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author | Mickaël Salaün <mic@digikod.net> | 2022-05-06 18:10:54 +0200 |
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committer | Mickaël Salaün <mic@digikod.net> | 2022-05-23 13:27:57 +0200 |
commit | 8ba0005ff418ec356e176b26eaa04a6ac755d05b (patch) | |
tree | dc29bec751ad38098161896f30da8dcd7933bee4 /security | |
parent | landlock: Create find_rule() from unmask_layers() (diff) | |
download | linux-8ba0005ff418ec356e176b26eaa04a6ac755d05b.tar.xz linux-8ba0005ff418ec356e176b26eaa04a6ac755d05b.zip |
landlock: Fix same-layer rule unions
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 112 bytes (2*14*4) 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.
Reviewed-by: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20220506161102.525323-5-mic@digikod.net
Cc: stable@vger.kernel.org
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Diffstat (limited to 'security')
-rw-r--r-- | security/landlock/fs.c | 78 | ||||
-rw-r--r-- | security/landlock/ruleset.h | 2 |
2 files changed, 54 insertions, 26 deletions
diff --git a/security/landlock/fs.c b/security/landlock/fs.c index 20953bff8fd5..c5749301b37d 100644 --- a/security/landlock/fs.c +++ b/security/landlock/fs.c @@ -207,45 +207,67 @@ find_rule(const struct landlock_ruleset *const domain, 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]; } + if (is_empty) + return true; } - return layer_mask; + return false; } static int check_access_path(const struct landlock_ruleset *const domain, const struct path *const path, const access_mask_t access_request) { - bool allowed = false; + layer_mask_t layer_masks[LANDLOCK_NUM_ACCESS_FS] = {}; + bool allowed = false, has_access = false; struct path walker_path; - layer_mask_t layer_mask; size_t i; if (!access_request) @@ -265,13 +287,20 @@ static int check_access_path(const struct landlock_ruleset *const domain, return -EACCES; /* Saves all layers handling a subset of requested accesses. */ - layer_mask = 0; for (i = 0; i < domain->num_layers; i++) { - if (domain->fs_access_masks[i] & access_request) - layer_mask |= BIT_ULL(i); + const unsigned long access_req = access_request; + unsigned long access_bit; + + for_each_set_bit(access_bit, &access_req, + ARRAY_SIZE(layer_masks)) { + if (domain->fs_access_masks[i] & BIT_ULL(access_bit)) { + layer_masks[access_bit] |= BIT_ULL(i); + has_access = true; + } + } } /* An access request not handled by the domain is allowed. */ - if (layer_mask == 0) + if (!has_access) return 0; walker_path = *path; @@ -283,14 +312,11 @@ static int check_access_path(const struct landlock_ruleset *const domain, while (true) { struct dentry *parent_dentry; - layer_mask = - unmask_layers(find_rule(domain, walker_path.dentry), - access_request, layer_mask); - if (layer_mask == 0) { + allowed = unmask_layers(find_rule(domain, walker_path.dentry), + access_request, &layer_masks); + if (allowed) /* Stops when a rule from each layer grants access. */ - allowed = true; break; - } jump_up: if (walker_path.dentry == walker_path.mnt->mnt_root) { diff --git a/security/landlock/ruleset.h b/security/landlock/ruleset.h index 521af2848951..d43231b783e4 100644 --- a/security/landlock/ruleset.h +++ b/security/landlock/ruleset.h @@ -22,6 +22,8 @@ typedef u16 access_mask_t; /* Makes sure all filesystem access rights can be stored. */ static_assert(BITS_PER_TYPE(access_mask_t) >= LANDLOCK_NUM_ACCESS_FS); +/* Makes sure for_each_set_bit() and for_each_clear_bit() calls are OK. */ +static_assert(sizeof(unsigned long) >= sizeof(access_mask_t)); typedef u16 layer_mask_t; /* Makes sure all layers can be checked. */ |