4.20-stable review patch. If anyone has any objections, please let me know.
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From: Todd Kjos tkjos@android.com
commit 80cd795630d6526ba729a089a435bf74a57af927 upstream.
44d8047f1d8 ("binder: use standard functions to allocate fds") exposed a pre-existing issue in the binder driver.
fdget() is used in ksys_ioctl() as a performance optimization. One of the rules associated with fdget() is that ksys_close() must not be called between the fdget() and the fdput(). There is a case where this requirement is not met in the binder driver which results in the reference count dropping to 0 when the device is still in use. This can result in use-after-free or other issues.
If userpace has passed a file-descriptor for the binder driver using a BINDER_TYPE_FDA object, then kys_close() is called on it when handling a binder_ioctl(BC_FREE_BUFFER) command. This violates the assumptions for using fdget().
The problem is fixed by deferring the close using task_work_add(). A new variant of __close_fd() was created that returns a struct file with a reference. The fput() is deferred instead of using ksys_close().
Fixes: 44d8047f1d87a ("binder: use standard functions to allocate fds") Suggested-by: Al Viro viro@zeniv.linux.org.uk Signed-off-by: Todd Kjos tkjos@google.com Cc: stable stable@vger.kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- drivers/android/binder.c | 63 +++++++++++++++++++++++++++++++++++++++++++++-- fs/file.c | 29 +++++++++++++++++++++ include/linux/fdtable.h | 1 3 files changed, 91 insertions(+), 2 deletions(-)
--- a/drivers/android/binder.c +++ b/drivers/android/binder.c @@ -72,6 +72,7 @@ #include <linux/spinlock.h> #include <linux/ratelimit.h> #include <linux/syscalls.h> +#include <linux/task_work.h>
#include <uapi/linux/android/binder.h>
@@ -2160,6 +2161,64 @@ static bool binder_validate_fixup(struct return (fixup_offset >= last_min_offset); }
+/** + * struct binder_task_work_cb - for deferred close + * + * @twork: callback_head for task work + * @fd: fd to close + * + * Structure to pass task work to be handled after + * returning from binder_ioctl() via task_work_add(). + */ +struct binder_task_work_cb { + struct callback_head twork; + struct file *file; +}; + +/** + * binder_do_fd_close() - close list of file descriptors + * @twork: callback head for task work + * + * It is not safe to call ksys_close() during the binder_ioctl() + * function if there is a chance that binder's own file descriptor + * might be closed. This is to meet the requirements for using + * fdget() (see comments for __fget_light()). Therefore use + * task_work_add() to schedule the close operation once we have + * returned from binder_ioctl(). This function is a callback + * for that mechanism and does the actual ksys_close() on the + * given file descriptor. + */ +static void binder_do_fd_close(struct callback_head *twork) +{ + struct binder_task_work_cb *twcb = container_of(twork, + struct binder_task_work_cb, twork); + + fput(twcb->file); + kfree(twcb); +} + +/** + * binder_deferred_fd_close() - schedule a close for the given file-descriptor + * @fd: file-descriptor to close + * + * See comments in binder_do_fd_close(). This function is used to schedule + * a file-descriptor to be closed after returning from binder_ioctl(). + */ +static void binder_deferred_fd_close(int fd) +{ + struct binder_task_work_cb *twcb; + + twcb = kzalloc(sizeof(*twcb), GFP_KERNEL); + if (!twcb) + return; + init_task_work(&twcb->twork, binder_do_fd_close); + __close_fd_get_file(fd, &twcb->file); + if (twcb->file) + task_work_add(current, &twcb->twork, true); + else + kfree(twcb); +} + static void binder_transaction_buffer_release(struct binder_proc *proc, struct binder_buffer *buffer, binder_size_t *failed_at) @@ -2299,7 +2358,7 @@ static void binder_transaction_buffer_re } fd_array = (u32 *)(parent_buffer + (uintptr_t)fda->parent_offset); for (fd_index = 0; fd_index < fda->num_fds; fd_index++) - ksys_close(fd_array[fd_index]); + binder_deferred_fd_close(fd_array[fd_index]); } break; default: pr_err("transaction release %d bad object type %x\n", @@ -3912,7 +3971,7 @@ static int binder_apply_fd_fixups(struct } else if (ret) { u32 *fdp = (u32 *)(t->buffer->data + fixup->offset);
- ksys_close(*fdp); + binder_deferred_fd_close(*fdp); } list_del(&fixup->fixup_entry); kfree(fixup); --- a/fs/file.c +++ b/fs/file.c @@ -640,6 +640,35 @@ out_unlock: } EXPORT_SYMBOL(__close_fd); /* for ksys_close() */
+/* + * variant of __close_fd that gets a ref on the file for later fput + */ +int __close_fd_get_file(unsigned int fd, struct file **res) +{ + struct files_struct *files = current->files; + struct file *file; + struct fdtable *fdt; + + spin_lock(&files->file_lock); + fdt = files_fdtable(files); + if (fd >= fdt->max_fds) + goto out_unlock; + file = fdt->fd[fd]; + if (!file) + goto out_unlock; + rcu_assign_pointer(fdt->fd[fd], NULL); + __put_unused_fd(files, fd); + spin_unlock(&files->file_lock); + get_file(file); + *res = file; + return filp_close(file, files); + +out_unlock: + spin_unlock(&files->file_lock); + *res = NULL; + return -ENOENT; +} + void do_close_on_exec(struct files_struct *files) { unsigned i; --- a/include/linux/fdtable.h +++ b/include/linux/fdtable.h @@ -121,6 +121,7 @@ extern void __fd_install(struct files_st unsigned int fd, struct file *file); extern int __close_fd(struct files_struct *files, unsigned int fd); +extern int __close_fd_get_file(unsigned int fd, struct file **res);
extern struct kmem_cache *files_cachep;