Re: [PATCH v2 1/3] seccomp: add SECCOMP_USER_NOTIF_FLAG_CONTINUE

On Thu, Oct 10, 2019 at 02:45:38PM -0700, Kees Cook wrote:

On Fri, Sep 20, 2019 at 10:30:05AM +0200, Christian Brauner wrote:

...

• • Similar precautions should be applied when stacking SECCOMP_RET_USER_NOTIF.
• • For SECCOMP_RET_USER_NOTIF filters acting on the same syscall the uppermost
• • filter takes precedence. This means that the uppermost
• • SECCOMP_RET_USER_NOTIF filter can override any SECCOMP_IOCTL_NOTIF_SEND from
• • lower filters essentially allowing all syscalls to pass by using
• • SECCOMP_USER_NOTIF_FLAG_CONTINUE. Note that SECCOMP_RET_USER_NOTIF can

                                                      ^^^^^^^^^^^^^^

This is meant to read RET_TRACE, yes?

Yes. :)

...

• • equally be overriden by SECCOMP_USER_NOTIF_FLAG_CONTINUE.

I rewrote this paragraph with that corrected and swapping some "upper/lower" to "most recently added" etc:

• • Similar precautions should be applied when stacking SECCOMP_RET_USER_NOTIF
• • or SECCOMP_RET_TRACE. For SECCOMP_RET_USER_NOTIF filters acting on the
• • same syscall, the most recently added filter takes precedence. This means
• • that the new SECCOMP_RET_USER_NOTIF filter can override any
• • SECCOMP_IOCTL_NOTIF_SEND from earlier filters, essentially allowing all
• • such filtered syscalls to be executed by sending the response
• • SECCOMP_USER_NOTIF_FLAG_CONTINUE. Note that SECCOMP_RET_TRACE can equally
• • be overriden by SECCOMP_USER_NOTIF_FLAG_CONTINUE.

Ultimately, I think this caveat is fine. RET_USER_NOTIF and RET_TRACE are both used from the "process manager" use-case. The benefits of "continue" semantics here outweighs the RET_USER_NOTIF and RET_TRACE "bypass". If we end up in a situation where we need to deal with some kind of nesting where this is a problem in practice, we can revisit this.

Applied to my for-next/seccomp. Thanks!

Thanks! Christian