linux-kernel - Re: [PATCH] security: keys: perform capable check only on privileged operations

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Message-ID: <CAHC9VhTcso+RTEOkGOCDxyMscznEXrUhp+quDWvATUhEzEOhRQ@mail.gmail.com>
Date:   Fri, 19 May 2023 17:07:51 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Christian Göttsche <cgzones@...glemail.com>
Cc:     selinux@...r.kernel.org, David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, keyrings@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] security: keys: perform capable check only on privileged operations

On Thu, May 11, 2023 at 8:33 AM Christian Göttsche
<cgzones@...glemail.com> wrote:
>
> If the current task fails the check for the queried capability via
> `capable(CAP_SYS_ADMIN)` LSMs like SELinux generate a denial message.
> Issuing such denial messages unnecessarily can lead to a policy author
> granting more privileges to a subject than needed to silence them.
>
> Reorder CAP_SYS_ADMIN checks after the check whether the operation is
> actually privileged.
>
> Signed-off-by: Christian Göttsche <cgzones@...glemail.com>
> ---
>  security/keys/keyctl.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> index d54f73c558f7..19be69fa4d05 100644
> --- a/security/keys/keyctl.c
> +++ b/security/keys/keyctl.c
> @@ -980,14 +980,19 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
>         ret = -EACCES;
>         down_write(&key->sem);
>
> -       if (!capable(CAP_SYS_ADMIN)) {
> +       {
> +               bool is_privileged_op = false;
> +
>                 /* only the sysadmin can chown a key to some other UID */
>                 if (user != (uid_t) -1 && !uid_eq(key->uid, uid))
> -                       goto error_put;
> +                       is_privileged_op = true;
>
>                 /* only the sysadmin can set the key's GID to a group other
>                  * than one of those that the current process subscribes to */
>                 if (group != (gid_t) -1 && !gid_eq(gid, key->gid) && !in_group_p(gid))
> +                       is_privileged_op = true;
> +
> +               if (is_privileged_op && !capable(CAP_SYS_ADMIN))
>                         goto error_put;
>         }

Hmm.  Using braces just to create a new scope is a bit hacky; I'll
admit to using it to quickly create new local variables, but I only do
so in debug/test situations, not production code.

What if you move the CAP_SYS_ADMIN check down into the if-conditional
where the code checks to see if CAP_SYS_ADMIN is needed when changing
the UID?  It should be possible to structure the CAP_SYS_ADMIN check
such that it is only executed if needed.  It's a little more
complicated in the GID case, but I believe it should be doable.

-- 
paul-moore.com