Message-ID: <56d15841-e2c1-2d58-59b8-3a6a09b23b4a@digikod.net>
Date:   Fri, 10 Jul 2020 14:57:09 +0200
From:   Mickaël Salaün <mic@...ikod.net>
To:     Christian Brauner <christian.brauner@...ntu.com>,
        Arnd Bergmann <arnd@...db.de>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...capital.net>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        James Morris <jmorris@...ei.org>, Jann Horn <jannh@...gle.com>,
        Jeff Dike <jdike@...toit.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Mickaël Salaün <mickael.salaun@....gouv.fr>,
        Richard Weinberger <richard@....at>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Shuah Khan <shuah@...nel.org>,
        Vincent Dagonneau <vincent.dagonneau@....gouv.fr>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH v19 08/12] landlock: Add syscall implementation


On 09/07/2020 19:47, Christian Brauner wrote:
> On Thu, Jul 09, 2020 at 07:26:18PM +0200, Arnd Bergmann wrote:
>> On Wed, Jul 8, 2020 at 7:50 PM Mickaël Salaün <mic@...ikod.net> wrote:
>>> On 08/07/2020 15:49, Arnd Bergmann wrote:
>>>> On Wed, Jul 8, 2020 at 3:04 PM Mickaël Salaün <mic@...ikod.net> wrote:
>>>>> On 08/07/2020 10:57, Arnd Bergmann wrote:
>>>>>> On Tue, Jul 7, 2020 at 8:10 PM Mickaël Salaün <mic@...ikod.net> wrote:
>>>>>>
>>>>>> It looks like all you need here today is a single argument bit, plus
>>>>>> possibly some room for extensibility. I would suggest removing all
>>>>>> the extra bits and using a syscall like
>>>>>>
>>>>>> SYSCALL_DEFINE1(landlock_create_ruleset, u32, flags);
>>>>>>
>>>>>> I don't really see how this needs any variable-length arguments,
>>>>>> it really doesn't do much.
>>>>>
>>>>> We need the attr_ptr/attr_size pattern because the number of ruleset
>>>>> properties will increase (e.g. network access mask).
>>>>
>>>> But how many bits do you think you will *actually* need in total that
>>>> this needs to be a two-dimensional set of flags? At the moment you
>>>> only have a single bit that you interpret.
>>>
>>> I think there is a misunderstanding. For this syscall I wasn't talking
>>> about the "options" field but about the "handled_access_fs" field which
>>> has 14 bits dedicated to control access to the file system:
>>> https://landlock.io/linux-doc/landlock-v19/security/landlock/user.html#filesystem-flags
>>
>> Ok, got it. I didn't read far enough there.
>>
>>> The idea is to add other handled_access_* fields for other kernel object
>>> types (e.g. network, process, etc.).
>>>
>>> The "options" field is fine as a raw __u32 syscall argument.
>>
>> I'd still like to avoid having it variable-length and structured though.
>> How about having a __u32 "options" flag, plus an indirect argument
>> with 32 fixed-length (all 32 bit or all 64 bit) flag words, each of which
>> corresponds to one of the option bits?
>>
>> It's still fairly complex that way, but not as much as the version
>> you have right now that can be extended in multiple dimensions.

It seems simpler at first glance, but I prefer to follow the same
implementation as clone3(2) and openat2(2) because it is the same
(generic) struct argument parsing, which may then be handled by seccomp
and others, while adding a variable struct size depending on some flags
seems more complex to handle correctly and error-prone.

>>
>> This could possibly also help avoid the need for the get_features
> 
> What is this fresh hell again, please?

As explained in the commit message, this command/syscall enables to get
the Landlock supported features (i.e. option flags, commands, etc.),
which is required for backward and forward compatibility, and
best-effort security, while having strict kernel errors when
encountering unknown features. Cf.
https://landlock.io/linux-doc/landlock-v19/security/landlock/user.html
See the following arguments.

> 
>> syscall: If user space just passes the bitmap of all the access flags
>> it wants to use in a fixed-size structure, the kernel can update the
>> bits to mask out the ones it does not understand and write back
>> that bitmap as the result of create_ruleset().
>>
>>>>>> To be on the safe side, you might split up the flags into either the
>>>>>> upper/lower 16 bits or two u32 arguments, to allow both compatible
>>>>>> (ignored by older kernels if flag is set) and incompatible (return error
>>>>>> when an unknown flag is set) bits.
>>>>>
>>>>> This may be a good idea in general, but in the case of Landlock, because
>>>>> this kind of (discretionary) sandboxing should be a best-effort security
>>>>> feature, we should avoid incompatible behavior. In practice, every
>>>>> unknown bit returns an error because userland can probe for available
>>>>> bits thanks to the get_features command. This kind of (in)compatibility
>>>>> can then be handled by userland.
>>>>
>>>> If there are not going to be incompatible extensions, then just ignore
>>>> all unknown bits and never return an error but get rid of the user
>>>> space probing that just complicates the interface.
>>>
>>> There was multiple discussions about ABI compatibility, especially
>>> inspired by open(2) vs. openat2(2), and ignoring flags seems to be a bad
>>> idea. In the "sandboxer" example, we first probe the supported features
>>> and then mask unknown bits (i.e. access rights) at run time in userland.
>>> This strategy is quite straightforward, backward compatible and
>>> future-proof.
>>
>> For behavior changing flags, I agree they should be seen as
>> incompatible flags (i.e. return an error if an unknown bit is set).
>>
>> However, for the flags you pass in in an allowlist, treating them
>> as compatible (i.e. ignore any unknown flags, allowing everything
>> you are not forbidding already) seems completely reasonable
>> to me. Do you foresee user space doing anything other than masking
>> out the bits that the kernel doesn't know about? If not, then doing
>> it in the  kernel should always be simpler.

There is some use cases where userspace would benefit from this:

Because of backward and forward compatibility, one important case is for
common usage libraries, to help and encourage developers to not ignore
all security features if some are missing. Indeed, if from the start we
have get_features(), userland logic will be able to leverage it to
seamlessly enforce access-controls as much as possible whatever is the
running kernel.

For userland, being able to not trigger kernel errors (because userland
knows what is supported) enables to log them as real anomalies, which
can then help for debugging and understanding issues.

Landlock may also be combined with seccomp, but with different filters
according to the number of features the running kernel supports (e.g. an
app could filter some syscalls if they can't be restricted by the
running kernel through Landlock). Another point is that while seccomp
could be used as an alternative for some missing access-control from
Landlock, using seccomp for this could have a different impact on
performances (i.e. running BPF filters for each syscalls vs. controlling
access to kernel objects).

Another use case is that a system integrator should be aware that some
security features required by applications are not available. This may
be implemented as logging some warnings, or displaying the supported
security features as Chromium does (i.e. chrome://sandbox/).

This is also useful for (self)tests to ensure that a specific security
feature works as intended.

To sum up, get_features() and the strict kernel error codes enable
application developers to have a contract with the kernel. Multiple
use-cases can then be managed in userspace.


>>
>>>>> I suggest this syscall signature:
>>>>> SYSCALL_DEFINE3(landlock_create_ruleset, __u32, options, const struct
>>>>> landlock_attr_ruleset __user *, ruleset_ptr, size_t, ruleset_size);
>>>>
>>>> The other problem here is that indirect variable-size structured arguments
>>>> are a pain to instrument with things like strace or seccomp, so you
>>>> should first try to use a fixed argument list, and fall back to a fixed
>>>> structure if that fails.
>>>
>>> I agree that it is not perfect with the current tools but this kind of
>>> extensible structs are becoming common and well defined (e.g. openat2).
>>> Moreover there is some work going on for seccomp to support "extensible
>>> argument" syscalls: https://lwn.net/Articles/822256/
>>
>> openat2() is already more complex than we'd ideally want, I think we
>> should try hard to make new syscalls simpler than that, following the
>> rule that any interface should be as simple as possible, but no simpler.
> 
> Extensible structs are targeted at system calls that are either known to
> grow a lot of features or we already have prior versions that have
> accumulated quite a lot of features or that by their nature need to be
> more complex.
> openat2() is not really complex per se (At least not yet. It will likely
> grow quite a bit in the future...). The kernel now has infrastructure
> since clone3() and later generalized with openat2() and is well-equipped
> with a consistent api to deal with such syscalls so I don't see how this
> is really an issue in the first place. Yes, syscalls should be kept
> as simple as possible but we don't need to lock us into a "structs as
> arguments" are inherently bad mindset. That will also cause us to end up
> with crappy syscalls that are awkward to use for userspace.
> (Second-level pointers is a whole different issue of course.)

I agree.

> 
> (Arnd, you should also note that we're giving a talk at kernel summit
> about new syscall conventions and I'm syncing with Florian who'll be
> talking about the userspace side and requirements of this.)

Good! What do you think about these 4 new Landlock syscalls?

> 
> Christian
> 
>>
>>>>>>> +static int syscall_add_rule_path_beneath(const void __user *const attr_ptr,
>>>>>>> +               const size_t attr_size)
>>>>>>> +{
>>>>>>> +       struct landlock_attr_path_beneath attr_path_beneath;
>>>>>>> +       struct path path;
>>>>>>> +       struct landlock_ruleset *ruleset;
>>>>>>> +       int err;
>>>>>>
>>>>>> Similarly, it looks like this wants to be
>>>>>>
>>>>>> SYSCALL_DEFINE3(landlock_add_rule_path_beneath, int, ruleset, int,
>>>>>> path, __u32, flags)
>>>>>>
>>>>>> I don't see any need to extend this in a way that wouldn't already
>>>>>> be served better by adding another system call. You might argue
>>>>>> that 'flags' and 'allowed_access' could be separate, with the latter
>>>>>> being an indirect in/out argument here, like
>>>>>>
>>>>>> SYSCALL_DEFINE4(landlock_add_rule_path_beneath, int, ruleset, int, path,
>>>>>>                            __u64 *, allowed_acces, __u32, flags)
>>>>>
>>>>> To avoid adding a new syscall for each new rule type (e.g. path_beneath,
>>>>> path_range, net_ipv4_range, etc.), I think it would be better to keep
>>>>> the attr_ptr/attr_size pattern and to explicitely set a dedicated option
>>>>> flag to specify the attr type.
>>>>>
>>>>> This would look like this:
>>>>> SYSCALL_DEFINE4(landlock_add_rule, __u32, options, int, ruleset, const
>>>>> void __user *, rule_ptr, size_t, rule_size);
>>>>>
>>>>> The rule_ptr could then point to multiple types like struct
>>>>> landlock_attr_path_beneath (without the current ruleset_fd field).
>>>>
>>>> This again introduces variable-sized structured data. How many different
>>>> kinds of rule types do you think there will be (most likely, and maybe an
>>>> upper bound)?
>>>
>>> I don't know how many rule types will come, but right now I think it may
>>> be less than 10.
>>
>> Ok,
>>
>>>> Could (some of) these be generalized to use the same data structure?
>>>
>>> I don't think so, file path and network addresses are an example of very
>>> different types.
>>
>> Clearly the target object is something different, but maybe there is
>> enough commonality to still make them fit into a more regular form.

Because they don't exist yet, it seems risky to assume that they could
fit into a common form.

>>
>> For the file system case, you have an identify for an object
>> (the file descriptor) and the  '__u64 allowed_access'. I would
>> expect that the 'allowed_access' concept is generic enough that
>> you can make it a direct argument (32 bit register arg, or pointer
>> to a __u64). Do you expect others to need something besides
>> an object identifier and a permission bitmask?

Yes, a file descriptor (i.e. parent_fd) may not make sense to identify
every rule types. For example, it does not help to identify a network
subnet.

Even if it may be "shared", the allowed_access semantic depends on the
object type. It should also remain a 64-bits value to hold enough access
rights. I think it is more consistent, safer, and simpler to keep it in
the same pointed struct.

>> Maybe it could
>> be something like
>>
>>  SYSCALL_DEFINE4(landlock_add_rule, int, ruleset, __u32, options,
>>                        const void __user *, object, const __u64 __user
>> *, allowed_access,
>>                        __u32, flags);
>>
>> with a fixed-length 'object' identifier type (file descriptor,
>> sockaddr_storage, ...) for each option.

This would freeze the pointed struct to a fixed size, which may lead to
redundant struct parsing in the kernel. For example, the current
extended struct enables to extend struct landlock_attr_path_beneath with
a "path" field, which could have the same usage as in openat(2) to avoid
multiple calls to open(2) and close(2) to add new paths. If we have a
fixed-size struct, adding such field would require to create a new type
and I think it would make the kernel code more complex.

The "options" and "flags" seems confusing. For optional future flags, I
think that the name "options" is more explicit than "flags". What about
an "options" (set to zero for now) and a "rule_type" field to clearly
differentiate the rule type identifier (and use an enum)?
This would look like this:
SYSCALL_DEFINE5(landlock_add_rule, int, ruleset, __u32, options, __u32,
rule_type, const void __user *, rule_ptr, size_t, rule_size);

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