linux-kernel - Re: [PATCH] seccomp: Add SECCOMP_CLONE

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Message-ID: <0411cd55-6a3b-423c-b0b6-cb5de34827ff@oracle.com>
Date: Thu, 4 Sep 2025 06:08:32 -0600
From: Tom Hromatka <tom.hromatka@...cle.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Kees Cook <kees@...nel.org>, Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, Sargun Dhillon <sargun@...gun.me>,
        Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH] seccomp: Add SECCOMP_CLONE_FILTER operation

On 9/3/25 4:44 PM, Alexei Starovoitov wrote:
> On Wed, Sep 3, 2025 at 1:52 PM Tom Hromatka <tom.hromatka@...cle.com> wrote:
>>
>> On 9/3/25 2:45 PM, Alexei Starovoitov wrote:
>>> On Wed, Sep 3, 2025 at 1:38 PM Tom Hromatka <tom.hromatka@...cle.com> wrote:
>>>>
>>>> +
>>>> +       spin_lock_irq(&current->sighand->siglock);
>>>> +       spin_lock_irq(&task->sighand->siglock);
>>>> +
>>>> +       if (atomic_read(&task->seccomp.filter_count) == 0) {
>>>> +               spin_unlock_irq(&task->sighand->siglock);
>>>> +               spin_unlock_irq(&current->sighand->siglock);
>>>
>>> did you copy this pattern from somewhere ?
>>> It's obviously buggy.
>>
>> I tried to mimic the logic in copy_seccomp() in kernel/fork.c,
>> but as you point out, I probably messed it up :).
>>
>> Do you have recommendations for a better design pattern?
> 
> Several things look wrong here.

Thanks so much for weighing in.

> Double _irq() is one obvious bug.

Makes sense.  I'll look through the kernel code to see if I can
find another place where two task structs are being locked at
the same time.  I've never had to do that before.

> Grabbing spin_lock to do atomic_read() is another oddity.

That would indeed be strange.

The spin_lock is needed to ensure that the source and target's
seccomp filters don't change out from underneath me.  Once I
read the target's seccomp filter count, I don't want another
thread to make any changes before I've updated the target's
filters.

Thanks!

Tom