[Devel] Re: [PATCH 2/4] Expose may_setuid() in user.h

[Serge E. Hallyn]

Quoting Serge E. Hallyn (serue at us.ibm.com):
> Quoting Dan Smith (danms at us.ibm.com):
> > Make this helper available to others.
> 
> No objection to exporting may_setuid, nor to creating may_setgid(),
> but I don't think may_setgid() is right.  See below
> 
> > Signed-off-by: Dan Smith <danms at us.ibm.com>
> > ---
> >  include/linux/user.h |    9 +++++++++
> >  kernel/user.c        |   16 +++++++++++++++-
> >  2 files changed, 24 insertions(+), 1 deletions(-)
> > 
> > diff --git a/include/linux/user.h b/include/linux/user.h
> > index 68daf84..713bae7 100644
> > --- a/include/linux/user.h
> > +++ b/include/linux/user.h
> > @@ -1 +1,10 @@
> > +#ifndef _LINUX_USER_H
> > +#define _LINUX_USER_H
> > +
> >  #include <asm/user.h>
> > +#include <linux/sched.h>
> > +
> > +extern int may_setuid(struct user_namespace *ns, uid_t uid);
> > +extern int may_setgid(struct group_info *groupinfo, gid_t gid);
> > +
> > +#endif
> > diff --git a/kernel/user.c b/kernel/user.c
> > index a535ed6..38b8b50 100644
> > --- a/kernel/user.c
> > +++ b/kernel/user.c
> > @@ -604,7 +604,7 @@ int checkpoint_user(struct ckpt_ctx *ctx, void *ptr)
> >  	return do_checkpoint_user(ctx, (struct user_struct *) ptr);
> >  }
> > 
> > -static int may_setuid(struct user_namespace *ns, uid_t uid)
> > +int may_setuid(struct user_namespace *ns, uid_t uid)
> >  {
> >  	/*
> >  	 * this next check will one day become
> > @@ -631,6 +631,20 @@ static int may_setuid(struct user_namespace *ns, uid_t uid)
> >  	return 0;
> >  }
> > 
> > +int may_setgid(struct group_info *groupinfo, gid_t gid)
> > +{
> > +	if (capable(CAP_SETGID))
> > +		return 1;
> 
> We should pass in a user_ns so we can eventually check for
> capable_to(ns, CAP_SETGID).  So the caller may not be
> CAP_SETGID in the root user namespace, but may have created
> the child user namespace and be privileged there.
> 
> > +	if (current_cred_xxx(group_info) != groupinfo)
> > +		return 0;
> 
> I think it's possible for two different group_info's to have the same
> member groups.
> 
> I think the thing to do is walk over all groups in group_info,
> and do in_egroup_p(g) for each.
> 
> It's also possible that groups 1, 4, and 5 are in current_group_info and 6 is
> current_egroup, while we're asking for a group_info with groups 1, 5 and 6, and
> egid of 4.  That would be legal, right?  Walking over the groups and doing
> in_egroup_p(g) should do that check.  Sure, it's n^2 on the # groups...  So
> we could eventually optimize it to exploit the fact that both groupinfos
> are sorted and keep last_used_g in both groupinfos...
> 
> > +	if (in_egroup_p(gid))
> > +		return 1;

Ah, nm - you don't actually much care about the groupinfos, that
was a sanity check?

You don't actually want to check the ctx->realcred.  I think you
just want to check in_egroup_p(gid), as that will chekc against
the credentials used to call sys_restart(), which is what you want.

-serge
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