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SubjectRe: [PATCH v26 07/12] landlock: Support filesystem access-control
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;
> +}

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