Skip to content

L
linux
Switch branch/tag
  1. 13 Mar, 2020 10 commits
  3. 11 Mar, 2020 2 commits
    • Ioanna Alifieraki's avatar
      Revert "ipc, sem: remove uneeded sem_undo_list lock usage in exit_sem()" · 84721237
      Ioanna Alifieraki authored 
      BugLink: https://bugs.launchpad.net/bugs/1858834

This reverts commit a9795584.

Commit a9795584 ("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 a9795584 ("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 a9795584 ("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: a9795584 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()")
Signed-off-by: default avatarIoanna Alifieraki <ioanna-maria.alifieraki@canonical.com>
Acked-by: default avatarManfred Spraul <manfred@colorfullife.com>
Acked-by: default avatarHerton 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit edf28f40)
Signed-off-by: default avatarIoanna Alifieraki <ioanna-maria.alifieraki@canonical.com>
Acked-by: default avatarColin Ian King <colin.king@canonical.com>
Acked-by: default avatarKhalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: default avatarKhalid Elmously <khalid.elmously@canonical.com>
      84721237
    • Jan Kara's avatar
      xfs: Sanity check flags of Q_XQUOTARM call · d380ba58
      Jan Kara authored 
      BugLink: https://bugs.launchpad.net/bugs/1864092

Flags passed to Q_XQUOTARM were not sanity checked for invalid values.
Fix that.

Fixes: 9da93f9b ("xfs: fix Q_XQUOTARM ioctl")
Reported-by: default avatarYang Xu <xuyang2018.jy@cn.fujitsu.com>
Signed-off-by: default avatarJan Kara <jack@suse.cz>
Reviewed-by: default avatarEric Sandeen <sandeen@redhat.com>
Reviewed-by: default avatarDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: default avatarDarrick J. Wong <darrick.wong@oracle.com>
(cherry picked from commit 3dd4d40b)
Signed-off-by: default avatarPo-Hsu Lin <po-hsu.lin@canonical.com>
Acked-by: default avatarColin Ian King <colin.king@canonical.com>
Acked-by: default avatarThadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: default avatarKhalid Elmously <khalid.elmously@canonical.com>
      d380ba58
  5. 28 Feb, 2020 8 commits
  7. 14 Feb, 2020 5 commits
  9. 29 Jan, 2020 15 commits
GitLab Nexedi Edition | About GitLab | About Nexedi | 沪ICP备2021021310号-2 | 沪ICP备2021021310号-7