Re: [PATCH v26 07/12] landlock: Support filesystem access-control

[Jann Horn <jannh@google.com>]

On Wed, Dec 9, 2020 at 8:28 PM Mickaël Salaün <mic@digikod.net> wrote:
> Thanks to the Landlock objects and ruleset, it is possible to identify
> inodes according to a process's domain.  To enable an unprivileged
> process to express a file hierarchy, it first needs to open a directory
> (or a file) and pass this file descriptor to the kernel through
> landlock_add_rule(2).  When checking if a file access request is
> allowed, we walk from the requested dentry to the real root, following
> the different mount layers.  The access to each "tagged" inodes are
> collected according to their rule layer level, and ANDed to create
> access to the requested file hierarchy.  This makes possible to identify
> a lot of files without tagging every inodes nor modifying the
> filesystem, while still following the view and understanding the user
> has from the filesystem.
>
> Add a new ARCH_EPHEMERAL_INODES for UML because it currently does not
> keep the same struct inodes for the same inodes whereas these inodes are
> in use.
>
> This commit adds a minimal set of supported filesystem access-control
> which doesn't enable to restrict all file-related actions.  This is the
> result of multiple discussions to minimize the code of Landlock to ease
> review.  Thanks to the Landlock design, extending this access-control
> without breaking user space will not be a problem.  Moreover, seccomp
> filters can be used to restrict the use of syscall families which may
> not be currently handled by Landlock.
[...]
> +static bool check_access_path_continue(
> +               const struct landlock_ruleset *const domain,
> +               const struct path *const path, const u32 access_request,
> +               u64 *const layer_mask)
> +{
[...]
> +       /*
> +        * An access is granted if, for each policy layer, at least one rule
> +        * encountered on the pathwalk grants the access, regardless of their
> +        * position in the layer stack.  We must then check not-yet-seen layers
> +        * for each inode, from the last one added to the first one.
> +        */
> +       for (i = 0; i < rule->num_layers; i++) {
> +               const struct landlock_layer *const layer = &rule->layers[i];
> +               const u64 layer_level = BIT_ULL(layer->level - 1);
> +
> +               if (!(layer_level & *layer_mask))
> +                       continue;
> +               if ((layer->access & access_request) != access_request)
> +                       return false;
> +               *layer_mask &= ~layer_level;

Hmm... shouldn't the last 5 lines be replaced by the following?

if ((layer->access & access_request) == access_request)
    *layer_mask &= ~layer_level;

And then, since this function would always return true, you could
change its return type to "void".


As far as I can tell, the current version will still, if a ruleset
looks like this:

/usr read+write
/usr/lib/ read

reject write access to /usr/lib, right?


> +       }
> +       return true;
> +}