Currently this list is protected with a simple spinlock, even
for reading from one. This is OK, but can be better.
Actually I want it to be better very much, since after replacing
the OpenVZ device permissions engine with the cgroup-based one
I noticed, that we set 12 default device permissions for each newly
created container (for /dev/null, full, terminals, ect devices),
and people sometimes have up to 20 perms more, so traversing the
~30-40 elements list under a spinlock doesn't seem very good.
Here's the liter RCU protection for white-list.
Signed-off-by: Pavel Emelyanov <[email protected]>
---
diff --git a/security/device_cgroup.c b/security/device_cgroup.c
index 4ea5836..9d940c3 100644
--- a/security/device_cgroup.c
+++ b/security/device_cgroup.c
@@ -41,6 +41,7 @@ struct dev_whitelist_item {
short type;
short access;
struct list_head list;
+ struct rcu_head rcu;
};
struct dev_cgroup {
@@ -110,11 +111,19 @@ static int dev_whitelist_add(struct dev_cgroup *dev_cgroup,
memcpy(whcopy, wh, sizeof(*whcopy));
spin_lock(&dev_cgroup->lock);
- list_add_tail(&whcopy->list, &dev_cgroup->whitelist);
+ list_add_tail_rcu(&whcopy->list, &dev_cgroup->whitelist);
spin_unlock(&dev_cgroup->lock);
return 0;
}
+static void whitelist_item_free(struct rcu_head *rcu)
+{
+ struct dev_whitelist_item *item;
+
+ item = container_of(rcu, struct dev_whitelist_item, rcu);
+ kfree(item);
+}
+
/*
* called under cgroup_lock()
* since the list is visible to other tasks, we need the spinlock also
@@ -138,8 +147,8 @@ static void dev_whitelist_rm(struct dev_cgroup *dev_cgroup,
remove:
walk->access &= ~wh->access;
if (!walk->access) {
- list_del(&walk->list);
- kfree(walk);
+ list_del_rcu(&walk->list);
+ call_rcu(&walk->rcu, whitelist_item_free);
}
}
spin_unlock(&dev_cgroup->lock);
@@ -246,15 +255,15 @@ static int devcgroup_seq_read(struct cgroup *cgroup, struct cftype *cft,
struct dev_whitelist_item *wh;
char maj[MAJMINLEN], min[MAJMINLEN], acc[ACCLEN];
- spin_lock(&devcgroup->lock);
- list_for_each_entry(wh, &devcgroup->whitelist, list) {
+ rcu_read_lock();
+ list_for_each_entry_rcu(wh, &devcgroup->whitelist, list) {
set_access(acc, wh->access);
set_majmin(maj, wh->major);
set_majmin(min, wh->minor);
seq_printf(m, "%c %s:%s %s\n", type_to_char(wh->type),
maj, min, acc);
}
- spin_unlock(&devcgroup->lock);
+ rcu_read_unlock();
return 0;
}
@@ -516,8 +525,8 @@ int devcgroup_inode_permission(struct inode *inode, int mask)
if (!dev_cgroup)
return 0;
- spin_lock(&dev_cgroup->lock);
- list_for_each_entry(wh, &dev_cgroup->whitelist, list) {
+ rcu_read_lock();
+ list_for_each_entry_rcu(wh, &dev_cgroup->whitelist, list) {
if (wh->type & DEV_ALL)
goto acc_check;
if ((wh->type & DEV_BLOCK) && !S_ISBLK(inode->i_mode))
@@ -533,10 +542,10 @@ acc_check:
continue;
if ((mask & MAY_READ) && !(wh->access & ACC_READ))
continue;
- spin_unlock(&dev_cgroup->lock);
+ rcu_read_unlock();
return 0;
}
- spin_unlock(&dev_cgroup->lock);
+ rcu_read_unlock();
return -EPERM;
}
@@ -552,7 +561,7 @@ int devcgroup_inode_mknod(int mode, dev_t dev)
if (!dev_cgroup)
return 0;
- spin_lock(&dev_cgroup->lock);
+ rcu_read_lock();
list_for_each_entry(wh, &dev_cgroup->whitelist, list) {
if (wh->type & DEV_ALL)
goto acc_check;
@@ -567,9 +576,9 @@ int devcgroup_inode_mknod(int mode, dev_t dev)
acc_check:
if (!(wh->access & ACC_MKNOD))
continue;
- spin_unlock(&dev_cgroup->lock);
+ rcu_read_unlock();
return 0;
}
- spin_unlock(&dev_cgroup->lock);
+ rcu_read_unlock();
return -EPERM;
}
Quoting Pavel Emelyanov ([email protected]):
> Currently this list is protected with a simple spinlock, even
> for reading from one. This is OK, but can be better.
>
> Actually I want it to be better very much, since after replacing
> the OpenVZ device permissions engine with the cgroup-based one
> I noticed, that we set 12 default device permissions for each newly
> created container (for /dev/null, full, terminals, ect devices),
> and people sometimes have up to 20 perms more, so traversing the
> ~30-40 elements list under a spinlock doesn't seem very good.
>
> Here's the liter RCU protection for white-list.
>
> Signed-off-by: Pavel Emelyanov <[email protected]>
>
> ---
>
> diff --git a/security/device_cgroup.c b/security/device_cgroup.c
> index 4ea5836..9d940c3 100644
> --- a/security/device_cgroup.c
> +++ b/security/device_cgroup.c
> @@ -41,6 +41,7 @@ struct dev_whitelist_item {
> short type;
> short access;
> struct list_head list;
> + struct rcu_head rcu;
> };
>
> struct dev_cgroup {
> @@ -110,11 +111,19 @@ static int dev_whitelist_add(struct dev_cgroup *dev_cgroup,
>
> memcpy(whcopy, wh, sizeof(*whcopy));
> spin_lock(&dev_cgroup->lock);
> - list_add_tail(&whcopy->list, &dev_cgroup->whitelist);
> + list_add_tail_rcu(&whcopy->list, &dev_cgroup->whitelist);
> spin_unlock(&dev_cgroup->lock);
> return 0;
> }
>
> +static void whitelist_item_free(struct rcu_head *rcu)
> +{
> + struct dev_whitelist_item *item;
> +
> + item = container_of(rcu, struct dev_whitelist_item, rcu);
> + kfree(item);
> +}
> +
> /*
> * called under cgroup_lock()
> * since the list is visible to other tasks, we need the spinlock also
> @@ -138,8 +147,8 @@ static void dev_whitelist_rm(struct dev_cgroup *dev_cgroup,
> remove:
> walk->access &= ~wh->access;
> if (!walk->access) {
> - list_del(&walk->list);
> - kfree(walk);
> + list_del_rcu(&walk->list);
> + call_rcu(&walk->rcu, whitelist_item_free);
The only thing I'd suggest is that a call_rcu() really isn't necessary.
You'd avoid the rcu_head in each dev_whitelist_item if you just did
synchronize_rcu();
kfree(walk);
here. Downside is you're keeping the cgroup_lock() a little longer
then...
But that's just an idea. Whether you do that or not,
Acked-by: Serge Hallyn <[email protected]>
Thanks for doing this.
-serge
> }
> }
> spin_unlock(&dev_cgroup->lock);
> @@ -246,15 +255,15 @@ static int devcgroup_seq_read(struct cgroup *cgroup, struct cftype *cft,
> struct dev_whitelist_item *wh;
> char maj[MAJMINLEN], min[MAJMINLEN], acc[ACCLEN];
>
> - spin_lock(&devcgroup->lock);
> - list_for_each_entry(wh, &devcgroup->whitelist, list) {
> + rcu_read_lock();
> + list_for_each_entry_rcu(wh, &devcgroup->whitelist, list) {
> set_access(acc, wh->access);
> set_majmin(maj, wh->major);
> set_majmin(min, wh->minor);
> seq_printf(m, "%c %s:%s %s\n", type_to_char(wh->type),
> maj, min, acc);
> }
> - spin_unlock(&devcgroup->lock);
> + rcu_read_unlock();
>
> return 0;
> }
> @@ -516,8 +525,8 @@ int devcgroup_inode_permission(struct inode *inode, int mask)
> if (!dev_cgroup)
> return 0;
>
> - spin_lock(&dev_cgroup->lock);
> - list_for_each_entry(wh, &dev_cgroup->whitelist, list) {
> + rcu_read_lock();
> + list_for_each_entry_rcu(wh, &dev_cgroup->whitelist, list) {
> if (wh->type & DEV_ALL)
> goto acc_check;
> if ((wh->type & DEV_BLOCK) && !S_ISBLK(inode->i_mode))
> @@ -533,10 +542,10 @@ acc_check:
> continue;
> if ((mask & MAY_READ) && !(wh->access & ACC_READ))
> continue;
> - spin_unlock(&dev_cgroup->lock);
> + rcu_read_unlock();
> return 0;
> }
> - spin_unlock(&dev_cgroup->lock);
> + rcu_read_unlock();
>
> return -EPERM;
> }
> @@ -552,7 +561,7 @@ int devcgroup_inode_mknod(int mode, dev_t dev)
> if (!dev_cgroup)
> return 0;
>
> - spin_lock(&dev_cgroup->lock);
> + rcu_read_lock();
> list_for_each_entry(wh, &dev_cgroup->whitelist, list) {
> if (wh->type & DEV_ALL)
> goto acc_check;
> @@ -567,9 +576,9 @@ int devcgroup_inode_mknod(int mode, dev_t dev)
> acc_check:
> if (!(wh->access & ACC_MKNOD))
> continue;
> - spin_unlock(&dev_cgroup->lock);
> + rcu_read_unlock();
> return 0;
> }
> - spin_unlock(&dev_cgroup->lock);
> + rcu_read_unlock();
> return -EPERM;
> }