[PATCH] binderfs: implement sysctls

[Christian Brauner <christian@brauner.io>]

This implements three sysctls that have very specific goals:

1. /proc/sys/fs/binder/max:
   Allow global root to globally limit the number of allocatable binder
   devices.
2. /proc/sys/fs/binder/nr:
   Allow global root to easily detect how many binder devices are currently
   in use across all binderfs mounts.
3. /proc/sys/fs/binder/reserved:
   Ensure that global root can reserve binder devices for the initial
   binderfs mount in the initial ipc namespace to prevent DOS attacks.

This is equivalent to sysctls of devpts.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 drivers/android/binderfs.c | 81 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 79 insertions(+), 2 deletions(-)

diff --git a/drivers/android/binderfs.c b/drivers/android/binderfs.c
index 7496b10532aa..5ff015f82314 100644
--- a/drivers/android/binderfs.c
+++ b/drivers/android/binderfs.c
@@ -64,6 +64,71 @@ struct binderfs_info {
 
 };
 
+/* Global default limit on the number of binder devices. */
+static int device_limit = 4096;
+
+/*
+ * Number of binder devices reserved for the initial binderfs mount in the
+ * initial ipc namespace.
+ */
+static int device_reserve = 1024;
+
+/* Dummy sysctl minimum. */
+static int device_limit_min;
+
+/* Cap sysctl at BINDERFS_MAX_MINOR. */
+static int device_limit_max = BINDERFS_MAX_MINOR;
+
+/* Current number of allocated binder devices. */
+static atomic_t device_count = ATOMIC_INIT(0);
+
+static struct ctl_table binderfs_table[] = {
+	{
+		.procname	= "max",
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.data		= &device_limit,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &device_limit_min,
+		.extra2		= &device_limit_max,
+	},
+	{
+		.procname	= "reserve",
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.data		= &device_reserve,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &device_limit_min,
+		.extra2		= &device_limit_max,
+	},
+	{
+		.procname	= "nr",
+		.maxlen		= sizeof(int),
+		.mode		= 0444,
+		.data		= &device_count,
+		.proc_handler	= proc_dointvec,
+	},
+	{}
+};
+
+static struct ctl_table binderfs_fs_table[] = {
+	{
+		.procname	= "binder",
+		.mode		= 0555,
+		.child		= binderfs_table,
+	},
+	{}
+};
+
+static struct ctl_table binderfs_root_table[] = {
+	{
+		.procname	= "fs",
+		.mode		= 0555,
+		.child		= binderfs_fs_table,
+	},
+	{}
+};
+
 static inline struct binderfs_info *BINDERFS_I(const struct inode *inode)
 {
 	return inode->i_sb->s_fs_info;
@@ -107,13 +172,21 @@ static int binderfs_binder_device_create(struct inode *ref_inode,
 	struct inode *inode = NULL;
 	struct super_block *sb = ref_inode->i_sb;
 	struct binderfs_info *info = sb->s_fs_info;
+	bool use_reserved = (info->ipc_ns == &init_ipc_ns);
 
 	/* Reserve new minor number for the new device. */
 	mutex_lock(&binderfs_minors_mutex);
-	minor = ida_alloc_max(&binderfs_minors, BINDERFS_MAX_MINOR, GFP_KERNEL);
+	if (atomic_inc_return(&device_count) <
+	    (device_limit - (use_reserved ? 0 : device_reserve)))
+		minor = ida_alloc_max(&binderfs_minors, BINDERFS_MAX_MINOR,
+				      GFP_KERNEL);
+	else
+		minor = -ENOSPC;
 	mutex_unlock(&binderfs_minors_mutex);
-	if (minor < 0)
+	if (minor < 0) {
+		atomic_dec(&device_count);
 		return minor;
+	}
 
 	ret = -ENOMEM;
 	device = kzalloc(sizeof(*device), GFP_KERNEL);
@@ -187,6 +260,7 @@ static int binderfs_binder_device_create(struct inode *ref_inode,
 	kfree(name);
 	kfree(device);
 	mutex_lock(&binderfs_minors_mutex);
+	atomic_dec(&device_count);
 	ida_free(&binderfs_minors, minor);
 	mutex_unlock(&binderfs_minors_mutex);
 	iput(inode);
@@ -239,6 +313,7 @@ static void binderfs_evict_inode(struct inode *inode)
 		return;
 
 	mutex_lock(&binderfs_minors_mutex);
+	atomic_dec(&device_count);
 	ida_free(&binderfs_minors, device->miscdev.minor);
 	mutex_unlock(&binderfs_minors_mutex);
 
@@ -536,6 +611,8 @@ static int __init init_binderfs(void)
 		binderfs_mnt = NULL;
 		unregister_filesystem(&binder_fs_type);
 		unregister_chrdev_region(binderfs_dev, BINDERFS_MAX_MINOR);
+	} else {
+		register_sysctl_table(binderfs_root_table);
 	}
 
 	return ret;
-- 
2.19.1