Re: [PATCH v8 3/9] landlock: Refactor check_access_path_dual() into is_access_to_paths_allowed()

["Günther Noack" <gnoack3000@gmail.com>]

On Wed, Oct 05, 2022 at 08:54:08PM +0200, Mickaël Salaün wrote:
> On 01/10/2022 17:49, Günther Noack wrote:
> > * Rename it to is_access_to_paths_allowed()
> > * Make it return true iff the access is allowed
> > * Calculate the EXDEV/EACCES error code in the one place where it's needed
> 
> Can you please replace these bullet points with (one-sentence) paragraphs?

Done.

> > Suggested-by: Mickaël Salaün <mic@digikod.net>
> > Signed-off-by: Günther Noack <gnoack3000@gmail.com>
> > ---
> >   security/landlock/fs.c | 89 +++++++++++++++++++++---------------------
> >   1 file changed, 44 insertions(+), 45 deletions(-)
> > 
> > diff --git a/security/landlock/fs.c b/security/landlock/fs.c
> > index a9dbd99d9ee7..083dd3d359de 100644
> > --- a/security/landlock/fs.c
> > +++ b/security/landlock/fs.c
> > @@ -430,7 +430,7 @@ is_eacces(const layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS],
> >   }
> >   /**
> > - * check_access_path_dual - Check accesses for requests with a common path
> > + * is_access_to_paths_allowed - Check accesses for requests with a common path
> >    *
> >    * @domain: Domain to check against.
> >    * @path: File hierarchy to walk through.
> > @@ -465,14 +465,10 @@ is_eacces(const layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS],
> >    * allow the request.
> >    *
> >    * Returns:
> > - * - 0 if the access request is granted;
> > - * - -EACCES if it is denied because of access right other than
> > - *   LANDLOCK_ACCESS_FS_REFER;
> > - * - -EXDEV if the renaming or linking would be a privileged escalation
> > - *   (according to each layered policies), or if LANDLOCK_ACCESS_FS_REFER is
> > - *   not allowed by the source or the destination.
> > + * - true if the access request is granted;
> > + * - false otherwise
> 
> Missing final dot.

Done.

> >    */
> > -static int check_access_path_dual(
> > +static bool is_access_to_paths_allowed(
> >   	const struct landlock_ruleset *const domain,
> >   	const struct path *const path,
> >   	const access_mask_t access_request_parent1,
> > @@ -492,17 +488,17 @@ static int check_access_path_dual(
> >   	(*layer_masks_child2)[LANDLOCK_NUM_ACCESS_FS] = NULL;
> >   	if (!access_request_parent1 && !access_request_parent2)
> > -		return 0;
> > +		return true;
> >   	if (WARN_ON_ONCE(!domain || !path))
> > -		return 0;
> > +		return true;
> >   	if (is_nouser_or_private(path->dentry))
> > -		return 0;
> > +		return true;
> >   	if (WARN_ON_ONCE(domain->num_layers < 1 || !layer_masks_parent1))
> > -		return -EACCES;
> > +		return false;
> >   	if (unlikely(layer_masks_parent2)) {
> >   		if (WARN_ON_ONCE(!dentry_child1))
> > -			return -EACCES;
> > +			return false;
> >   		/*
> >   		 * For a double request, first check for potential privilege
> >   		 * escalation by looking at domain handled accesses (which are
> > @@ -513,7 +509,7 @@ static int check_access_path_dual(
> >   		is_dom_check = true;
> >   	} else {
> >   		if (WARN_ON_ONCE(dentry_child1 || dentry_child2))
> > -			return -EACCES;
> > +			return false;
> >   		/* For a simple request, only check for requested accesses. */
> >   		access_masked_parent1 = access_request_parent1;
> >   		access_masked_parent2 = access_request_parent2;
> > @@ -622,24 +618,7 @@ static int check_access_path_dual(
> >   	}
> >   	path_put(&walker_path);
> > -	if (allowed_parent1 && allowed_parent2)
> > -		return 0;
> > -
> > -	/*
> > -	 * This prioritizes EACCES over EXDEV for all actions, including
> > -	 * renames with RENAME_EXCHANGE.
> > -	 */
> > -	if (likely(is_eacces(layer_masks_parent1, access_request_parent1) ||
> > -		   is_eacces(layer_masks_parent2, access_request_parent2)))
> > -		return -EACCES;
> > -
> > -	/*
> > -	 * Gracefully forbids reparenting if the destination directory
> > -	 * hierarchy is not a superset of restrictions of the source directory
> > -	 * hierarchy, or if LANDLOCK_ACCESS_FS_REFER is not allowed by the
> > -	 * source or the destination.
> > -	 */
> > -	return -EXDEV;
> > +	return allowed_parent1 && allowed_parent2;
> >   }
> >   static inline int check_access_path(const struct landlock_ruleset *const domain,
> > @@ -649,8 +628,10 @@ static inline int check_access_path(const struct landlock_ruleset *const domain,
> >   	layer_mask_t layer_masks[LANDLOCK_NUM_ACCESS_FS] = {};
> >   	access_request = init_layer_masks(domain, access_request, &layer_masks);
> > -	return check_access_path_dual(domain, path, access_request,
> > -				      &layer_masks, NULL, 0, NULL, NULL);
> > +	if (is_access_to_paths_allowed(domain, path, access_request,
> > +				       &layer_masks, NULL, 0, NULL, NULL))
> > +		return 0;
> > +	return -EACCES;
> >   }
> >   static inline int current_check_access_path(const struct path *const path,
> > @@ -711,8 +692,9 @@ static inline access_mask_t maybe_remove(const struct dentry *const dentry)
> >    * file.  While walking from @dir to @mnt_root, we record all the domain's
> >    * allowed accesses in @layer_masks_dom.
> >    *
> > - * This is similar to check_access_path_dual() but much simpler because it only
> > - * handles walking on the same mount point and only check one set of accesses.
> > + * This is similar to is_access_to_paths_allowed() but much simpler because it
> > + * only handles walking on the same mount point and only checks one set of
> > + * accesses.
> >    *
> >    * Returns:
> >    * - true if all the domain access rights are allowed for @dir;
> > @@ -857,10 +839,11 @@ static int current_check_refer_path(struct dentry *const old_dentry,
> >   		access_request_parent1 = init_layer_masks(
> >   			dom, access_request_parent1 | access_request_parent2,
> >   			&layer_masks_parent1);
> > -		return check_access_path_dual(dom, new_dir,
> > -					      access_request_parent1,
> > -					      &layer_masks_parent1, NULL, 0,
> > -					      NULL, NULL);
> > +		if (is_access_to_paths_allowed(
> > +			    dom, new_dir, access_request_parent1,
> > +			    &layer_masks_parent1, NULL, 0, NULL, NULL))
> > +			return 0;
> > +		return -EACCES;
> >   	}
> >   	access_request_parent1 |= LANDLOCK_ACCESS_FS_REFER;
> > @@ -886,11 +869,27 @@ static int current_check_refer_path(struct dentry *const old_dentry,
> >   	 * parent access rights.  This will be useful to compare with the
> >   	 * destination parent access rights.
> >   	 */
> > -	return check_access_path_dual(dom, &mnt_dir, access_request_parent1,
> > -				      &layer_masks_parent1, old_dentry,
> > -				      access_request_parent2,
> > -				      &layer_masks_parent2,
> > -				      exchange ? new_dentry : NULL);
> > +	if (is_access_to_paths_allowed(
> > +		    dom, &mnt_dir, access_request_parent1, &layer_masks_parent1,
> > +		    old_dentry, access_request_parent2, &layer_masks_parent2,
> > +		    exchange ? new_dentry : NULL))
> > +		return 0;
> > +
> > +	/*
> > +	 * This prioritizes EACCES over EXDEV for all actions, including
> > +	 * renames with RENAME_EXCHANGE.
> > +	 */
> > +	if (likely(is_eacces(&layer_masks_parent1, access_request_parent1) ||
> > +		   is_eacces(&layer_masks_parent2, access_request_parent2)))
> > +		return -EACCES;
> > +
> > +	/*
> > +	 * Gracefully forbids reparenting if the destination directory
> > +	 * hierarchy is not a superset of restrictions of the source directory
> > +	 * hierarchy, or if LANDLOCK_ACCESS_FS_REFER is not allowed by the
> > +	 * source or the destination.
> > +	 */
> > +	return -EXDEV;
> >   }
> >   /* Inode hooks */

--