On Tue, Apr 28, 2020 at 8:54 AM David Howells <dhowells@xxxxxxxxxx> wrote:
>
> selinux: Fix use of KEY_NEED_* instead of KEY__* perms
>
> selinux_key_permission() is passing the KEY_NEED_* permissions to
> avc_has_perm() instead of the KEY__* values. It happens to work because
> the values are all coincident.
>
> Fixes: d720024e94de ("[PATCH] selinux: add hooks for key subsystem")
> Reported-by: Paul Moore <paul@xxxxxxxxxxxxxx>
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> ---
> security/selinux/hooks.c | 23 +++++++++++++++++++++--
> 1 file changed, 21 insertions(+), 2 deletions(-)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 0b4e32161b77..4b6624e5dab4 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -6539,20 +6539,39 @@ static void selinux_key_free(struct key *k)
> kfree(ksec);
> }
>
> +static unsigned int selinux_keyperm_to_av(unsigned int need_perm)
> +{
> + switch (need_perm) {
> + case KEY_NEED_VIEW: return KEY__VIEW;
> + case KEY_NEED_READ: return KEY__READ;
> + case KEY_NEED_WRITE: return KEY__WRITE;
> + case KEY_NEED_SEARCH: return KEY__SEARCH;
> + case KEY_NEED_LINK: return KEY__LINK;
> + case KEY_NEED_SETATTR: return KEY__SETATTR;
> + default:
> + WARN_ON(1);
> + return 0;
> + }
> +}
> +
> static int selinux_key_permission(key_ref_t key_ref,
> const struct cred *cred,
> - unsigned perm)
> + unsigned need_perm)
> {
> struct key *key;
> struct key_security_struct *ksec;
> + unsigned int perm;
> u32 sid;
>
> /* if no specific permissions are requested, we skip the
> permission check. No serious, additional covert channels
> appear to be created. */
> - if (perm == 0)
> + if (need_perm == 0)
> return 0;
>
> + perm = selinux_keyperm_to_av(need_perm);
> + if (perm == 0)
> + return -EPERM;
> sid = cred_sid(cred);
>
> key = key_ref_to_ptr(key_ref);
1) Are we guaranteed that the caller only ever passes a single
KEY_NEED_* perm at a time (i.e. hook is never called with a bitmask
of multiple permissions)? Where is that guarantee enforced?
2) We had talked about adding a BUILD_BUG_ON() or other build-time
guard to ensure that new KEY_NEED_* permissions
are not added without updating SELinux. We already have similar
constructs for catching new capabilities (#if CAP_LAST_CAP > 63 #error
...), socket address families (#if PF_MAX > 45 #error ...), RTM_* and
XFRM_MSG* values.
