[PATCH v10 5/8] rust: security: add abstraction for secctx

[Alice Ryhl]

On Sun, Sep 15, 2024 at 10:58 PM Kees Cook <kees at kernel.org> wrote:
>
> On Sun, Sep 15, 2024 at 02:31:31PM +0000, Alice Ryhl wrote:
> > Add an abstraction for viewing the string representation of a security
> > context.
>
> Hm, this may collide with "LSM: Move away from secids" is going to happen.
> https://lore.kernel.org/all/20240830003411.16818-1-casey@schaufler-ca.com/
>
> This series is not yet landed, but in the future, the API changes should
> be something like this, though the "lsmblob" name is likely to change to
> "lsmprop"?
> security_cred_getsecid()   -> security_cred_getlsmblob()
> security_secid_to_secctx() -> security_lsmblob_to_secctx()

Thanks for the heads up. I'll make sure to look into how this
interacts with those changes.

> > This is needed by Rust Binder because it has a feature where a process
> > can view the string representation of the security context for incoming
> > transactions. The process can use that to authenticate incoming
> > transactions, and since the feature is provided by the kernel, the
> > process can trust that the security context is legitimate.
> >
> > This abstraction makes the following assumptions about the C side:
> > * When a call to `security_secid_to_secctx` is successful, it returns a
> >   pointer and length. The pointer references a byte string and is valid
> >   for reading for that many bytes.
>
> Yes. (len includes trailing C-String NUL character.)

I suppose the NUL character implies that this API always returns a
non-zero length? I could simplify the patch a little bit by not
handling empty strings.

It looks like the CONFIG_SECURITY=n case returns -EOPNOTSUPP, so we
don't get an empty string from that case, at least.

> > * The string may be referenced until `security_release_secctx` is
> >   called.
>
> Yes.
>
> > * If CONFIG_SECURITY is set, then the three methods mentioned in
> >   rust/helpers are available without a helper. (That is, they are not a
> >   #define or `static inline`.)
>
> Yes.
>
> >
> > Reviewed-by: Benno Lossin <benno.lossin at proton.me>
> > Reviewed-by: Martin Rodriguez Reboredo <yakoyoku at gmail.com>
> > Reviewed-by: Trevor Gross <tmgross at umich.edu>
> > Reviewed-by: Gary Guo <gary at garyguo.net>
> > Signed-off-by: Alice Ryhl <aliceryhl at google.com>
>
> Reviewed-by: Kees Cook <kees at kernel.org>

Thanks for the review!

Alice