Re: [PATCH v7 0/7] Add support for O_MAYEXEC

[Mickaël Salaün]

On 11/08/2020 01:03, Jann Horn wrote:
> On Tue, Aug 11, 2020 at 12:43 AM Mickaël Salaün <mic@xxxxxxxxxxx> wrote:
>> On 10/08/2020 22:21, Al Viro wrote:
>>> On Mon, Aug 10, 2020 at 10:11:53PM +0200, Mickaël Salaün wrote:
>>>> It seems that there is no more complains nor questions. Do you want me
>>>> to send another series to fix the order of the S-o-b in patch 7?
>>>
>>> There is a major question regarding the API design and the choice of
>>> hooking that stuff on open().  And I have not heard anything resembling
>>> a coherent answer.
>>
>> Hooking on open is a simple design that enables processes to check files
>> they intend to open, before they open them. From an API point of view,
>> this series extends openat2(2) with one simple flag: O_MAYEXEC. The
>> enforcement is then subject to the system policy (e.g. mount points,
>> file access rights, IMA, etc.).
>>
>> Checking on open enables to not open a file if it does not meet some
>> requirements, the same way as if the path doesn't exist or (for whatever
>> reasons, including execution permission) if access is denied.
> 
> You can do exactly the same thing if you do the check in a separate
> syscall though.
> 
> And it provides a greater degree of flexibility; for example, you can
> use it in combination with fopen() without having to modify the
> internals of fopen() or having to use fdopen().
> 
>> It is a
>> good practice to check as soon as possible such properties, and it may
>> enables to avoid (user space) time-of-check to time-of-use (TOCTOU)
>> attacks (i.e. misuse of already open resources).
> 
> The assumption that security checks should happen as early as possible
> can actually cause security problems. For example, because seccomp was
> designed to do its checks as early as possible, including before
> ptrace, we had an issue for a long time where the ptrace API could be
> abused to bypass seccomp filters.
> 
> Please don't decide that a check must be ordered first _just_ because
> it is a security check. While that can be good for limiting attack
> surface, it can also create issues when the idea is applied too
> broadly.

I'd be interested with such security issue examples.

I hope that delaying checks will not be an issue for mechanisms such as
IMA or IPE:
https://lore.kernel.org/lkml/1544699060.6703.11.camel@xxxxxxxxxxxxx/

Any though Mimi, Deven, Chrome OS folks?

> 
> I don't see how TOCTOU issues are relevant in any way here. If someone
> can turn a script that is considered a trusted file into an untrusted
> file and then maliciously change its contents, you're going to have
> issues either way because the modifications could still happen after
> openat(); if this was possible, the whole thing would kind of fall
> apart. And if that isn't possible, I don't see any TOCTOU.

Sure, and if the scripts are not protected in some way there is no point
to check anything.

> 
>> It is important to keep
>> in mind that the use cases we are addressing consider that the (user
>> space) script interpreters (or linkers) are trusted and unaltered (i.e.
>> integrity/authenticity checked). These are similar sought defensive
>> properties as for SUID/SGID binaries: attackers can still launch them
>> with malicious inputs (e.g. file paths, file descriptors, environment
>> variables, etc.), but the binaries can then have a way to check if they
>> can extend their trust to some file paths.
>>
>> Checking file descriptors may help in some use cases, but not the ones
>> motivating this series.
> 
> It actually provides a superset of the functionality that your
> existing patches provide.

It also brings new issues with multiple file descriptor origins (e.g.
memfd_create).

> 
>> Checking (already) opened resources could be a
>> *complementary* way to check execute permission, but it is not in the
>> scope of this series.