Re: [RFC PATCH v1 0/2] Add O_DENY_WRITE (complement AT_EXECVE_CHECK)

[Roberto Sassu <roberto.sassu@huaweicloud.com>]

On Thu, 2025-08-28 at 10:14 +1000, Aleksa Sarai wrote:
> On 2025-08-26, Mickaël Salaün <mic@digikod.net> wrote:
> > On Tue, Aug 26, 2025 at 11:07:03AM +0200, Christian Brauner wrote:
> > > Nothing has changed in that regard and I'm not interested in stuffing
> > > the VFS APIs full of special-purpose behavior to work around the fact
> > > that this is work that needs to be done in userspace. Change the apps,
> > > stop pushing more and more cruft into the VFS that has no business
> > > there.
> > 
> > It would be interesting to know how to patch user space to get the same
> > guarantees...  Do you think I would propose a kernel patch otherwise?
> 
> You could mmap the script file with MAP_PRIVATE. This is the *actual*
> protection the kernel uses against overwriting binaries (yes, ETXTBSY is
> nice but IIRC there are ways to get around it anyway). Of course, most
> interpreters don't mmap their scripts, but this is a potential solution.
> If the security policy is based on validating the script text in some
> way, this avoids the TOCTOU.
> 
> Now, in cases where you have IMA or something and you only permit signed
> binaries to execute, you could argue there is a different race here (an
> attacker creates a malicious script, runs it, and then replaces it with
> a valid script's contents and metadata after the fact to get
> AT_EXECVE_CHECK to permit the execution). However, I'm not sure that

Uhm, let's consider measurement, I'm more familiar with.

I think the race you wanted to express was that the attacker replaces
the good script, verified with AT_EXECVE_CHECK, with the bad script
after the IMA verification but before the interpreter reads it.

Fortunately, IMA is able to cope with this situation, since this race
can happen for any file open, where of course a file can be not read-
locked.

If the attacker tries to concurrently open the script for write in this
race window, IMA will report this event (called violation) in the
measurement list, and during remote attestation it will be clear that
the interpreter did not read what was measured.

We just need to run the violation check for the BPRM_CHECK hook too
(then, probably for us the O_DENY_WRITE flag or alternative solution
would not be needed, for measurement).

Please, let us know when you apply patches like 2a010c412853 ("fs:
don't block i_writecount during exec"). We had a discussion [1], but
probably I missed when it was decided to be applied (I saw now it was
in the same thread, but didn't get that at the time). We would have
needed to update our code accordingly. In the future, we will try to
clarify better our expectations from the VFS.

Thanks

Roberto

[1]: https://lore.kernel.org/linux-fsdevel/88d5a92379755413e1ec3c981d9a04e6796da110.camel@huaweicloud.com/#t

> this is even possible with IMA (can an unprivileged user even set
> security.ima?). But even then, I would expect users that really need
> this would also probably use fs-verity or dm-verity that would block
> this kind of attack since it would render the files read-only anyway.
> 
> This is why a more detailed threat model of what kinds of attacks are
> relevant is useful. I was there for the talk you gave and subsequent
> discussion at last year's LPC, but I felt that your threat model was
> not really fleshed out at all. I am still not sure what capabilities you
> expect the attacker to have nor what is being used to authenticate
> binaries (other than AT_EXECVE_CHECK). Maybe I'm wrong with my above
> assumptions, but I can't know without knowing what threat model you have
> in mind, *in detail*.
> 
> For example, if you are dealing with an attacker that has CAP_SYS_ADMIN,
> there are plenty of ways for an attacker to execute their own code
> without using interpreters (create a new tmpfs with fsopen(2) for
> instance). Executable memfds are even easier and don't require
> privileges on most systems (yes, you can block them with vm.memfd_noexec
> but CAP_SYS_ADMIN can disable that -- and there's always fsopen(2) or
> mount(2)).
> 
> (As an aside, it's a shame that AT_EXECVE_CHECK burned one of the
> top-level AT_* bits for a per-syscall flag -- the block comment I added
> in b4fef22c2fb9 ("uapi: explain how per-syscall AT_* flags should be
> allocated") was meant to avoid this happening but it seems you and the
> reviewers missed that...)
>