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SubjectRe: [PATCH v1] seccomp: Release filter when copy_process() fails.
On Mon, Aug 22, 2022 at 01:44:36PM -0700, Kuniyuki Iwashima wrote:
> 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 ref count.
>
> 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, let's call seccomp_filter_release() in copy_process()
> and add a WARN_ON_ONCE() in free_task() for future debugging.

Thanks for tracking this down! I think I'd prefer to avoid changing the
semantics around the existing seccomp refcount lifetime, so what about
just moving copy_seccomp() below the last possible error path?


diff --git a/kernel/fork.c b/kernel/fork.c
index 90c85b17bf69..e7f4e7f1e01e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2409,12 +2409,6 @@ static __latent_entropy struct task_struct *copy_process(

spin_lock(&current->sighand->siglock);

- /*
- * Copy seccomp details explicitly here, in case they were changed
- * before holding sighand lock.
- */
- copy_seccomp(p);
-
rv_task_fork(p);

rseq_fork(p, clone_flags);
@@ -2431,6 +2425,14 @@ static __latent_entropy struct task_struct *copy_process(
goto bad_fork_cancel_cgroup;
}

+ /* No more failures paths after this point. */
+
+ /*
+ * Copy seccomp details explicitly here, in case they were changed
+ * before holding sighand lock.
+ */
+ copy_seccomp(p);
+
init_task_pid_links(p);
if (likely(p->pid)) {
ptrace_init_task(p, (clone_flags & CLONE_PTRACE) || trace);

Totally untested, but I think it would fix this?

-Kees

--
Kees Cook

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