From: Christian Brauner christian.brauner@ubuntu.com
commit f0fe2c0f050d31babcad7d65f1d550d462a40064 upstream.
This is a necessary follow up to the first fix I proposed and we merged in 2669b8b0c79 ("binder: prevent UAF for binderfs devices"). I have been overly optimistic that the simple fix I proposed would work. But alas, ihold() + iput() won't work since the inodes won't survive the destruction of the superblock. So all we get with my prior fix is a different race with a tinier race-window but it doesn't solve the issue. Fwiw, the problem lies with generic_shutdown_super(). It even has this cozy Al-style comment:
if (!list_empty(&sb->s_inodes)) { printk("VFS: Busy inodes after unmount of %s. " "Self-destruct in 5 seconds. Have a nice day...\n", sb->s_id); }
On binder_release(), binder_defer_work(proc, BINDER_DEFERRED_RELEASE) is called which punts the actual cleanup operation to a workqueue. At some point, binder_deferred_func() will be called which will end up calling binder_deferred_release() which will retrieve and cleanup the binder_context attach to this struct binder_proc.
If we trace back where this binder_context is attached to binder_proc we see that it is set in binder_open() and is taken from the struct binder_device it is associated with. This obviously assumes that the struct binder_device that context is attached to is _never_ freed. While that might be true for devtmpfs binder devices it is most certainly wrong for binderfs binder devices.
So, assume binder_open() is called on a binderfs binder devices. We now stash away the struct binder_context associated with that struct binder_devices: proc->context = &binder_dev->context; /* binderfs stashes devices in i_private */ if (is_binderfs_device(nodp)) { binder_dev = nodp->i_private; info = nodp->i_sb->s_fs_info; binder_binderfs_dir_entry_proc = info->proc_log_dir; } else { . . . proc->context = &binder_dev->context;
Now let's assume that the binderfs instance for that binder devices is shutdown via umount() and/or the mount namespace associated with it goes away. As long as there is still an fd open for that binderfs binder device things are fine. But let's assume we now close the last fd for that binderfs binder device. Now binder_release() is called and punts to the workqueue. Assume that the workqueue has quite a bit of stuff to do and doesn't get to cleaning up the struct binder_proc and the associated struct binder_context with it for that binderfs binder device right away. In the meantime, the VFS is killing the super block and is ultimately calling sb->evict_inode() which means it will call binderfs_evict_inode() which does:
static void binderfs_evict_inode(struct inode *inode) { struct binder_device *device = inode->i_private; struct binderfs_info *info = BINDERFS_I(inode);
clear_inode(inode);
if (!S_ISCHR(inode->i_mode) || !device) return;
mutex_lock(&binderfs_minors_mutex); --info->device_count; ida_free(&binderfs_minors, device->miscdev.minor); mutex_unlock(&binderfs_minors_mutex);
kfree(device->context.name); kfree(device); }
thereby freeing the struct binder_device including struct binder_context.
Now the workqueue finally has time to get around to cleaning up struct binder_proc and is now trying to access the associate struct binder_context. Since it's already freed it will OOPs.
Fix this by introducing a refounct on binder devices.
This is an alternative fix to 51d8a7eca677 ("binder: prevent UAF read in print_binder_transaction_log_entry()").
Fixes: 3ad20fe393b3 ("binder: implement binderfs") Fixes: 2669b8b0c798 ("binder: prevent UAF for binderfs devices") Fixes: 03e2e07e3814 ("binder: Make transaction_log available in binderfs") Related : 51d8a7eca677 ("binder: prevent UAF read in print_binder_transaction_log_entry()") Cc: stable@vger.kernel.org Signed-off-by: Christian Brauner christian.brauner@ubuntu.com Acked-by: Todd Kjos tkjos@google.com Link: https://lore.kernel.org/r/20200303164340.670054-1-christian.brauner@ubuntu.c... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- drivers/android/binder.c | 12 +++++++++--- drivers/android/binder_internal.h | 15 ++------------- drivers/android/binderfs.c | 7 +++++-- 3 files changed, 16 insertions(+), 18 deletions(-)
--- a/drivers/android/binder.c +++ b/drivers/android/binder.c @@ -5219,13 +5219,14 @@ static int binder_open(struct inode *nod proc->default_priority = task_nice(current); /* binderfs stashes devices in i_private */ if (is_binderfs_device(nodp)) { - binder_dev = binderfs_device_get(nodp->i_private); + binder_dev = nodp->i_private; info = nodp->i_sb->s_fs_info; binder_binderfs_dir_entry_proc = info->proc_log_dir; } else { binder_dev = container_of(filp->private_data, struct binder_device, miscdev); } + refcount_inc(&binder_dev->ref); proc->context = &binder_dev->context; binder_alloc_init(&proc->alloc);
@@ -5420,6 +5421,12 @@ static void binder_deferred_release(stru context->binder_context_mgr_node = NULL; } mutex_unlock(&context->context_mgr_node_lock); + device = container_of(proc->context, struct binder_device, context); + if (refcount_dec_and_test(&device->ref)) { + kfree(context->name); + kfree(device); + } + proc->context = NULL; binder_inner_proc_lock(proc); /* * Make sure proc stays alive after we @@ -5483,8 +5490,6 @@ static void binder_deferred_release(stru outgoing_refs, active_transactions);
binder_proc_dec_tmpref(proc); - device = container_of(proc->context, struct binder_device, context); - binderfs_device_put(device); }
static void binder_deferred_func(struct work_struct *work) @@ -6078,6 +6083,7 @@ static int __init init_binder_device(con binder_device->miscdev.minor = MISC_DYNAMIC_MINOR; binder_device->miscdev.name = name;
+ refcount_set(&binder_device->ref, 1); binder_device->context.binder_context_mgr_uid = INVALID_UID; binder_device->context.name = name; mutex_init(&binder_device->context.context_mgr_node_lock); --- a/drivers/android/binder_internal.h +++ b/drivers/android/binder_internal.h @@ -8,6 +8,7 @@ #include <linux/list.h> #include <linux/miscdevice.h> #include <linux/mutex.h> +#include <linux/refcount.h> #include <linux/stddef.h> #include <linux/types.h> #include <linux/uidgid.h> @@ -33,21 +34,9 @@ struct binder_device { struct miscdevice miscdev; struct binder_context context; struct inode *binderfs_inode; + refcount_t ref; };
-static inline struct binder_device *binderfs_device_get(struct binder_device *dev) -{ - if (dev->binderfs_inode) - ihold(dev->binderfs_inode); - return dev; -} - -static inline void binderfs_device_put(struct binder_device *dev) -{ - if (dev->binderfs_inode) - iput(dev->binderfs_inode); -} - /** * binderfs_mount_opts - mount options for binderfs * @max: maximum number of allocatable binderfs binder devices --- a/drivers/android/binderfs.c +++ b/drivers/android/binderfs.c @@ -154,6 +154,7 @@ static int binderfs_binder_device_create if (!name) goto err;
+ refcount_set(&device->ref, 1); device->binderfs_inode = inode; device->context.binder_context_mgr_uid = INVALID_UID; device->context.name = name; @@ -257,8 +258,10 @@ static void binderfs_evict_inode(struct ida_free(&binderfs_minors, device->miscdev.minor); mutex_unlock(&binderfs_minors_mutex);
- kfree(device->context.name); - kfree(device); + if (refcount_dec_and_test(&device->ref)) { + kfree(device->context.name); + kfree(device); + } }
/**