Re: [PATCH v5 6/6] seccomp: add SECCOMP_EXT_ACT_TSYNC and SECCOMP_FILTER_TSYNC

[Andy Lutomirski]

On Thu, May 22, 2014 at 4:05 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> Applying restrictive seccomp filter programs to large or diverse
> codebases often requires handling threads which may be started early in
> the process lifetime (e.g., by code that is linked in). While it is
> possible to apply permissive programs prior to process start up, it is
> difficult to further restrict the kernel ABI to those threads after that
> point.
>
> This change adds a new seccomp extension action for synchronizing thread
> group seccomp filters and a prctl() for accessing that functionality,
> as well as a flag for SECCOMP_EXT_ACT_FILTER to perform sync at filter
> installation time.
>
> When calling prctl(PR_SECCOMP_EXT, SECCOMP_EXT_ACT, SECCOMP_EXT_ACT_FILTER,
> flags, filter) with flags containing SECCOMP_FILTER_TSYNC, or when calling
> prctl(PR_SECCOMP_EXT, SECCOMP_EXT_ACT, SECCOMP_EXT_ACT_TSYNC, 0, 0), it
> will attempt to synchronize all threads in current's threadgroup to its
> seccomp filter program. This is possible iff all threads are using a filter
> that is an ancestor to the filter current is attempting to synchronize to.
> NULL filters (where the task is running as SECCOMP_MODE_NONE) are also
> treated as ancestors allowing threads to be transitioned into
> SECCOMP_MODE_FILTER. If prctrl(PR_SET_NO_NEW_PRIVS, ...) has been set on the
> calling thread, no_new_privs will be set for all synchronized threads too.
> On success, 0 is returned. On failure, the pid of one of the failing threads
> will be returned, with as many filters installed as possible.

Is there a use case for adding a filter and synchronizing filters
being separate operations?  If not, I think this would be easier to
understand and to use if there was just a single operation.

If you did that, you'd have to decide whether to continue requiring
that all the other threads have a filter that's an ancestor of the
current thread's filter.

--Andy
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/