linux-cve-announce - CVE-2024-36894: usb: gadget: f_fs: Fix race between aio

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Message-ID: <2024053034-CVE-2024-36894-b44b@gregkh>
Date: Thu, 30 May 2024 17:28:46 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2024-36894: usb: gadget: f_fs: Fix race between aio_cancel() and AIO request complete

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

usb: gadget: f_fs: Fix race between aio_cancel() and AIO request complete

FFS based applications can utilize the aio_cancel() callback to dequeue
pending USB requests submitted to the UDC.  There is a scenario where the
FFS application issues an AIO cancel call, while the UDC is handling a
soft disconnect.  For a DWC3 based implementation, the callstack looks
like the following:

    DWC3 Gadget                               FFS Application
dwc3_gadget_soft_disconnect()              ...
  --> dwc3_stop_active_transfers()
    --> dwc3_gadget_giveback(-ESHUTDOWN)
      --> ffs_epfile_async_io_complete()   ffs_aio_cancel()
        --> usb_ep_free_request()            --> usb_ep_dequeue()

There is currently no locking implemented between the AIO completion
handler and AIO cancel, so the issue occurs if the completion routine is
running in parallel to an AIO cancel call coming from the FFS application.
As the completion call frees the USB request (io_data->req) the FFS
application is also referencing it for the usb_ep_dequeue() call.  This can
lead to accessing a stale/hanging pointer.

commit b566d38857fc ("usb: gadget: f_fs: use io_data->status consistently")
relocated the usb_ep_free_request() into ffs_epfile_async_io_complete().
However, in order to properly implement locking to mitigate this issue, the
spinlock can't be added to ffs_epfile_async_io_complete(), as
usb_ep_dequeue() (if successfully dequeuing a USB request) will call the
function driver's completion handler in the same context.  Hence, leading
into a deadlock.

Fix this issue by moving the usb_ep_free_request() back to
ffs_user_copy_worker(), and ensuring that it explicitly sets io_data->req
to NULL after freeing it within the ffs->eps_lock.  This resolves the race
condition above, as the ffs_aio_cancel() routine will not continue
attempting to dequeue a request that has already been freed, or the
ffs_user_copy_work() not freeing the USB request until the AIO cancel is
done referencing it.

This fix depends on
  commit b566d38857fc ("usb: gadget: f_fs: use io_data->status
  consistently")

The Linux kernel CVE team has assigned CVE-2024-36894 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 3.15 with commit 2e4c7553cd6f and fixed in 6.6.31 with commit 73c05ad46bb4
	Issue introduced in 3.15 with commit 2e4c7553cd6f and fixed in 6.8.10 with commit d74618308232
	Issue introduced in 3.15 with commit 2e4c7553cd6f and fixed in 6.9 with commit 24729b307eef

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-36894
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	drivers/usb/gadget/function/f_fs.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/73c05ad46bb4fbbdb346004651576d1c8dbcffbb
	https://git.kernel.org/stable/c/d7461830823242702f5d84084bcccb25159003f4
	https://git.kernel.org/stable/c/24729b307eefcd7c476065cd7351c1a018082c19