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Eli Billauer 923fe07f1f usb: core: Solve race condition in anchor cleanup functions 
[ Upstream commit fbc299437c ]

usb_kill_anchored_urbs() is commonly used to cancel all URBs on an
anchor just before releasing resources which the URBs rely on. By doing
so, users of this function rely on that no completer callbacks will take
place from any URB on the anchor after it returns.

However if this function is called in parallel with __usb_hcd_giveback_urb
processing a URB on the anchor, the latter may call the completer
callback after usb_kill_anchored_urbs() returns. This can lead to a
kernel panic due to use after release of memory in interrupt context.

The race condition is that __usb_hcd_giveback_urb() first unanchors the URB
and then makes the completer callback. Such URB is hence invisible to
usb_kill_anchored_urbs(), allowing it to return before the completer has
been called, since the anchor's urb_list is empty.

Even worse, if the racing completer callback resubmits the URB, it may
remain in the system long after usb_kill_anchored_urbs() returns.

Hence list_empty(&anchor->urb_list), which is used in the existing
while-loop, doesn't reliably ensure that all URBs of the anchor are gone.

A similar problem exists with usb_poison_anchored_urbs() and
usb_scuttle_anchored_urbs().

This patch adds an external do-while loop, which ensures that all URBs
are indeed handled before these three functions return. This change has
no effect at all unless the race condition occurs, in which case the
loop will busy-wait until the racing completer callback has finished.
This is a rare condition, so the CPU waste of this spinning is
negligible.

The additional do-while loop relies on usb_anchor_check_wakeup(), which
returns true iff the anchor list is empty, and there is no
__usb_hcd_giveback_urb() in the system that is in the middle of the
unanchor-before-complete phase. The @suspend_wakeups member of
struct usb_anchor is used for this purpose, which was introduced to solve
another problem which the same race condition causes, in commit
6ec4147e7b ("usb-anchor: Delay usb_wait_anchor_empty_timeout wake up
till completion is done").

The surely_empty variable is necessary, because usb_anchor_check_wakeup()
must be called with the lock held to prevent races. However the spinlock
must be released and reacquired if the outer loop spins with an empty
URB list while waiting for the unanchor-before-complete passage to finish:
The completer callback may very well attempt to take the very same lock.

To summarize, using usb_anchor_check_wakeup() means that the patched
functions can return only when the anchor's list is empty, and there is
no invisible URB being processed. Since the inner while loop finishes on
the empty list condition, the new do-while loop will terminate as well,
except for when the said race condition occurs.

Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Acked-by: Oliver Neukum <oneukum@suse.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/20200731054650.30644-1-eli.billauer@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
2020-10-30 10:38:32 +01:00
arch powerpc/powernv/dump: Fix race while processing OPAL dump 2020-10-30 10:38:25 +01:00
block block: ratelimit handle_bad_sector() message 2020-10-30 10:38:26 +01:00
certs export.h: remove VMLINUX_SYMBOL() and VMLINUX_SYMBOL_STR() 2018-08-22 23:21:44 +09:00
crypto crypto: algif_skcipher - EBUSY on aio should be an error 2020-10-29 09:55:01 +01:00
Documentation x86/fpu: Allow multiple bits in clearcpuid= parameter 2020-10-29 09:55:00 +01:00
drivers usb: core: Solve race condition in anchor cleanup functions 2020-10-30 10:38:32 +01:00
firmware Fix built-in early-load Intel microcode alignment 2020-01-23 08:21:29 +01:00
fs reiserfs: Fix memory leak in reiserfs_parse_options() 2020-10-30 10:38:31 +01:00
include scsi: target: core: Add CONTROL field for trace events 2020-10-30 10:38:29 +01:00
init printk: queue wake_up_klogd irq_work only if per-CPU areas are ready 2020-07-22 09:32:13 +02:00
ipc ipc/util.c: sysvipc_find_ipc() incorrectly updates position index 2020-05-20 08:18:40 +02:00
kernel PM: hibernate: remove the bogus call to get_gendisk() in software_resume() 2020-10-30 10:38:28 +01:00
lib lib/crc32.c: fix trivial typo in preprocessor condition 2020-10-30 10:38:21 +01:00
LICENSES LICENSES: Remove CC-BY-SA-4.0 license text 2018-10-18 11:28:50 +02:00
mm mm, oom_adj: don't loop through tasks in __set_oom_adj when not necessary 2020-10-29 09:55:15 +01:00
net ipvs: Fix uninit-value in do_ip_vs_set_ctl() 2020-10-30 10:38:31 +01:00
samples misc: vop: add round_up(x,4) for vring_size to avoid kernel panic 2020-10-30 10:38:29 +01:00
scripts checkpatch: fix the usage of capture group ( ... ) 2020-09-09 19:04:32 +02:00
security ima: Don't ignore errors from crypto_shash_update() 2020-10-29 09:54:59 +01:00
sound ALSA: seq: oss: Avoid mutex lock for a long-time ioctl 2020-10-29 09:55:11 +01:00
tools perf intel-pt: Fix "context_switch event has no tid" error 2020-10-30 10:38:19 +01:00
usr initramfs: restore default compression behavior 2020-04-13 10:44:59 +02:00
virt KVM: arm64: Assume write fault on S1PTW permission fault on instruction fetch 2020-10-01 13:14:54 +02:00
.clang-format clang-format: Set IndentWrappedFunctionNames false 2018-08-01 18:38:51 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Add hch to .get_maintainer.ignore 2015-08-21 14:30:10 -07:00
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore Kbuild updates for v4.17 (2nd) 2018-04-15 17:21:30 -07:00
.mailmap libnvdimm-for-4.19_misc 2018-08-25 18:13:10 -07:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS 9p: remove Ron Minnich from MAINTAINERS 2018-08-17 16:20:26 -07:00
Kbuild Kbuild updates for v4.15 2017-11-17 17:45:29 -08:00
Kconfig kconfig: move the "Executable file formats" menu to fs/Kconfig.binfmt 2018-08-02 08:06:55 +09:00
MAINTAINERS Documentation/llvm: add documentation on building w/ Clang/LLVM 2020-09-26 18:01:31 +02:00
Makefile Linux 4.19.153 2020-10-29 09:55:18 +01:00
README Docs: Added a pointer to the formatted docs to README 2018-03-21 09:02:53 -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.
See Documentation/00-INDEX for a list of what is contained in each file.

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.