Re: [PATCH v4 10/30] selinux: implement get, set and remove acl hook

[Paul Moore <paul@paul-moore.com>]

On Thu, Sep 29, 2022 at 11:31 AM Christian Brauner <brauner@kernel.org> wrote:
>
> The current way of setting and getting posix acls through the generic
> xattr interface is error prone and type unsafe. The vfs needs to
> interpret and fixup posix acls before storing or reporting it to
> userspace. Various hacks exist to make this work. The code is hard to
> understand and difficult to maintain in it's current form. Instead of
> making this work by hacking posix acls through xattr handlers we are
> building a dedicated posix acl api around the get and set inode
> operations. This removes a lot of hackiness and makes the codepaths
> easier to maintain. A lot of background can be found in [1].
>
> So far posix acls were passed as a void blob to the security and
> integrity modules. Some of them like evm then proceed to interpret the
> void pointer and convert it into the kernel internal struct posix acl
> representation to perform their integrity checking magic. This is
> obviously pretty problematic as that requires knowledge that only the
> vfs is guaranteed to have and has lead to various bugs. Add a proper
> security hook for setting posix acls and pass down the posix acls in
> their appropriate vfs format instead of hacking it through a void
> pointer stored in the uapi format.
>
> I spent considerate time in the security module infrastructure and
> audited all codepaths. SELinux has no restrictions based on the posix
> acl values passed through it. The capability hook doesn't need to be
> called either because it only has restrictions on security.* xattrs. So
> these are all fairly simply hooks for SELinux.
>
> Link: https://lore.kernel.org/all/20220801145520.1532837-1-brauner@kernel.org [1]
> Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
> ---
>
> Notes:
>     /* v2 */
>     unchanged
>
>     /* v3 */
>     Paul Moore <paul@paul-moore.com>:
>     - Add get, and remove acl hook
>
>     /* v4 */
>     unchanged
>
>  security/selinux/hooks.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)

One small nitpick below, but looks good regardless.

Acked-by: Paul Moore <paul@paul-moore.com>

> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 79573504783b..0e3cd67e5e92 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -3239,6 +3239,27 @@ static int selinux_inode_setxattr(struct user_namespace *mnt_userns,
>                             &ad);
>  }
>
> +static int selinux_inode_set_acl(struct user_namespace *mnt_userns,
> +                                struct dentry *dentry, const char *acl_name,
> +                                struct posix_acl *kacl)
> +{
> +       return dentry_has_perm(current_cred(), dentry, FILE__SETATTR);
> +}
> +
> +static int selinux_inode_get_acl(struct user_namespace *mnt_userns,
> +                                struct dentry *dentry, const char *acl_name)
> +{
> +       const struct cred *cred = current_cred();
> +
> +       return dentry_has_perm(cred, dentry, FILE__GETATTR);
> +}

Both the set and remove hooks use current_cred() directly in the call
to dentry_has_perm(), you might as well do the same in the get hook.


> +static int selinux_inode_remove_acl(struct user_namespace *mnt_userns,
> +                                   struct dentry *dentry, const char *acl_name)
> +{
> +       return dentry_has_perm(current_cred(), dentry, FILE__SETATTR);
> +}

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