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Ioanna Alifieraki aa4f749f81 Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()" 
commit edf28f4061afe4c2d9eb1c3323d90e882c1d6800 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 17:22:20 +01:00
Documentation fbdev: fix numbering of fbcon options 2020-02-24 08:36:42 +01:00
LICENSES LICENSES: Rename other to deprecated 2019-05-03 06:34:32 -06:00
arch x86/cpu/amd: Enable the fixed Instructions Retired counter IRPERF 2020-02-28 17:22:19 +01:00
block block, bfq: do not plug I/O for bfq_queues with no proc refs 2020-02-24 08:36:31 +01:00
certs PKCS#7: Refactor verify_pkcs7_signature() 2019-08-05 18:40:18 -04:00
crypto crypto: essiv - fix AEAD capitalization and preposition use in help text 2020-02-24 08:36:50 +01:00
drivers ACPI: PM: s2idle: Check fixed wakeup events in acpi_s2idle_wake() 2020-02-28 17:22:20 +01:00
fs jbd2: fix ocfs2 corrupt when clearing block group bits 2020-02-28 17:22:18 +01:00
include ACPI: PM: s2idle: Check fixed wakeup events in acpi_s2idle_wake() 2020-02-28 17:22:20 +01:00
init Revert "um: Enable CONFIG_CONSTRUCTORS" 2020-02-01 09:34:53 +00:00
ipc Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()" 2020-02-28 17:22:20 +01:00
kernel trigger_next should increase position index 2020-02-24 08:36:59 +01:00
lib lib/scatterlist.c: adjust indentation in __sg_alloc_table 2020-02-24 08:37:00 +01:00
mm mm/mmu_gather: invalidate TLB correctly on batch allocation failure and flush 2020-02-11 04:35:42 -08:00
net sunrpc: Fix potential leaks in sunrpc_cache_unhash() 2020-02-24 08:36:55 +01:00
samples samples/bpf: Set -fno-stack-protector when building BPF programs 2020-02-24 08:36:36 +01:00
scripts bpf, btf: Always output invariant hit in pahole DWARF to BTF transform 2020-02-24 08:36:55 +01:00
security selinux: ensure we cleanup the internal AVC counters on error in avc_update() 2020-02-24 08:36:39 +01:00
sound ASoC: sun8i-codec: Fix setting DAI data format 2020-02-28 17:22:13 +01:00
tools selftests/eeh: Bump EEH wait time to 60s 2020-02-24 08:36:57 +01:00
usr gen_initramfs_list.sh: fix 'bad variable name' error 2020-01-09 10:20:00 +01:00
virt KVM: arm64: Treat emulated TVAL TimerValue as a signed 32-bit integer 2020-02-14 16:34:18 -05:00
.clang-format clang-format: Update with the latest for_each macro list 2019-08-31 10:00:51 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore Modules updates for v5.4 2019-09-22 10:34:46 -07:00
.mailmap ARM: SoC fixes 2019-11-10 13:41:59 -08:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS MAINTAINERS: Remove Simon as Renesas SoC Co-Maintainer 2019-10-10 08:12:51 -07:00
Kbuild kbuild: do not descend to ./Kbuild when cleaning 2019-08-21 21:03:58 +09:00
Kconfig docs: kbuild: convert docs to ReST and rename to *.rst 2019-06-14 14:21:21 -06:00
MAINTAINERS MAINTAINERS: Update drm/i915 bug filing URL 2020-02-28 17:22:19 +01:00
Makefile Linux 5.4.22 2020-02-24 08:37:04 +01:00
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.