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Date: Fri, 14 Dec 2018 15:58:21 -0800
From: Todd Kjos <tkjos@...roid.com>
To: tkjos@...gle.com, gregkh@...uxfoundation.org, arve@...roid.com,
devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
maco@...gle.com, viro@...iv.linux.org.uk
Cc: joel@...lfernandes.org, kernel-team@...roid.com
Subject: [PATCH v3] 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@...iv.linux.org.uk>
Signed-off-by: Todd Kjos <tkjos@...gle.com>
---
v2:
- simplified code
v3:
- implemented Al Viro's suggestion to pass struct file instead of fd
- added __close_fd_get_file() to close the fd, but reference the file
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 c525b164d39d2f..c4ee11d883dd93 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>
@@ -2184,6 +2185,64 @@ 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)
@@ -2323,7 +2382,7 @@ 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",
@@ -3942,7 +4001,7 @@ 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 7ffd6e9d103d64..8d059d8973e9fc 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 41615f38bcff3e..f07c55ea0c22f1 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;
--
2.20.0.405.gbc1bbc6f85-goog
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