Re: [PATCH v3 seccomp 3/5] seccomp/cache: Lookup syscall allowlist for fast path

[Kees Cook <keescook@chromium.org>]

On Thu, Oct 08, 2020 at 07:17:39PM -0500, YiFei Zhu wrote:
> On Wed, Sep 30, 2020 at 4:32 PM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Wed, Sep 30, 2020 at 10:19:14AM -0500, YiFei Zhu wrote:
> > > From: YiFei Zhu <yifeifz2@illinois.edu>
> > >
> > > The fast (common) path for seccomp should be that the filter permits
> > > the syscall to pass through, and failing seccomp is expected to be
> > > an exceptional case; it is not expected for userspace to call a
> > > denylisted syscall over and over.
> > >
> > > This first finds the current allow bitmask by iterating through
> > > syscall_arches[] array and comparing it to the one in struct
> > > seccomp_data; this loop is expected to be unrolled. It then
> > > does a test_bit against the bitmask. If the bit is set, then
> > > there is no need to run the full filter; it returns
> > > SECCOMP_RET_ALLOW immediately.
> > >
> > > Co-developed-by: Dimitrios Skarlatos <dskarlat@cs.cmu.edu>
> > > Signed-off-by: Dimitrios Skarlatos <dskarlat@cs.cmu.edu>
> > > Signed-off-by: YiFei Zhu <yifeifz2@illinois.edu>
> >
> > I'd like the content/ordering of this and the emulator patch to be reorganized a bit.
> > I'd like to see the infrastructure of the cache added first (along with
> > the "always allow" test logic in this patch), with the emulator missing:
> > i.e. the patch is a logical no-op: no behavior changes because nothing
> > ever changes the cache bits, but all the operational logic, structure
> > changes, etc, is in place. Then the next patch would be replacing the
> > no-op with the emulator.
> >
> > > ---
> > >  kernel/seccomp.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 52 insertions(+)
> > >
> > > diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> > > index f09c9e74ae05..bed3b2a7f6c8 100644
> > > --- a/kernel/seccomp.c
> > > +++ b/kernel/seccomp.c
> > > @@ -172,6 +172,12 @@ struct seccomp_cache_filter_data { };
> > >  static inline void seccomp_cache_prepare(struct seccomp_filter *sfilter)
> > >  {
> > >  }
> > > +
> > > +static inline bool seccomp_cache_check(const struct seccomp_filter *sfilter,
> >
> > bikeshedding: "cache check" doesn't tell me anything about what it's
> > actually checking for. How about calling this seccomp_is_constant_allow() or
> > something that reflects both the "bool" return ("is") and what that bool
> > means ("should always be allowed").
> 
> We have a naming conflict here. I'm about to rename
> seccomp_emu_is_const_allow to seccomp_is_const_allow. Adding another
> seccomp_is_constant_allow is confusing. Suggestions?
> 
> I think I would prefer to change seccomp_cache_check to
> seccomp_cache_check_allow. While in this patch set seccomp_cache_check
> does imply the filter is "constant" allow, argument-processing cache
> may change this, and specifying an "allow" in the name specifies the
> 'what that bool means ("should always be allowed")'.

Yeah, that seems good.

-- 
Kees Cook