Re: [PATCH v1 04/11] landlock: Fix same-layer rule unions

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On Mon, Feb 21, 2022 at 4:15 PM Mickaël Salaün <mic@xxxxxxxxxxx> wrote:
>
> From: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx>
>
> The original behavior was to check if the full set of requested accesses
> was allowed by at least a rule of every relevant layer.  This didn't
> take into account requests for multiple accesses and same-layer rules
> allowing the union of these accesses in a complementary way.  As a
> result, multiple accesses requested on a file hierarchy matching rules
> that, together, allowed these accesses, but without a unique rule
> allowing all of them, was illegitimately denied.  This case should be
> rare in practice and it can only be triggered by the path_rename or
> file_open hook implementations.
>
> For instance, if, for the same layer, a rule allows execution
> beneath /a/b and another rule allows read beneath /a, requesting access
> to read and execute at the same time for /a/b should be allowed for this
> layer.
>
> This was an inconsistency because the union of same-layer rule accesses
> was already allowed if requested once at a time anyway.
>
> This fix changes the way allowed accesses are gathered over a path walk.
> To take into account all these rule accesses, we store in a matrix all
> layer granting the set of requested accesses, according to the handled
> accesses.  To avoid heap allocation, we use an array on the stack which
> is 2*13 bytes.  A following commit bringing the LANDLOCK_ACCESS_FS_REFER
> access right will increase this size to reach 84 bytes (2*14*3) in case
> of link or rename actions.
>
> Add a new layout1.layer_rule_unions test to check that accesses from
> different rules pertaining to the same layer are ORed in a file
> hierarchy.  Also test that it is not the case for rules from different
> layers.
>
> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/20220221212522.320243-5-mic@xxxxxxxxxxx
> ---
>  security/landlock/fs.c                     |  77 ++++++++++-----
>  security/landlock/ruleset.h                |   2 +
>  tools/testing/selftests/landlock/fs_test.c | 107 +++++++++++++++++++++
>  3 files changed, 160 insertions(+), 26 deletions(-)
>
> diff --git a/security/landlock/fs.c b/security/landlock/fs.c
> index 0bcb27f2360a..9662f9fb3cd0 100644
> --- a/security/landlock/fs.c
> +++ b/security/landlock/fs.c
> @@ -204,45 +204,66 @@ static inline const struct landlock_rule *find_rule(
>         return rule;
>  }
>
> -static inline layer_mask_t unmask_layers(
> -               const struct landlock_rule *const rule,
> -               const access_mask_t access_request, layer_mask_t layer_mask)
> +/*
> + * @layer_masks is read and may be updated according to the access request and
> + * the matching rule.
> + *
> + * Returns true if the request is allowed (i.e. relevant layer masks for the
> + * request are empty).
> + */
> +static inline bool unmask_layers(const struct landlock_rule *const rule,
> +               const access_mask_t access_request,
> +               layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS])
>  {
>         size_t layer_level;
>
> +       if (!access_request || !layer_masks)
> +               return true;
>         if (!rule)
> -               return layer_mask;
> +               return false;
>
>         /*
>          * An access is granted if, for each policy layer, at least one rule
> -        * encountered on the pathwalk grants the requested accesses,
> -        * regardless of their position in the layer stack.  We must then check
> +        * encountered on the pathwalk grants the requested access,
> +        * regardless of its position in the layer stack.  We must then check
>          * the remaining layers for each inode, from the first added layer to
> -        * the last one.
> +        * the last one.  When there is multiple requested accesses, for each
> +        * policy layer, the full set of requested accesses may not be granted
> +        * by only one rule, but by the union (binary OR) of multiple rules.
> +        * E.g. /a/b <execute> + /a <read> = /a/b <execute + read>
>          */
>         for (layer_level = 0; layer_level < rule->num_layers; layer_level++) {
>                 const struct landlock_layer *const layer =
>                         &rule->layers[layer_level];
>                 const layer_mask_t layer_bit = BIT_ULL(layer->level - 1);
> +               const unsigned long access_req = access_request;
> +               unsigned long access_bit;
> +               bool is_empty;
>
> -               /* Checks that the layer grants access to the full request. */
> -               if ((layer->access & access_request) == access_request) {
> -                       layer_mask &= ~layer_bit;
> -
> -                       if (layer_mask == 0)
> -                               return layer_mask;
> +               /*
> +                * Records in @layer_masks which layer grants access to each
> +                * requested access.
> +                */
> +               is_empty = true;
> +               for_each_set_bit(access_bit, &access_req,
> +                               ARRAY_SIZE(*layer_masks)) {
> +                       if (layer->access & BIT_ULL(access_bit))
> +                               (*layer_masks)[access_bit] &= ~layer_bit;
> +                       is_empty = is_empty && !(*layer_masks)[access_bit];


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