[PATCH 4.19 0/7] fix a race in release_task when flushing the dentry

[Wen Yang]

The dentries such as /proc/<pid>/ns/ have the DCACHE_OP_DELETE flag, they 
should be deleted when the process exits. 

Suppose the following race appears： 

release_task                 dput 
-> proc_flush_task 
                             -> dentry->d_op->d_delete(dentry) 
-> __exit_signal 
                             -> dentry->d_lockref.count--  and return. 

In the proc_flush_task(), if another process is using this dentry, it will
not be deleted. At the same time, in dput(), d_op->d_delete() can be executed
before __exit_signal(pid has not been hashed), d_delete returns false, so
this dentry still cannot be deleted.

This dentry will always be cached (although its count is 0 and the
DCACHE_OP_DELETE flag is set), its parent denry will also be cached too, and
these dentries can only be deleted when drop_caches is manually triggered.

This will result in wasted memory. What's more troublesome is that these
dentries reference pid, according to the commit f333c700c610 ("pidns: Add a
limit on the number of pid namespaces"), if the pid cannot be released, it
may result in the inability to create a new pid_ns.

This issue was introduced by 60347f6716aa ("pid namespaces: prepare
proc_flust_task() to flush entries from multiple proc trees"), exposed by
f333c700c610 ("pidns: Add a limit on the number of pid namespaces"), and then
fixed by 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc").


Christian Brauner (1):
  clone: add CLONE_PIDFD

Eric W. Biederman (5):
  proc: Rename in proc_inode rename sysctl_inodes sibling_inodes
  proc: Generalize proc_sys_prune_dcache into proc_prune_siblings_dcache
  proc: Clear the pieces of proc_inode that proc_evict_inode cares about
  proc: Use d_invalidate in proc_prune_siblings_dcache
  proc: Use a list of inodes to flush from proc

Joel Fernandes (Google) (1):
  pidfd: add polling support

 fs/proc/base.c             | 111 ++++++++++++-------------------------
 fs/proc/inode.c            |  67 +++++++++++++++++++++--
 fs/proc/internal.h         |   4 +-
 fs/proc/proc_sysctl.c      |  45 ++-------------
 include/linux/pid.h        |   6 ++
 include/linux/proc_fs.h    |   4 +-
 include/uapi/linux/sched.h |   1 +
 kernel/exit.c              |   4 +-
 kernel/fork.c              | 133 +++++++++++++++++++++++++++++++++++++++++++--
 kernel/pid.c               |   3 +
 kernel/signal.c            |  11 ++++
 11 files changed, 262 insertions(+), 127 deletions(-)

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1.8.3.1