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Christian Brauner f0fe2c0f05 binder: prevent UAF for binderfs devices II 
This is a necessary follow up to the first fix I proposed and we merged
in 2669b8b0c7 ("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 51d8a7eca6 ("binder: prevent UAF read in
print_binder_transaction_log_entry()").

Fixes: 3ad20fe393 ("binder: implement binderfs")
Fixes: 2669b8b0c7 ("binder: prevent UAF for binderfs devices")
Fixes: 03e2e07e38 ("binder: Make transaction_log available in binderfs")
Related : 51d8a7eca6 ("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.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2020-03-03 19:58:37 +01:00
arch csky updates for 5.6-rc3 2020-02-23 09:37:41 -08:00
block block-5.6-2020-02-05 2020-02-06 06:15:23 +00:00
certs certs: Add wrapper function to check blacklisted binary hash 2019-11-12 12:25:50 +11:00
crypto Merge branch 'next-integrity' of git://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-integrity 2020-02-20 15:15:16 -08:00
Documentation fs/zonefs patches for 5.6-rc3 2020-02-22 11:38:20 -08:00
drivers binder: prevent UAF for binderfs devices II 2020-03-03 19:58:37 +01:00
fs for-5.6-rc2-tag 2020-02-23 09:43:50 -08:00
include Two fixes for the irq core code which are follow ups to the recent MSI 2020-02-22 17:25:46 -08:00
init Various fixes: 2020-02-11 16:39:18 -08:00
ipc Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()" 2020-02-21 11:22:15 -08:00
kernel Two fixes for the irq core code which are follow ups to the recent MSI 2020-02-22 17:25:46 -08:00
lib lib/stackdepot.c: fix global out-of-bounds in stack_slabs 2020-02-21 11:22:15 -08:00
LICENSES LICENSES: Rename other to deprecated 2019-05-03 06:34:32 -06:00
mm arm64 fixes for -rc3 2020-02-21 16:03:36 -08:00
net Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2020-02-21 11:59:51 -08:00
samples Kbuild updates for v5.6 (2nd) 2020-02-09 16:05:50 -08:00
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security Merge branch 'next-integrity' of git://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-integrity 2020-02-20 15:15:16 -08:00
sound ASoC: Fixes for v5.6 2020-02-18 13:33:39 +01:00
tools Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2020-02-21 11:59:51 -08:00
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virt KVM: Disable preemption in kvm_get_running_vcpu() 2020-02-12 12:19:35 +01:00
.clang-format clang-format: Update with the latest for_each macro list 2019-08-31 10:00:51 +02:00
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.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore modpost: dump missing namespaces into a single modules.nsdeps file 2019-11-11 20:10:01 +09:00
.mailmap A handful of small documentation fixes that wandered in. 2020-02-07 13:03:10 -08:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS open: introduce openat2(2) syscall 2020-01-18 09:19:18 -05:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig docs: kbuild: convert docs to ReST and rename to *.rst 2019-06-14 14:21:21 -06:00
MAINTAINERS csky updates for 5.6-rc3 2020-02-23 09:37:41 -08:00
Makefile Linux 5.6-rc3 2020-02-23 16:17:42 -08:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.