This is a note to let you know that I've just added the patch titled
binder: fix use-after-free due to ksys_close() during fdget()
to my char-misc git tree which can be found at git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git in the char-misc-next branch.
The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)
The patch will also be merged in the next major kernel release during the merge window.
If you have any questions about this process, please let me know.
From 80cd795630d6526ba729a089a435bf74a57af927 Mon Sep 17 00:00:00 2001
From: Todd Kjos tkjos@android.com Date: Fri, 14 Dec 2018 15:58:21 -0800 Subject: binder: fix use-after-free due to ksys_close() during fdget()
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(-)
diff --git a/drivers/android/binder.c b/drivers/android/binder.c index d653e8a474fc..210940bd0457 100644 --- 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>
@@ -2170,6 +2171,64 @@ static bool binder_validate_fixup(struct binder_buffer *b, 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) @@ -2309,7 +2368,7 @@ static void binder_transaction_buffer_release(struct binder_proc *proc, } 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", @@ -3928,7 +3987,7 @@ static int binder_apply_fd_fixups(struct binder_transaction *t) } 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); diff --git a/fs/file.c b/fs/file.c index 7ffd6e9d103d..8d059d8973e9 100644 --- a/fs/file.c +++ b/fs/file.c @@ -640,6 +640,35 @@ int __close_fd(struct files_struct *files, unsigned fd) } 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; diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h index 41615f38bcff..f07c55ea0c22 100644 --- a/include/linux/fdtable.h +++ b/include/linux/fdtable.h @@ -121,6 +121,7 @@ extern void __fd_install(struct files_struct *files, 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;