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From:   Jann Horn <jannh@google.com>
Date:   Mon, 15 Jun 2020 11:36:22 +0200
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Subject: Re: [RFC PATCH] seccomp: Add extensibility mechanism to read notifications
To:     Sargun Dhillon <sargun@sargun.me>
Cc:     Kees Cook <keescook@chromium.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Tycho Andersen <tycho@tycho.ws>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Linux Containers <containers@lists.linux-foundation.org>,
        Linux API <linux-api@vger.kernel.org>
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On Sat, Jun 13, 2020 at 9:26 AM Sargun Dhillon <sargun@sargun.me> wrote:
> This introduces an extensibility mechanism to receive seccomp
> notifications. It uses read(2), as opposed to using an ioctl. The listener
> must be first configured to write the notification via the
> SECCOMP_IOCTL_NOTIF_CONFIG ioctl with the fields that the user is
> interested in.
>
> This is different than the old SECCOMP_IOCTL_NOTIF_RECV method as it allows
> for more flexibility. It allows the user to opt into certain fields, and
> not others. This is nice for users who want to opt into some fields like
> thread group leader. In the future, this mechanism can be used to expose
> file descriptors to users,

Please don't touch the caller's file descriptor table from read/write
handlers, only from ioctl handlers. A process should always be able to
read from files supplied by an untrusted user without having to worry
about new entries mysteriously popping up in its fd table.

> such as a representation of the process's
> memory. It also has good forwards and backwards compatibility guarantees.
> Users with programs compiled against newer headers will work fine on older
> kernels as long as they don't opt into any sizes, or optional fields that
> are only available on newer kernels.
>
> The ioctl method relies on an extensible struct[1]. This extensible struct
> is slightly misleading[2] as the ioctl number changes when we extend it.
> This breaks backwards compatibility with older kernels even if we're not
> asking for any fields that we do not need. In order to deal with this, the
> ioctl number would need to be dynamic, or the user would need to pass the
> size they're expecting, and we would need to implemented "extended syscall"
> semantics in ioctl. This potentially causes issue to future work of
> kernel-assisted copying for ioctl user buffers.

I don't see the issue. Can't you replace "switch (cmd)" with "switch
(cmd & ~IOCSIZE_MASK)" and then check the size separately?