Re: [PATCH] selinux: do not remap unknown SIDs to the unlabeled context

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

On Tuesday, June 09, 2015 09:09:52 AM Stephen Smalley wrote:
> SELinux remaps invalid SIDs to the unlabeled SID/context in order
> to provide sane handling of objects whose SIDs become invalid upon
> a policy reload (e.g. removal of a type from policy).  However,
> this can also hide bugs and yield unexpected behavior, e.g. as described
> in https://bugzilla.redhat.com/show_bug.cgi?id=1224211, if a program
> sets SO_PASSSEC on a Unix stream socket, it will receive a SCM_SECURITY
> control message with the unlabeled context because the secid is not
> properly set/propagated for Unix stream sends, only for Unix datagram
> sends, but the automatic remapping of any invalid SID to the unlabeled
> context still produces a context to be returned when SO_PASSSEC is
> set on the socket.  Since commit 12b29f34558b9b45a2c6eabd4f3c6be939a3980f
> ("selinux: support deferred mapping of contexts") changed SELinux to not
> remove invalid SIDs from the SID table but rather to retain them with a
> copy of the unmapped context string so that the SID could be made valid
> again if a subsequent policy reload made the context valid again, we no
> longer need to map unknown SIDs to the unlabeled context, only SIDs that
> have unmapped context strings.
> 
> With this change applied, we get saner behavior for SCM_SECURITY on
> Unix stream sockets:  the kernel will not put any SCM_SECURITY control
> message at all rather than putting one with an unlabeled context.  If
> we want to support SCM_SECURITY on Unix stream sockets, that can be
> taken up as a separate change.  Regardless, this change will help catch
> cases where a secid/SID is never set (0) or contain a value beyond the
> set of allocated SIDs (e.g. never initialized and contains garbage).  The
> change does not break the support for deferred mapping of contexts; one
> can still insert a policy module that defines a type, label a file with
> that type, remove the policy module (i.e. load a policy that does not
> contain the type), check that the file's label is remapped to the
> unlabeled context, re-insert the policy module that defined the type,
> and see that the file's label is properly restored and valid.
> 
> Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
> ---
>  security/selinux/ss/sidtab.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)

This looks good to me thank for sending this patch.

Since SO_PASSSEC isn't widely used and/or documented I don't think there is 
any rush to push this for v4.2 since we've already done two pull requests for 
v4.2.  I'm also a little leery of changing such core behavior with little to 
no time in linux-next (although I will admit to being skeptical of the testing 
value of linux-next).  Unless someone can provide a very compelling reason why 
this needs to go into v4.2 I'm going to queue this up in my next-queue branch 
and apply it after the merge window is closed.

> diff --git a/security/selinux/ss/sidtab.c b/security/selinux/ss/sidtab.c
> index 5840a35..3bd992c 100644
> --- a/security/selinux/ss/sidtab.c
> +++ b/security/selinux/ss/sidtab.c
> @@ -98,7 +98,10 @@ static struct context *sidtab_search_core(struct sidtab
> *s, u32 sid, int force) if (force && cur && sid == cur->sid &&
> cur->context.len)
>  		return &cur->context;
> 
> -	if (cur == NULL || sid != cur->sid || cur->context.len) {
> +	if (cur == NULL || sid != cur->sid)
> +		return NULL;
> +
> +	if (cur->context.len) {
>  		/* Remap invalid SIDs to the unlabeled SID. */
>  		sid = SECINITSID_UNLABELED;
>  		hvalue = SIDTAB_HASH(sid);

-- 
paul moore
www.paul-moore.com