pid_namespace: make zap_pid_ns_processes() freezable

Overview

In the Linux kernel, the following vulnerability has been resolved:

pid_namespace: make zap_pid_ns_processes() freezable

When the init of a pid namespace exits, zap_pid_ns_processes() waits until all other pids in the namespace are freed. A tracer outside the namespace that does not reap its traced zombies keeps those pids allocated, so this wait has no upper bound, and while it lasts every suspend attempt fails:

Freezing user space processes failed after 20.006 seconds (2 tasks refusing to freeze, wq_busy=0):

Neither wait is freezable. The first loop sleeps in do_wait(), which returns -ERESTARTSYS on the freezer's fake signal; the loop then clears TIF_SIGPENDING and waits again without ever freezing. The final loop sleeps in TASK_INTERRUPTIBLE, and since nothing clears TIF_SIGPENDING there, schedule() returns immediately after the first freeze attempt and the task spins until the wait ends.

Sleep in TASK_INTERRUPTIBLE | TASK_FREEZABLE in do_wait(), so that the freezer can freeze tasks waiting there directly, and in TASK_IDLE | TASK_FREEZABLE in the final loop, like coredump_task_exit() does.

This was hit with Chromium-based browsers: crashpad ptrace-attaches the threads of a crashed renderer, which is a pid namespace init, and does not reap those killed during the dump.

Affected

• Linux >= 3eb07c8c8adb6f0572baba844ba2d9e501654316 < 064a6761d6f6ac55d090d3eaca649577ee736713
• Linux >= 3eb07c8c8adb6f0572baba844ba2d9e501654316 < e8dafad7de9ede813af2d48cc04b7c88fef16682
• Linux >= 3eb07c8c8adb6f0572baba844ba2d9e501654316 < 4b47107f1a80c809c80093e872bcaccdbae13db4
• Linux >= 3eb07c8c8adb6f0572baba844ba2d9e501654316 < 2e10b8a74f55b5aae33db105d3e2d3ae759c865a
• Linux >= 3eb07c8c8adb6f0572baba844ba2d9e501654316 < 7deb5770840339fe88755bf27b92f74edc26ee87
• Linux >= 3eb07c8c8adb6f0572baba844ba2d9e501654316 < 81933038f5e273d17ed22792f906889393b204c5
• Linux 2.6.24

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.