[PATCH] binder: restrict BINDER_NLGRP_REPORT to CAP_NET_ADMIN and fix binderfs minor leak

[Hui Peng <benquike@gmail.com>]

Fix three security and resource-accounting issues in `binder_netlink.c`
and `binderfs.c`:

1. In `drivers/android/binder_netlink.c`,
   `binder_nl_mcgrps[BINDER_NLGRP_REPORT]` is declared without `.flags =
   GENL_MCAST_CAP_NET_ADMIN`, allowing any unprivileged local user to
   subscribe to the `BINDER_NLGRP_REPORT` Generic Netlink multicast
   group and monitor system-wide binder transaction error reports
   (including sender/target PIDs and transaction metadata).
2. In `drivers/android/binderfs.c`, `binderfs_binder_ctl_create()`
   allocates an IDA minor via `ida_alloc_max(&binderfs_minors, ...)`,
   and if `d_alloc_name(root, "binder-control")` fails, jumps to `out:`
   (which calls `kfree(device)`) without freeing `minor` from
   `binderfs_minors`.
3. When the `binder-control` inode (`device->context.name == NULL`) is
   evicted in `binderfs_evict_inode()`, `--info->device_count` is
   decremented even though `binderfs_binder_ctl_create()` never
   incremented `info->device_count`, underflowing `info->device_count`.
   Only decrement `info->device_count` when `device->context.name` is
   non-NULL.

Fixes: 63740349eba7 ("binder: introduce transaction reports via netlink")
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>

---
 drivers/android/binder_netlink.c | 2 +-
 drivers/android/binderfs.c       | 9 +++++++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/android/binder_netlink.c b/drivers/android/binder_netlink.c
index 81e8432b5904..3f1a4210aa52 100644
--- a/drivers/android/binder_netlink.c
+++ b/drivers/android/binder_netlink.c
@@ -16,7 +16,7 @@ static const struct genl_split_ops binder_nl_ops[] = {
 };
 
 static const struct genl_multicast_group binder_nl_mcgrps[] = {
-	[BINDER_NLGRP_REPORT] = { "report", },
+	[BINDER_NLGRP_REPORT] = { "report", .flags = GENL_MCAST_CAP_NET_ADMIN },
 };
 
 struct genl_family binder_nl_family __ro_after_init = {
diff --git a/drivers/android/binderfs.c b/drivers/android/binderfs.c
index 361d69f756f5..661fda3ecb59 100644
--- a/drivers/android/binderfs.c
+++ b/drivers/android/binderfs.c
@@ -258,7 +258,8 @@ static void binderfs_evict_inode(struct inode *inode)
 		return;
 
 	mutex_lock(&binderfs_minors_mutex);
-	--info->device_count;
+	if (device->context.name)
+		--info->device_count;
 	ida_free(&binderfs_minors, device->miscdev.minor);
 	mutex_unlock(&binderfs_minors_mutex);
 
@@ -430,8 +431,12 @@ static int binderfs_binder_ctl_create(struct super_block *sb)
 	device->miscdev.minor = minor;
 
 	dentry = d_alloc_name(root, "binder-control");
-	if (!dentry)
+	if (!dentry) {
+		mutex_lock(&binderfs_minors_mutex);
+		ida_free(&binderfs_minors, minor);
+		mutex_unlock(&binderfs_minors_mutex);
 		goto out;
+	}
 
 	inode->i_private = device;
 	info->control_dentry = dentry;