2022-02-18 00:31:49

by Demi Marie Obenour

[permalink] [raw]
Subject: Re: [PATCH] SELinux: Always allow FIOCLEX and FIONCLEX

On 2/15/22 15:34, Paul Moore wrote:
> On Mon, Feb 14, 2022 at 2:11 AM Jeffrey Vander Stoep <[email protected]> wrote:
>> On Tue, Feb 8, 2022 at 3:18 PM William Roberts <[email protected]> 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.
Demi Marie Obenour (she/her/hers)

OpenPGP_0xB288B55FFF9C22C1.asc (4.85 kB)
OpenPGP public key
OpenPGP_signature (849.00 B)
OpenPGP digital signature
Download all attachments