Date:	Wed, 2 Sep 2015 15:54:53 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Andreas Gruenbacher <andreas.gruenbacher@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-nfs@...r.kernel.org, linux-api@...r.kernel.org,
	linux-cifs@...r.kernel.org, linux-security-module@...r.kernel.org,
	Andreas Gruenbacher <agruenba@...hat.com>
Subject: Re: [RFC v6 08/40] richacl: Compute maximum file masks from an acl

On Wed, Sep 02, 2015 at 03:54:08PM -0400, bfields wrote:
> On Tue, Aug 04, 2015 at 01:53:06PM +0200, Andreas Gruenbacher wrote:
> > Compute upper bound owner, group, and other file masks with as few
> > permissions as possible without denying any permissions that the NFSv4
> > acl in a richacl grants.
> > 
> > This algorithm is used when a file inherits an acl at create time and
> > when an acl is set via a mechanism that does not provide file masks
> > (such as setting an acl via nfsd).  When user-space sets an acl via
> > setxattr, the extended attribute already includes the file masks.
> > 
> > Setting an acl also sets the file mode permission bits: they are
> > determined by the file masks; see richacl_masks_to_mode().
> > 
> > Signed-off-by: Andreas Gruenbacher <agruenba@...hat.com>
> > Reviewed-by: J. Bruce Fields <bfields@...hat.com>
> > ---
> >  fs/richacl_base.c       | 156 ++++++++++++++++++++++++++++++++++++++++++++++++
> >  include/linux/richacl.h |   1 +
> >  2 files changed, 157 insertions(+)
> > 
> > diff --git a/fs/richacl_base.c b/fs/richacl_base.c
> > index 063dbe4..8426600 100644
> > --- a/fs/richacl_base.c
> > +++ b/fs/richacl_base.c
> > @@ -182,3 +182,159 @@ richacl_want_to_mask(unsigned int want)
> >  	return mask;
> >  }
> >  EXPORT_SYMBOL_GPL(richacl_want_to_mask);
> > +
> > +/*
> > + * Note: functions like richacl_allowed_to_who(), richacl_group_class_allowed(),
> > + * and richacl_compute_max_masks() iterate through the entire acl in reverse
> > + * order as an optimization.
> > + *
> > + * In the standard algorithm, aces are considered in forward order.  When a
> > + * process matches an ace, the permissions in the ace are either allowed or
> > + * denied depending on the ace type.  Once a permission has been allowed or
> > + * denied, it is no longer considered in further aces.
> > + *
> > + * By iterating through the acl in reverse order, we can compute the same
> > + * result without having to keep track of which permissions have been allowed
> > + * and denied already.
> > + */
> > +
> > +/**
> > + * richacl_allowed_to_who  -  permissions allowed to a specific who value
> > + *
> > + * Compute the maximum mask values allowed to a specific who value, taking
> > + * everyone@ aces into account.
> > + */
> > +static unsigned int richacl_allowed_to_who(struct richacl *acl,
> > +					   struct richace *who)
> > +{
> > +	struct richace *ace;
> > +	unsigned int allowed = 0;
> > +
> > +	richacl_for_each_entry_reverse(ace, acl) {
> > +		if (richace_is_inherit_only(ace))
> > +			continue;
> > +		if (richace_is_same_identifier(ace, who) ||
> > +		    richace_is_everyone(ace)) {
> > +			if (richace_is_allow(ace))
> > +				allowed |= ace->e_mask;
> > +			else if (richace_is_deny(ace))
> > +				allowed &= ~ace->e_mask;
> > +		}
> > +	}
> > +	return allowed;
> > +}
> > +
> > +/**
> > + * richacl_group_class_allowed  -  maximum permissions of the group class
> > + *
> > + * Compute the maximum mask values allowed to a process in the group class
> > + * (i.e., a process which is not the owner but is in the owning group or
> > + * matches a user or group acl entry).  This includes permissions granted or
> > + * denied by everyone@ aces.
> > + *
> > + * See richacl_compute_max_masks().
> > + */
> > +static unsigned int richacl_group_class_allowed(struct richacl *acl)
> > +{
> > +	struct richace *ace;
> > +	unsigned int everyone_allowed = 0, group_class_allowed = 0;
> > +	int had_group_ace = 0;
> > +
> > +	richacl_for_each_entry_reverse(ace, acl) {
> > +		if (richace_is_inherit_only(ace) ||
> > +		    richace_is_owner(ace))
> > +			continue;
> > +
> > +		if (richace_is_everyone(ace)) {
> > +			if (richace_is_allow(ace))
> > +				everyone_allowed |= ace->e_mask;
> > +			else if (richace_is_deny(ace))
> > +				everyone_allowed &= ~ace->e_mask;
> > +		} else {
> > +			group_class_allowed |=
> > +				richacl_allowed_to_who(acl, ace);
> > +
> > +			if (richace_is_group(ace))
> > +				had_group_ace = 1;
> > +		}
> > +	}
> > +	/*
> > +	 * If the acl doesn't contain any group@ aces, richacl_allowed_to_who()
> > +	 * wasn't called for the owning group.  We could make that call now, but
> > +	 * we already know the result (everyone_allowed).
> > +	 */
> > +	if (!had_group_ace)
> > +		group_class_allowed |= everyone_allowed;
> > +	return group_class_allowed;
> > +}
> > +
> > +/**
> > + * richacl_compute_max_masks  -  compute upper bound masks
> > + *
> > + * Computes upper bound owner, group, and other masks so that none of
> > + * the mask flags allowed by the acl are disabled (for any user with any
> > + * group membership).
> > + */
> > +void richacl_compute_max_masks(struct richacl *acl, kuid_t owner)
> > +{
> > +	unsigned int gmask = ~0;
> > +	struct richace *ace;
> > +
> > +	/*
> > +	 * @gmask contains all permissions which the group class is ever
> > +	 * allowed.  We use it to avoid adding permissions to the group mask
> > +	 * from everyone@ allow aces which the group class is always denied
> > +	 * through other aces.  For example, the following acl would otherwise
> > +	 * result in a group mask of rw:
> > +	 *
> > +	 *	group@:w::deny
> > +	 *	everyone@:rw::allow
> > +	 *
> > +	 * Avoid computing @gmask for acls which do not include any group class
> > +	 * deny aces: in such acls, the group class is never denied any
> > +	 * permissions from everyone@ allow aces, and the group class cannot
> > +	 * have fewer permissions than the other class.
> > +	 */
> > +
> > +restart:
> > +	acl->a_owner_mask = 0;
> > +	acl->a_group_mask = 0;
> > +	acl->a_other_mask = 0;
> > +
> > +	richacl_for_each_entry_reverse(ace, acl) {
> > +		if (richace_is_inherit_only(ace))
> > +			continue;
> > +
> > +		if (richace_is_owner(ace) ||
> > +		    (richace_is_unix_user(ace) &&
> > +		     uid_eq(ace->e_id.uid, owner))) {
> > +			if (richace_is_allow(ace))
> > +				acl->a_owner_mask |= ace->e_mask;
> > +			else if (richace_is_deny(ace))
> > +				acl->a_owner_mask &= ~ace->e_mask;
> > +		} else if (richace_is_everyone(ace)) {
> > +			if (richace_is_allow(ace)) {
> > +				acl->a_owner_mask |= ace->e_mask;
> > +				acl->a_group_mask |= ace->e_mask & gmask;
> > +				acl->a_other_mask |= ace->e_mask;
> > +			} else if (richace_is_deny(ace)) {
> > +				acl->a_owner_mask &= ~ace->e_mask;
> > +				acl->a_group_mask &= ~ace->e_mask;
> > +				acl->a_other_mask &= ~ace->e_mask;
> > +			}
> > +		} else {
> > +			if (richace_is_allow(ace)) {
> > +				acl->a_owner_mask |= ace->e_mask & gmask;
> > +				acl->a_group_mask |= ace->e_mask & gmask;
> 
> I think we do that because we don't (we can't) know whether the owner
> might match this ace, so we assume that it will match, as that's what
> gives us the maximum.
> 
> But on first glance this is a little counterintuitive and maybe worth a
> comment.

(By the way, feel free to add a

	Reviewed-by: J. Bruce Fields <bfields@...hat.com>

for these first 8 patches.)

--b.

> 
> --b.
> 
> > +			} else if (richace_is_deny(ace) && gmask == ~0) {
> > +				gmask = richacl_group_class_allowed(acl);
> > +				if (likely(gmask != ~0))
> > +					/* should always be true */
> > +					goto restart;
> > +			}
> > +		}
> > +	}
> > +
> > +	acl->a_flags &= ~(RICHACL_WRITE_THROUGH | RICHACL_MASKED);
> > +}
> > +EXPORT_SYMBOL_GPL(richacl_compute_max_masks);
> > diff --git a/include/linux/richacl.h b/include/linux/richacl.h
> > index f4ba113..3d719db 100644
> > --- a/include/linux/richacl.h
> > +++ b/include/linux/richacl.h
> > @@ -303,5 +303,6 @@ extern void richace_copy(struct richace *, const struct richace *);
> >  extern int richacl_masks_to_mode(const struct richacl *);
> >  extern unsigned int richacl_mode_to_mask(mode_t);
> >  extern unsigned int richacl_want_to_mask(unsigned int);
> > +extern void richacl_compute_max_masks(struct richacl *, kuid_t);
> >  
> >  #endif /* __RICHACL_H */
> > -- 
> > 2.5.0
> > 
> > --
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