RE: [PATCH RFC v2 3/5] LSM: Security module checking for side-channel dangers

[Schaufler, Casey]

> -----Original Message-----
> From: Jann Horn [mailto:jannh@xxxxxxxxxx]
> Sent: Friday, August 17, 2018 4:53 PM
> To: Schaufler, Casey <casey.schaufler@xxxxxxxxx>
> Cc: Kernel Hardening <kernel-hardening@xxxxxxxxxxxxxxxxxx>; kernel list
> <linux-kernel@xxxxxxxxxxxxxxx>; linux-security-module <linux-security-
> module@xxxxxxxxxxxxxxx>; selinux@xxxxxxxxxxxxx; Hansen, Dave
> <dave.hansen@xxxxxxxxx>; Dock, Deneen T <deneen.t.dock@xxxxxxxxx>;
> kristen@xxxxxxxxxxxxxxx; Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
> Subject: Re: [PATCH RFC v2 3/5] LSM: Security module checking for side-
> channel dangers
> 
> On Sat, Aug 18, 2018 at 12:17 AM Casey Schaufler
> <casey.schaufler@xxxxxxxxx> wrote:
> >
> > From: Casey Schaufler <cschaufler@xxxxxxxxxxxxxxxxxxxxx>
> >
> > The sidechannel LSM checks for cases where a side-channel
> > attack may be dangerous based on security attributes of tasks.
> > This includes:
> >         Effective UID of the tasks is different
> >         Capablity sets are different
> >         Tasks are in different namespaces
> > An option is also provided to assert that task are never
> > to be considered safe. This is high paranoia, and expensive
> > as well.
> >
> > Signed-off-by: Casey Schaufler <casey.schaufler@xxxxxxxxx>
> [...]
> > +#ifdef CONFIG_SECURITY_SIDECHANNEL_UIDS
> > +static int safe_by_uid(struct task_struct *p)
> > +{
> > +       const struct cred *ccred = current_real_cred();
> > +       const struct cred *pcred = get_task_cred(p);
> > +
> > +       /*
> > +        * Credential checks. Considered safe if:
> > +        *      UIDs are the same
> > +        */
> > +       if (ccred != pcred && ccred->euid.val != pcred->euid.val)
> > +               return -EACCES;
> > +       return 0;
> > +}
> 
> This function looks bogus. get_task_cred() bumps the refcount on the
> returned cred struct pointer, but you don't drop it. You probably want
> to use something that doesn't fiddle with the refcount at all here to
> avoid cacheline bouncing - possibly a raw rcu_dereference_protected()
> if there are no better helpers.
> 
> Same thing for the other get_task_cred() calls further down in the patch.

Thanks. Looks like I whacked out v2 a bit hastily.