Re: [PATCH 1/4] seccomp: add SECCOMP_RET_USER_NOTIF_ALLOW

[Christian Brauner <christian.brauner@ubuntu.com>]

On Wed, Sep 18, 2019 at 12:07:12PM -0600, Tycho Andersen wrote:
> On Wed, Sep 18, 2019 at 10:30:00AM -0700, Kees Cook wrote:
> > On Wed, Sep 18, 2019 at 10:48:30AM +0200, Christian Brauner wrote:
> > > This allows the seccomp notifier to continue a syscall. A positive
> > > discussion about this feature was triggered by a post to the
> > > ksummit-discuss mailing list (cf. [3]) and took place during KSummit
> > > (cf. [1]) and again at the containers/checkpoint-restore
> > > micro-conference at Linux Plumbers.
> > > 
> > > Recently we landed seccomp support for SECCOMP_RET_USER_NOTIF (cf. [4])
> > > which enables a process (watchee) to retrieve an fd for its seccomp
> > > filter. This fd can then be handed to another (usually more privileged)
> > > process (watcher). The watcher will then be able to receive seccomp
> > > messages about the syscalls having been performed by the watchee.
> > > 
> > > This feature is heavily used in some userspace workloads. For example,
> > > it is currently used to intercept mknod() syscalls in user namespaces
> > > aka in containers.
> > > The mknod() syscall can be easily filtered based on dev_t. This allows
> > > us to only intercept a very specific subset of mknod() syscalls.
> > > Furthermore, mknod() is not possible in user namespaces toto coelo and
> > > so intercepting and denying syscalls that are not in the whitelist on
> > > accident is not a big deal. The watchee won't notice a difference.
> > > 
> > > In contrast to mknod(), a lot of other syscall we intercept (e.g.
> > > setxattr()) cannot be easily filtered like mknod() because they have
> > > pointer arguments. Additionally, some of them might actually succeed in
> > > user namespaces (e.g. setxattr() for all "user.*" xattrs). Since we
> > > currently cannot tell seccomp to continue from a user notifier we are
> > > stuck with performing all of the syscalls in lieu of the container. This
> > > is a huge security liability since it is extremely difficult to
> > > correctly assume all of the necessary privileges of the calling task
> > > such that the syscall can be successfully emulated without escaping
> > > other additional security restrictions (think missing CAP_MKNOD for
> > > mknod(), or MS_NODEV on a filesystem etc.). This can be solved by
> > > telling seccomp to resume the syscall.
> > > 
> > > One thing that came up in the discussion was the problem that another
> > > thread could change the memory after userspace has decided to let the
> > > syscall continue which is a well known TOCTOU with seccomp which is
> > > present in other ways already.
> > > The discussion showed that this feature is already very useful for any
> > > syscall without pointer arguments. For any accidentally intercepted
> > > non-pointer syscall it is safe to continue.
> > > For syscalls with pointer arguments there is a race but for any cautious
> > > userspace and the main usec cases the race doesn't matter. The notifier
> > > is intended to be used in a scenario where a more privileged watcher
> > > supervises the syscalls of lesser privileged watchee to allow it to get
> > > around kernel-enforced limitations by performing the syscall for it
> > > whenever deemed save by the watcher. Hence, if a user tricks the watcher
> > > into allowing a syscall they will either get a deny based on
> > > kernel-enforced restrictions later or they will have changed the
> > > arguments in such a way that they manage to perform a syscall with
> > > arguments that they would've been allowed to do anyway.
> > > In general, it is good to point out again, that the notifier fd was not
> > > intended to allow userspace to implement a security policy but rather to
> > > work around kernel security mechanisms in cases where the watcher knows
> > > that a given action is safe to perform.
> > > 
> > > /* References */
> > > [1]: https://linuxplumbersconf.org/event/4/contributions/560
> > > [2]: https://linuxplumbersconf.org/event/4/contributions/477
> > > [3]: https://lore.kernel.org/r/20190719093538.dhyopljyr5ns33qx@brauner.io
> > > [4]: commit 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
> > > 
> > > Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> > > Cc: Kees Cook <keescook@chromium.org>
> > > Cc: Andy Lutomirski <luto@amacapital.net>
> > > Cc: Will Drewry <wad@chromium.org>
> > > Cc: Tycho Andersen <tycho@tycho.ws>
> > > CC: Tyler Hicks <tyhicks@canonical.com>
> > > Cc: Jann Horn <jannh@google.com>
> > > ---
> > >  include/uapi/linux/seccomp.h |  2 ++
> > >  kernel/seccomp.c             | 24 ++++++++++++++++++++----
> > >  2 files changed, 22 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> > > index 90734aa5aa36..2c23b9aa6383 100644
> > > --- a/include/uapi/linux/seccomp.h
> > > +++ b/include/uapi/linux/seccomp.h
> > > @@ -76,6 +76,8 @@ struct seccomp_notif {
> > >  	struct seccomp_data data;
> > >  };
> > >  
> > > +#define SECCOMP_RET_USER_NOTIF_ALLOW 0x00000001
> > 
> > nit: I'd like to avoid confusion here about what "family" these flags
> > belong to. "SECCOMP_RET_..." is used for the cBPF filter return action
> > value, so let's instead call this:
> > 
> > #define SECCOMP_USER_NOTIF_CONTINUE	BIT(0)
> 
> +1, I was thinking maybe even SECCOMP_USER_NOTIF_FLAG_CONTINUE.

I'll flip a coin between yours and Kees suggestion. :)

> 
> But the whole series (minus the patch that already exists) looks good
> to me if we make this change:
> 
> Reviewed-by: Tycho Andersen <tycho@tycho.ws>

Thanks for the review! :)
Christian