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Ioanna Alifieraki cf85f00f87 Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()" 
commit edf28f4061 upstream.

This reverts commit a979558448.

Commit a979558448 ("ipc,sem: remove uneeded sem_undo_list lock usage
in exit_sem()") removes a lock that is needed.  This leads to a process
looping infinitely in exit_sem() and can also lead to a crash.  There is
a reproducer available in [1] and with the commit reverted the issue
does not reproduce anymore.

Using the reproducer found in [1] is fairly easy to reach a point where
one of the child processes is looping infinitely in exit_sem between
for(;;) and if (semid == -1) block, while it's trying to free its last
sem_undo structure which has already been freed by freeary().

Each sem_undo struct is on two lists: one per semaphore set (list_id)
and one per process (list_proc).  The list_id list tracks undos by
semaphore set, and the list_proc by process.

Undo structures are removed either by freeary() or by exit_sem().  The
freeary function is invoked when the user invokes a syscall to remove a
semaphore set.  During this operation freeary() traverses the list_id
associated with the semaphore set and removes the undo structures from
both the list_id and list_proc lists.

For this case, exit_sem() is called at process exit.  Each process
contains a struct sem_undo_list (referred to as "ulp") which contains
the head for the list_proc list.  When the process exits, exit_sem()
traverses this list to remove each sem_undo struct.  As in freeary(),
whenever a sem_undo struct is removed from list_proc, it is also removed
from the list_id list.

Removing elements from list_id is safe for both exit_sem() and freeary()
due to sem_lock().  Removing elements from list_proc is not safe;
freeary() locks &un->ulp->lock when it performs
list_del_rcu(&un->list_proc) but exit_sem() does not (locking was
removed by commit a979558448 ("ipc,sem: remove uneeded sem_undo_list
lock usage in exit_sem()").

This can result in the following situation while executing the
reproducer [1] : Consider a child process in exit_sem() and the parent
in freeary() (because of semctl(sid[i], NSEM, IPC_RMID)).

 - The list_proc for the child contains the last two undo structs A and
   B (the rest have been removed either by exit_sem() or freeary()).

 - The semid for A is 1 and semid for B is 2.

 - exit_sem() removes A and at the same time freeary() removes B.

 - Since A and B have different semid sem_lock() will acquire different
   locks for each process and both can proceed.

The bug is that they remove A and B from the same list_proc at the same
time because only freeary() acquires the ulp lock. When exit_sem()
removes A it makes ulp->list_proc.next to point at B and at the same
time freeary() removes B setting B->semid=-1.

At the next iteration of for(;;) loop exit_sem() will try to remove B.

The only way to break from for(;;) is for (&un->list_proc ==
&ulp->list_proc) to be true which is not. Then exit_sem() will check if
B->semid=-1 which is and will continue looping in for(;;) until the
memory for B is reallocated and the value at B->semid is changed.

At that point, exit_sem() will crash attempting to unlink B from the
lists (this can be easily triggered by running the reproducer [1] a
second time).

To prove this scenario instrumentation was added to keep information
about each sem_undo (un) struct that is removed per process and per
semaphore set (sma).

          CPU0                                CPU1
  [caller holds sem_lock(sma for A)]      ...
  freeary()                               exit_sem()
  ...                                     ...
  ...                                     sem_lock(sma for B)
  spin_lock(A->ulp->lock)                 ...
  list_del_rcu(un_A->list_proc)           list_del_rcu(un_B->list_proc)

Undo structures A and B have different semid and sem_lock() operations
proceed.  However they belong to the same list_proc list and they are
removed at the same time.  This results into ulp->list_proc.next
pointing to the address of B which is already removed.

After reverting commit a979558448 ("ipc,sem: remove uneeded
sem_undo_list lock usage in exit_sem()") the issue was no longer
reproducible.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1694779

Link: http://lkml.kernel.org/r/20191211191318.11860-1-ioanna-maria.alifieraki@canonical.com
Fixes: a979558448 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()")
Signed-off-by: Ioanna Alifieraki <ioanna-maria.alifieraki@canonical.com>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
Acked-by: Herton R. Krzesinski <herton@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: <malat@debian.org>
Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Jay Vosburgh <jay.vosburgh@canonical.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2020-02-28 16:38:49 +01:00
arch x86/cpu/amd: Enable the fixed Instructions Retired counter IRPERF 2020-02-28 16:38:48 +01:00
block block: fix 32 bit overflow in __blkdev_issue_discard() 2020-02-01 09:37:12 +00:00
certs export.h: remove VMLINUX_SYMBOL() and VMLINUX_SYMBOL_STR() 2018-08-22 23:21:44 +09:00
crypto crypto: api - Fix race condition in crypto_spawn_alg 2020-02-11 04:34:05 -08:00
Documentation PM / devfreq: Add new name attribute for sysfs 2020-02-05 14:43:34 +00:00
drivers serdev: ttyport: restore client ops on deregistration 2020-02-28 16:38:49 +01:00
firmware Fix built-in early-load Intel microcode alignment 2020-01-23 08:21:29 +01:00
fs jbd2: fix ocfs2 corrupt when clearing block group bits 2020-02-28 16:38:48 +01:00
include serdev: ttyport: restore client ops on deregistration 2020-02-28 16:38:49 +01:00
init fork: fix some -Wmissing-prototypes warnings 2019-12-05 09:21:04 +01:00
ipc Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()" 2020-02-28 16:38:49 +01:00
kernel trigger_next should increase position index 2020-02-24 08:34:52 +01:00
lib lib/scatterlist.c: adjust indentation in __sg_alloc_table 2020-02-24 08:34:52 +01:00
LICENSES LICENSES: Remove CC-BY-SA-4.0 license text 2018-10-18 11:28:50 +02:00
mm mm/page_alloc.c: fix uninitialized memmaps on a partially populated last section 2020-02-11 04:34:18 -08:00
net bpf: Return -EBADRQC for invalid map type in __bpf_tx_xdp_map 2020-02-24 08:34:47 +01:00
samples samples/bpf: Don't try to remove user's homedir on clean 2020-02-11 04:34:05 -08:00
scripts kbuild: use -S instead of -E for precise cc-option test in Kconfig 2020-02-24 08:34:50 +01:00
security selinux: ensure we cleanup the internal AVC counters on error in avc_update() 2020-02-24 08:34:43 +01:00
sound ASoC: sun8i-codec: Fix setting DAI data format 2020-02-28 16:38:44 +01:00
tools selftests: bpf: Reset global state between reuseport test runs 2020-02-24 08:34:51 +01:00
usr kbuild: clean compressed initramfs image 2019-10-07 18:57:16 +02:00
virt KVM: arm64: pmu: Don't increment SW_INCR if PMCR.E is unset 2020-02-14 16:33:26 -05:00
.clang-format clang-format: Set IndentWrappedFunctionNames false 2018-08-01 18:38:51 +02:00
.cocciconfig
.get_maintainer.ignore
.gitattributes
.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 MAINTAINERS: Update drm/i915 bug filing URL 2020-02-28 16:38:49 +01:00
Makefile Linux 4.19.106 2020-02-24 08:34:54 +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.