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    Subject[PATCH v2 4.9 00/10] fix a race in release_task when flushing the dentry
    Date
    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").


    Alexey Dobriyan (1):
    proc: use %u for pid printing and slightly less stack

    Andreas Gruenbacher (1):
    proc: Pass file mode to proc_pid_make_inode

    Christian Brauner (1):
    clone: add CLONE_PIDFD

    Eric W. Biederman (6):
    proc: Better ownership of files for non-dumpable tasks in user
    namespaces
    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 | 242 ++++++++++++++++++++-------------------------
    fs/proc/fd.c | 20 +---
    fs/proc/inode.c | 67 ++++++++++++-
    fs/proc/internal.h | 22 ++---
    fs/proc/namespaces.c | 3 +-
    fs/proc/proc_sysctl.c | 45 ++-------
    fs/proc/self.c | 6 +-
    fs/proc/thread_self.c | 5 +-
    include/linux/pid.h | 5 +
    include/linux/proc_fs.h | 4 +-
    include/uapi/linux/sched.h | 1 +
    kernel/exit.c | 5 +-
    kernel/fork.c | 131 +++++++++++++++++++++++-
    kernel/pid.c | 3 +
    kernel/signal.c | 11 +++
    security/selinux/hooks.c | 1 +
    16 files changed, 343 insertions(+), 228 deletions(-)

    --
    1.8.3.1

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