[PATCH v3] cred: Propagate security_prepare_creds() error code

[Frederick Lawler]

On 6/13/22 11:44 PM, Eric W. Biederman wrote:
> Frederick Lawler <fred at cloudflare.com> writes:
> 
>> Hi Eric,
>>
>> On 6/13/22 12:04 PM, Eric W. Biederman wrote:
>>> Frederick Lawler <fred at cloudflare.com> writes:
>>>
>>>> While experimenting with the security_prepare_creds() LSM hook, we
>>>> noticed that our EPERM error code was not propagated up the callstack.
>>>> Instead ENOMEM is always returned.  As a result, some tools may send a
>>>> confusing error message to the user:
>>>>
>>>> $ unshare -rU
>>>> unshare: unshare failed: Cannot allocate memory
>>>>
>>>> A user would think that the system didn't have enough memory, when
>>>> instead the action was denied.
>>>>
>>>> This problem occurs because prepare_creds() and prepare_kernel_cred()
>>>> return NULL when security_prepare_creds() returns an error code. Later,
>>>> functions calling prepare_creds() and prepare_kernel_cred() return
>>>> ENOMEM because they assume that a NULL meant there was no memory
>>>> allocated.
>>>>
>>>> Fix this by propagating an error code from security_prepare_creds() up
>>>> the callstack.
>>> Why would it make sense for security_prepare_creds to return an error
>>> code other than ENOMEM?
>>>   > That seems a bit of a violation of what that function is supposed to do
>>>
>>
>> The API allows LSM authors to decide what error code is returned from the
>> cred_prepare hook. security_task_alloc() is a similar hook, and has its return
>> code propagated.
> 
> It is not an api.  It is an implementation detail of the linux kernel.
> It is a set of convenient functions that do a job.
> 
> The general rule is we don't support cases without an in-tree user.  I
> don't see an in-tree user.
> 
>> I'm proposing we follow security_task_allocs() pattern, and add visibility for
>> failure cases in prepare_creds().
> 
> I am asking why we would want to.  Especially as it is not an API, and I
> don't see any good reason for anything but an -ENOMEM failure to be
> supported.
>
We're writing a LSM BPF policy, and not a new LSM. Our policy aims to 
solve unprivileged unshare, similar to Debian's patch [1]. We're in a 
position such that we can't use that patch because we can't block _all_ 
of our applications from performing an unshare. We prefer a granular 
approach. LSM BPF seems like a good choice.

Because LSM BPF exposes these hooks, we should probably treat them as an 
API. From that perspective, userspace expects unshare to return a EPERM 
when the call is denied permissions.

> Without an in-tree user that cares it is probably better to go the
> opposite direction and remove the possibility of return anything but
> memory allocation failure.  That will make it clearer to implementors
> that a general error code is not supported and this is not a location
> to implement policy, this is only a hook to allocate state for the LSM.
> 

That's a good point, and it's possible we're using the wrong hook for 
the policy. Do you know of other hooks we can look into?

>>> I have probably missed a very interesting discussion where that was
>>> mentioned but I don't see link to the discussion or anything explaining
>>> why we want to do that in this change.
>>>
>>
>> AFAIK, this is the start of the discussion.
> 
> You were on v3 and had an out of tree piece of code so I assumed someone
> had at least thought about why you want to implement policy in a piece
> of code whose only purpose is to allocate memory to store state.
> 

No worries.

> Eric
> 
> 
> 

Links:
1: 
https://sources.debian.org/patches/linux/3.16.56-1+deb8u1/debian/add-sysctl-to-disallow-unprivileged-CLONE_NEWUSER-by-default.patch/