>> The way capabilities are checked in this patch make me very nervous.
>> 
>> We are not checking permissions at open time.  Every other location
>> of calling capable on file like objects has been show to be suceptible
>> to file descriptor pass attacks.
> Yes, I do understand the concern, this is valid for some cases! but we
> can't apply it on the ioctl API ?! please see below:
>
> All (perhaps not all) the current ioctl do not check for fd passing
> attacks! if a privileged do arbitrary ioctl on untrusted fds we are
> already owned... the dumb privileged process is the one to blame, right?
>
>
> Example:
> 1) fs/ext4/ioctl.c:ext4_ioctl()
>    they have:
>    inode_owner_or_capable() + capable() checks
>
>    for all the restricted ioctl()
>
> 2) fs/xfs/xfs_ioctl.c:xfs_file_ioctl()
>    they have:
>    capable() checks
>
> 3) fs/btrfs/ioctl.c:btrfs_ioctl()
>    they have capable() + inode_owner_or_capable()
>
> ... long list
>
> These are sensible API and they do not care at all about fd passing,
> so I don't think we should care either ?! or perhaps I'm missing
> something ?

- It is an easy mistake to make.
- We have not performed extensive audits of the capable calls at this
  time to veryify that fd passing is safe.
- Unless it is egregious we are likely to grandfather the existing usage
  in to avoid breaking userspace.

None of that is an excuse for kdbus to get it wrong once it has been
pointed out in review.
 
> The capable() is done as it is, and for the inode_owner_or_capable() you
> will notice that we followed the same logic and did use it in our
> kdbus_bus_uid_is_privileged() to stay safe and follow what other API are
> doing.

What others are doing makes it very hard to safely use allow those
ioctls in a tightly sandboxed application, as it is unpredictable
what the sandboxed ioctl can do with the file descriptor.

Further an application that calls setresuid at different times during
it's application will behave differently.  Which makes ioctls that do
not have consistent behavior after open time inappropriate for use in
userspace libraries.

Eric


> Thank you for the comments!
>
>
>> > See Documentation/kdbus.txt for more details.
>> >
>> > Signed-off-by: Daniel Mack <daniel@xxxxxxxxxx>
>> > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>> > ---
>> 
>> > diff --git a/drivers/misc/kdbus/bus.c b/drivers/misc/kdbus/bus.c
>> > new file mode 100644
>> > index 000000000000..6dcaf22f5d59
>> > --- /dev/null
>> > +++ b/drivers/misc/kdbus/bus.c
>> > @@ -0,0 +1,450 @@
>> 
>> > +/**
>> > + * kdbus_bus_cred_is_privileged() - check whether the given credentials in
>> > + *				    combination with the capabilities of the
>> > + *				    current thead are privileged on the bus
>> > + * @bus:		The bus to check
>> > + * @cred:		The credentials to match
>> > + *
>> > + * Return: true if the credentials are privileged, otherwise false.
>> > + */
>> > +bool kdbus_bus_cred_is_privileged(const struct kdbus_bus *bus,
>> > +				  const struct cred *cred)
>> > +{
>> > +	/* Capabilities are *ALWAYS* tested against the current thread, they're
>> > +	 * never remembered from conn-credentials. */
>> > +	if (ns_capable(&init_user_ns, CAP_IPC_OWNER))
>> > +		return true;
>> > +
>> > +	return uid_eq(bus->uid_owner, cred->fsuid);
>> > +}
>> > +
>> > +/**
>> > + * kdbus_bus_uid_is_privileged() - check whether the current user is a
>> > + *				   priviledged bus user
>> > + * @bus:		The bus to check
>> > + *
>> > + * Return: true if the current user has CAP_IPC_OWNER capabilities, or
>> > + * if it has the same UID as the user that created the bus. Otherwise,
>> > + * false is returned.
>> > + */
>> > +bool kdbus_bus_uid_is_privileged(const struct kdbus_bus *bus)
>> > +{
>> > +	return kdbus_bus_cred_is_privileged(bus, current_cred());
>> > +}
>> 
>> 
>> > +/**
>> > + * kdbus_bus_new() - create a new bus
>> > + * @domain:		The domain to work on
>> > + * @make:		Pointer to a struct kdbus_cmd_make containing the
>> > + *			details for the bus creation
>> > + * @name:		Name of the bus
>> > + * @bloom:		Bloom parameters for this bus
>> > + * @mode:		The access mode for the device node
>> > + * @uid:		The uid of the device node
>> > + * @gid:		The gid of the device node
>> > + * @bus:		Pointer to a reference where the new bus is stored
>> > + *
>> > + * This function will allocate a new kdbus_bus and link it to the given
>> > + * domain.
>> > + *
>> > + * Return: 0 on success, negative errno on failure.
>> > + */
>> > +int kdbus_bus_new(struct kdbus_domain *domain,
>> > +		  const struct kdbus_cmd_make *make,
>> > +		  const char *name,
>> > +		  const struct kdbus_bloom_parameter *bloom,
>> > +		  umode_t mode, kuid_t uid, kgid_t gid,
>> > +		  struct kdbus_bus **bus)
>> > +{
>> [snip]
>> > +
>> > +	if (!capable(CAP_IPC_OWNER) &&
>> > +	    atomic_inc_return(&b->user->buses) > KDBUS_USER_MAX_BUSES) {
>> > +		atomic_dec(&b->user->buses);
>> > +		ret = -EMFILE;
>> > +		goto exit_unref_user_unlock;
>> > +	}
>> > +
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