Re: [RFC v7 26/41] richacl: Apply the file masks to a richacl

["J. Bruce Fields" <bfields@fieldses.org>]

On Tue, Sep 22, 2015 at 03:11:08PM -0400, bfields wrote:
> On Sat, Sep 05, 2015 at 12:27:21PM +0200, Andreas Gruenbacher wrote:
> > Put all the pieces of the acl transformation puzzle together for
> > computing a richacl which has the file masks "applied" so that the
> > standard nfsv4 access check algorithm can be used on the richacl.
> > 
> > Signed-off-by: Andreas Gruenbacher <agruen@kernel.org>
> > ---
> >  fs/richacl_compat.c     | 110 ++++++++++++++++++++++++++++++++++++++++++++++++
> >  include/linux/richacl.h |   3 ++
> >  2 files changed, 113 insertions(+)
> > 
> > diff --git a/fs/richacl_compat.c b/fs/richacl_compat.c
> > index 412844c..9681efe 100644
> > --- a/fs/richacl_compat.c
> > +++ b/fs/richacl_compat.c
> > @@ -730,3 +730,113 @@ richacl_isolate_group_class(struct richacl_alloc *alloc)
> >  	}
> >  	return 0;
> >  }
> > +
> > +/**
> > + * __richacl_apply_masks  -  apply the file masks to all aces
> > + * @alloc:	acl and number of allocated entries
> > + *
> > + * Apply the owner mask to owner@ aces, the other mask to
> > + * everyone@ aces, and the group mask to all other aces.
> > + *
> > + * The previous transformations have brought the acl into a
> > + * form in which applying the masks will not lead to the
> > + * accidental loss of permissions anymore.
> > + */
> > +static int
> > +__richacl_apply_masks(struct richacl_alloc *alloc, kuid_t owner)
> > +{
> > +	struct richace *ace;
> > +
> > +	richacl_for_each_entry(ace, alloc->acl) {
> > +		unsigned int mask;
> > +
> > +		if (richace_is_inherit_only(ace) || !richace_is_allow(ace))
> > +			continue;
> > +		if (richace_is_owner(ace) ||
> > +		    (richace_is_unix_user(ace) && uid_eq(owner, ace->e_id.uid)))
> > +			mask = alloc->acl->a_owner_mask;
> 
> Is treating matching user aces like owner aces what you intended to do,
> and if so, why?

That does look wrong to me; in an example like:

	file owner bfields
	mask 0700, not WRITE_THROUGH
	bfields:rwx::allow

The permission algorithm grants nothing to anyone, but it looks to me
like richacl_apply_masks just leaves this as

	bfields:rwx::allow

but it would give the right result (an empty/deny-all ACL) if it weren't
for this odd case here.

--b.

> 
> --b.
> 
> > +		else if (richace_is_everyone(ace))
> > +			mask = alloc->acl->a_other_mask;
> > +		else
> > +			mask = alloc->acl->a_group_mask;
> > +		if (richace_change_mask(alloc, &ace, ace->e_mask & mask))
> > +			return -1;
> > +	}
> > +	return 0;
> > +}
> > +
> > +/**
> > + * richacl_apply_masks  -  apply the masks to the acl
> > + *
> > + * Transform @acl so that the standard NFSv4 permission check algorithm (which
> > + * is not aware of file masks) will compute the same access decisions as the
> > + * richacl permission check algorithm (which looks at the acl and the file
> > + * masks).
> > + *
> > + * This algorithm is split into several steps:
> > + *
> > + *   - Move everyone@ aces to the end of the acl.  This simplifies the other
> > + *     transformations, and allows the everyone@ allow ace at the end of the
> > + *     acl to eventually allow permissions to the other class only.
> > + *
> > + *   - Propagate everyone@ permissions up the acl.  This transformation makes
> > + *     sure that the owner and group class aces won't lose any permissions when
> > + *     we apply the other mask to the everyone@ allow ace at the end of the acl.
> > + *
> > + *   - Apply the file masks to all aces.
> > + *
> > + *   - Make sure the owner is granted the owner mask permissions.
> > + *
> > + *   - Make sure everyone is granted the other mask permissions.
> > + *
> > + *   - Make sure that the owner is not granted any permissions beyond the owner
> > + *     mask from group class aces or from everyone@.
> > + *
> > + *   - Make sure that the group class is not granted any permissions from
> > + *     everyone@.
> > + *
> > + * The algorithm is exact except for richacls which cannot be represented as an
> > + * acl alone: for example, given this acl:
> > + *
> > + *    group@:rw::allow
> > + *
> > + * when file masks corresponding to mode 0600 are applied, the owner would only
> > + * get rw access if he is a member of the owning group.  This algorithm would
> > + * produce an empty acl in this case.  We fix this case by modifying
> > + * richacl_permission() so that the group mask is always applied to group class
> > + * aces.  With this fix, the owner would not have any access (beyond the
> > + * implicit permissions always granted to owners).
> > + *
> > + * NOTE: Depending on the acl and file masks, this algorithm can increase the
> > + * number of aces by almost a factor of three in the worst case. This may make
> > + * the acl too large for some purposes.
> > + */
> > +int
> > +richacl_apply_masks(struct richacl **acl, kuid_t owner)
> > +{
> > +	if ((*acl)->a_flags & RICHACL_MASKED) {
> > +		struct richacl_alloc alloc = {
> > +			.acl = richacl_clone(*acl, GFP_KERNEL),
> > +			.count = (*acl)->a_count,
> > +		};
> > +		if (!alloc.acl)
> > +			return -ENOMEM;
> > +
> > +		if (richacl_move_everyone_aces_down(&alloc) ||
> > +		    richacl_propagate_everyone(&alloc) ||
> > +		    __richacl_apply_masks(&alloc, owner) ||
> > +		    richacl_set_owner_permissions(&alloc) ||
> > +		    richacl_set_other_permissions(&alloc) ||
> > +		    richacl_isolate_owner_class(&alloc) ||
> > +		    richacl_isolate_group_class(&alloc)) {
> > +			richacl_put(alloc.acl);
> > +			return -ENOMEM;
> > +		}
> > +
> > +		alloc.acl->a_flags &= ~(RICHACL_WRITE_THROUGH | RICHACL_MASKED);
> > +		richacl_put(*acl);
> > +		*acl = alloc.acl;
> > +	}
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(richacl_apply_masks);
> > diff --git a/include/linux/richacl.h b/include/linux/richacl.h
> > index 832b06c..a945f3c 100644
> > --- a/include/linux/richacl.h
> > +++ b/include/linux/richacl.h
> > @@ -332,4 +332,7 @@ extern struct richacl *richacl_inherit(const struct richacl *, int);
> >  extern int richacl_permission(struct inode *, const struct richacl *, int);
> >  extern struct richacl *richacl_create(struct inode *, struct inode *);
> >  
> > +/* richacl_compat.c */
> > +extern int richacl_apply_masks(struct richacl **, kuid_t);
> > +
> >  #endif /* __RICHACL_H */
> > -- 
> > 2.4.3
> > 
> > --
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