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

Mickaël Salaün
2020-08-11T08:49:59Z

On 11/08/2020 01:05, Al Viro wrote:
> On Tue, Aug 11, 2020 at 12:43:52AM +0200, Mickaël Salaün wrote:
>
>> Hooking on open is a simple design that enables processes to check files
>> they intend to open, before they open them.
>
> Which is a good thing, because...?
>
>> 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.).
>
> That's what "unspecified" means - as far as the kernel concerned, it's
> "something completely opaque, will let these hooks to play, semantics is
> entirely up to them".

I see it as an access controls mechanism; access may be granted or
denied, as for O_RDONLY, O_WRONLY or (non-Linux) O_EXEC. Even for common
access controls, there are capabilities to bypass them (i.e.
CAP_DAC_OVERRIDE), but multiple layers may enforce different
complementary policies.

>
>> 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. 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).
>
> ????? You explicitly assume a cooperating caller.

As said in the below (removed) reply, no, quite the contrary.

> If it can't be trusted
> to issue the check between open and use, or can be manipulated (ptraced,
> etc.) into not doing so, how can you rely upon the flag having been passed
> in the first place? And TOCTOU window is definitely not wider that way.

OK, I guess it would be considered a bug in the application (e.g. buggy
resource management between threads).

>
> If you want to have it done immediately after open(), bloody well do it
> immediately after open. If attacker has subverted your control flow to the
> extent that allows them to hit descriptor table in the interval between
> these two syscalls, you have already lost - they'll simply prevent that
> flag from being passed.
>
> What's the point of burying it inside openat2()? A convenient multiplexor
> to hook into? We already have one - it's called do_syscall_...
>

To check as soon as possible without opening something that should not
be opened in the first place.

Isn't a dedicated syscall a bit too much for this feature? What about
adding a new command/flag to fcntl(2)?