Re: [PATCH] SELinux: Always allow FIOCLEX and FIONCLEX

[Demi Marie Obenour <demiobenour@gmail.com>]

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On 2/15/22 15:34, Paul Moore wrote:
> On Mon, Feb 14, 2022 at 2:11 AM Jeffrey Vander Stoep <jeffv@google.com> wrote:
>> On Tue, Feb 8, 2022 at 3:18 PM William Roberts <bill.c.roberts@gmail.com> wrote:
>>>
>>> <snip>
>>>
>>> This is getting too long for me.
>>>
>>>>>
>>>>> I don't have a strong opinion either way.  If one were to allow this
>>>>> using a policy rule, it would result in a major policy breakage.  The
>>>>> rule would turn on extended perm checks across the entire system,
>>>>> which the SELinux Reference Policy isn't written for.  I can't speak
>>>>> to the Android policy, but I would imagine it would be the similar
>>>>> problem there too.
>>>>
>>>> Excuse me if I am wrong but AFAIK adding a xperm rule does not turn on
>>>> xperm checks across the entire system.
>>>
>>> It doesn't as you state below its target + class.
>>>
>>>>
>>>> If i am not mistaken it will turn on xperm checks only for the
>>>> operations that have the same source and target/target class.
>>>
>>> That's correct.
>>>
>>>>
>>>> This is also why i don't (with the exception TIOSCTI for termdev
>>>> chr_file) use xperms by default.
>>>>
>>>> 1. it is really easy to selectively filter ioctls by adding xperm rules
>>>> for end users (and since ioctls are often device/driver specific they
>>>> know best what is needed and what not)
>>>
>>>>>>> and FIONCLEX can be trivially bypassed unless fcntl(F_SETFD)
>>>>
>>>> 2. if you filter ioctls in upstream policy for example like i do with
>>>> TIOSCTI using for example (allowx foo bar (ioctl chr_file (not
>>>> (0xXXXX)))) then you cannot easily exclude additional ioctls later where source is
>>>> foo and target/tclass is bar/chr_file because there is already a rule in
>>>> place allowing the ioctl (and you cannot add rules)
>>>
>>> Currently, fcntl flag F_SETFD is never checked, it's silently allowed, but
>>> the equivalent FIONCLEX and FIOCLEX are checked. So if you wrote policy
>>> to block the FIO*CLEX flags, it would be bypassable through F_SETFD and
>>> FD_CLOEXEC. So the patch proposed makes the FIO flags behave like
>>> F_SETFD. Which means upstream policy users could drop this allow, which
>>> could then remove the target/class rule and allow all icotls. Which is easy
>>> to prevent and fix you could be a rule in to allowx 0 as documented in the
>>> wiki: https://selinuxproject.org/page/XpermRules
>>>
>>> The questions I think we have here are:
>>> 1. Do we agree that the behavior between SETFD and the FIO flags are equivalent?
>>>   I think they are.
>>> 2. Do we want the interfaces to behave the same?
>>>   I think they should.
>>> 3. Do upstream users of the policy construct care?
>>>   The patch is backwards compat, but I don't want their to be cruft
>>> floating around with extra allowxperm rules.
>>
>> I think this proposed change is fine from Android's perspective. It
>> implements in the kernel what we've already already put in place in
>> our policy - that all domains are allowed to use these IOCLTs.
>> https://cs.android.com/android/platform/superproject/+/master:system/sepolicy/public/domain.te;l=312
>>
>> It'll be a few years before we can clean up our policy since we need
>> to support older kernels, but that's fine.
> 
> Thanks for the discussion everyone, it sounds like everybody is okay
> with the change - that's good.  However, as I said earlier in this
> thread I think we need to put this behind a policy capability, how
> does POLICYDB_CAPABILITY_IOCTL_CLOEXEC/"ioctl_skip_cloexec" sound to
> everyone?
> 
> Demi, are you able to respin this patch with policy capability changes?

I can try, but this is something I am doing in my spare time and I
have no idea what adding a policy capability would involve.  While I
have written several policies myself, I believe this is the first time
I have dealt with policy capabilities outside of kernel log output.
So it will be a while before I can make a patch.  You would probably be
able to write a patch far more quickly and easily.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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