CVE-2022-50661 — seccomp: Move copy_seccomp() to no failure path.

Affected: Linux

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

seccomp: Move copy_seccomp() to no failure path.

Our syzbot instance reported memory leaks in do_seccomp() [0], similar to the report [1]. It shows that we miss freeing struct seccomp_filter and some objects included in it.

We can reproduce the issue with the program below [2] which calls one seccomp() and two clone() syscalls.

The first clone()d child exits earlier than its parent and sends a signal to kill it during the second clone(), more precisely before the fatal_signal_pending() test in copy_process(). When the parent receives the signal, it has to destroy the embryonic process and return -EINTR to user space. In the failure path, we have to call seccomp_filter_release() to decrement the filter's refcount.

Initially, we called it in free_task() called from the failure path, but the commit 3a15fb6ed92c ("seccomp: release filter after task is fully dead") moved it to release_task() to notify user space as early as possible that the filter is no longer used.

To keep the change and current seccomp refcount semantics, let's move copy_seccomp() just after the signal check and add a WARN_ON_ONCE() in free_task() for future debugging.

[0]: unreferenced object 0xffff8880063add00 (size 256): comm "repro_seccomp", pid 230, jiffies 4294687090 (age 9.914s) hex dump (first 32 bytes): 01 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 ................ ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff…

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CVE-2022-50661 — seccomp: Move copy_seccomp() to no failure path.