On 01/08/2019 19:35, Alexei Starovoitov wrote:
> On Wed, Jul 31, 2019 at 09:11:10PM +0200, Mickaël Salaün wrote:
>>
>>
>> On 31/07/2019 20:58, Alexei Starovoitov wrote:
>>> On Wed, Jul 31, 2019 at 11:46 AM Mickaël Salaün
>>> <[email protected]> wrote:
>>>>>> + for (i = 0; i < htab->n_buckets; i++) {
>>>>>> + head = select_bucket(htab, i);
>>>>>> + hlist_nulls_for_each_entry_safe(l, n, head, hash_node) {
>>>>>> + landlock_inode_remove_map(*((struct inode
>>>>>> **)l->key), map);
>>>>>> + }
>>>>>> + }
>>>>>> + htab_map_free(map);
>>>>>> +}
>>>>>
>>>>> user space can delete the map.
>>>>> that will trigger inode_htab_map_free() which will call
>>>>> landlock_inode_remove_map().
>>>>> which will simply itereate the list and delete from the list.
>>>>
>>>> landlock_inode_remove_map() removes the reference to the map (being
>>>> freed) from the inode (with an RCU lock).
>>>
>>> I'm going to ignore everything else for now and focus only on this bit,
>>> since it's fundamental issue to address before this discussion can
>>> go any further.
>>> rcu_lock is not a spin_lock. I'm pretty sure you know this.
>>> But you're arguing that it's somehow protecting from the race
>>> I mentioned above?
>>>
>>
>> I was just clarifying your comment to avoid misunderstanding about what
>> is being removed.
>>
>> As said in the full response, there is currently a race but, if I add a
>> bpf_map_inc() call when the map is referenced by inode->security, then I
>> don't see how a race could occur because such added map could only be
>> freed in a security_inode_free() (as long as it retains a reference to
>> this inode).
>
> then it will be a cycle and a map will never be deleted?
> closing map_fd should delete a map. It cannot be alive if it's not
> pinned in bpffs, there are no FDs that are holding it, and no progs using it.
> So the map deletion will iterate over inodes that belong to this map.
> In parallel security_inode_free() will be called that will iterate
> over its link list that contains elements from different maps.
> So the same link list is modified by two cpus.
> Where is a lock that protects from concurrent links list manipulations?
Ok, I think I got it. What about this fix?
diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 4fc7755042f0..3226e50b6211 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -1708,10 +1708,16 @@ static void inode_htab_map_free(struct bpf_map *map)
for (i = 0; i < htab->n_buckets; i++) {
head = select_bucket(htab, i);
- hlist_nulls_for_each_entry_safe(l, n, head, hash_node) {
+ rcu_read_lock();
+ hlist_nulls_for_each_entry_rcu(l, n, head, hash_node) {
landlock_inode_remove_map(*((struct inode **)l->key),
map);
}
+ rcu_read_unlock();
}
+ /*
+ * The last pending put_landlock_inode_map() may be called here, before
+ * the rcu_barrier() from htab_map_free().
+ */
htab_map_free(map);
}
diff --git a/security/landlock/common.h b/security/landlock/common.h
index b0ba3f31ac7d..535c6a4292b9 100644
--- a/security/landlock/common.h
+++ b/security/landlock/common.h
@@ -58,6 +58,11 @@ struct landlock_prog_set {
refcount_t usage;
};
+struct landlock_inode_security {
+ struct list_head list;
+ spinlock_t lock;
+};
+
struct landlock_inode_map {
struct list_head list;
struct rcu_head rcu_put;
diff --git a/security/landlock/hooks_fs.c b/security/landlock/hooks_fs.c
index 8c9d6a333111..b9bfd558f8b8 100644
--- a/security/landlock/hooks_fs.c
+++ b/security/landlock/hooks_fs.c
@@ -10,6 +10,7 @@
#include <linux/kernel.h> /* ARRAY_SIZE */
#include <linux/lsm_hooks.h>
#include <linux/rcupdate.h> /* synchronize_rcu() */
+#include <linux/spinlock.h>
#include <linux/stat.h> /* S_ISDIR */
#include <linux/stddef.h> /* offsetof */
#include <linux/types.h> /* uintptr_t */
@@ -251,13 +252,16 @@ static int hook_sb_pivotroot(const struct path *old_path,
/* inode helpers */
-static inline struct list_head *inode_landlock(const struct inode *inode)
+static inline struct landlock_inode_security *inode_landlock(
+ const struct inode *inode)
{
return inode->i_security + landlock_blob_sizes.lbs_inode;
}
int landlock_inode_add_map(struct inode *inode, struct bpf_map *map)
{
+ unsigned long flags;
+ struct landlock_inode_security *inode_sec = inode_landlock(inode);
struct landlock_inode_map *inode_map;
inode_map = kzalloc(sizeof(*inode_map), GFP_ATOMIC);
@@ -266,60 +270,66 @@ int landlock_inode_add_map(struct inode *inode, struct
bpf_map *map)
INIT_LIST_HEAD(&inode_map->list);
inode_map->map = map;
inode_map->inode = inode;
- list_add_tail(&inode_map->list, inode_landlock(inode));
+ spin_lock_irqsave(&inode_sec->lock, flags);
+ list_add_tail_rcu(&inode_map->list, &inode_sec->list);
+ spin_unlock_irqrestore(&inode_sec->lock, flags);
return 0;
}
static void put_landlock_inode_map(struct rcu_head *head)
{
struct landlock_inode_map *inode_map;
- int err;
inode_map = container_of(head, struct landlock_inode_map, rcu_put);
- err = bpf_inode_ptr_unlocked_htab_map_delete_elem(inode_map->map,
+ bpf_inode_ptr_unlocked_htab_map_delete_elem(inode_map->map,
&inode_map->inode, false);
- bpf_map_put(inode_map->map);
kfree(inode_map);
}
void landlock_inode_remove_map(struct inode *inode, const struct bpf_map *map)
{
+ unsigned long flags;
+ struct landlock_inode_security *inode_sec = inode_landlock(inode);
struct landlock_inode_map *inode_map;
- bool found = false;
+ spin_lock_irqsave(&inode_sec->lock, flags);
rcu_read_lock();
- list_for_each_entry_rcu(inode_map, inode_landlock(inode), list) {
+ list_for_each_entry_rcu(inode_map, &inode_sec->list, list) {
if (inode_map->map == map) {
- found = true;
list_del_rcu(&inode_map->list);
kfree_rcu(inode_map, rcu_put);
break;
}
}
rcu_read_unlock();
- WARN_ON(!found);
+ spin_unlock_irqrestore(&inode_sec->lock, flags);
}
/* inode hooks */
static int hook_inode_alloc_security(struct inode *inode)
{
- struct list_head *ll_inode = inode_landlock(inode);
+ struct landlock_inode_security *inode_sec = inode_landlock(inode);
- INIT_LIST_HEAD(ll_inode);
+ INIT_LIST_HEAD(&inode_sec->list);
+ spin_lock_init(&inode_sec->lock);
return 0;
}
static void hook_inode_free_security(struct inode *inode)
{
+ unsigned long flags;
+ struct landlock_inode_security *inode_sec = inode_landlock(inode);
struct landlock_inode_map *inode_map;
+ spin_lock_irqsave(&inode_sec->lock, flags);
rcu_read_lock();
- list_for_each_entry_rcu(inode_map, inode_landlock(inode), list) {
+ list_for_each_entry_rcu(inode_map, &inode_sec->list, list) {
list_del_rcu(&inode_map->list);
call_rcu(&inode_map->rcu_put, put_landlock_inode_map);
}
rcu_read_unlock();
+ spin_unlock_irqrestore(&inode_sec->lock, flags);
}
/* a directory inode contains only one dentry */
diff --git a/security/landlock/init.c b/security/landlock/init.c
index 35165fc8a595..1305255f5d2e 100644
--- a/security/landlock/init.c
+++ b/security/landlock/init.c
@@ -137,7 +137,7 @@ static int __init landlock_init(void)
}
struct lsm_blob_sizes landlock_blob_sizes __lsm_ro_after_init = {
- .lbs_inode = sizeof(struct list_head),
+ .lbs_inode = sizeof(struct landlock_inode_security),
};
DEFINE_LSM(LANDLOCK_NAME) = {
>
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>
> Please get rid of this. It's absolutely not appropriate on public mailing
> list.
> Next time I'd have to ignore emails that contain such disclaimers.
Unfortunately this message is automatically appended (server-side) to all my
professional emails...
