linux-kernel - Re: [PATCH v1] seccomp: Release filter when copy

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Date:   Mon, 22 Aug 2022 14:16:03 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Kuniyuki Iwashima <kuniyu@...zon.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>,
        Christian Brauner <brauner@...nel.org>,
        Kuniyuki Iwashima <kuni1840@...il.com>,
        linux-kernel@...r.kernel.org,
        syzbot+ab17848fe269b573eb71@...kaller.appspotmail.com,
        Ayushman Dutta <ayudutta@...zon.com>
Subject: Re: [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