linux-kernel - Re: [PATCH v1] binfmt_misc: fix crash when load/unload module

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Message-ID: <CAA5qM4A_pArXkyLvcXbJwp0xFSnDZJ+Md82ME1AOrt+5v-zaDg@mail.gmail.com>
Date:   Mon, 24 Jan 2022 10:23:17 -0800
From:   Tong Zhang <ztong0001@...il.com>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] binfmt_misc: fix crash when load/unload module

On Mon, Jan 24, 2022 at 2:40 AM Christian Brauner <brauner@...nel.org> wrote:
> The fix itself is obviously needed.
>
> However, afaict the previous patch introduced another bug and this patch
> right here doesn't fix it either.
>
> Namely, if you set CONFIG_SYSCTL=n and CONFIG_BINFMT_MISC={y,m}, then
> register_sysctl_mount_point() will return NULL causing modprobe
> binfmt_misc to fail. However, before 3ba442d5331f ("fs: move binfmt_misc
> sysctl to its own file") loading binfmt_misc would've succeeded even if
> fs/binfmt_misc wasn't created in kernel/sysctl.c. Afaict, that goes for
> both CONFIG_SYSCTL={y,n} since even in the CONFIG_SYSCTL=y case the
> kernel would've moved on if creating the sysctl header would've failed.
> And that makes sense since binfmt_misc is mountable wherever, not just
> at fs/binfmt_misc.
>
> All that indicates that the correct fix here would be to simply:
>
> binfmt_misc_header = register_sysctl_mount_point("fs/binfmt_misc");
>
> without checking for an error. That should fully restore the old
> behavior.
>

Thanks! That makes sense.
I modified the patch according to your comment, added another fix for
the return type issue and sent a v2.
Thanks again.
- Tong