[PoC][PATCH] bpf: Call return value check function in the JITed code

[Alexei Starovoitov]

On Wed, Nov 16, 2022 at 8:41 AM Roberto Sassu
<roberto.sassu at huaweicloud.com> wrote:
>
> On Wed, 2022-11-16 at 08:16 -0800, Alexei Starovoitov wrote:
> > On Wed, Nov 16, 2022 at 7:48 AM Roberto Sassu
> > <roberto.sassu at huaweicloud.com> wrote:
> > > +static bool is_ret_value_allowed(int ret, u32 ret_flags)
> > > +{
> > > +       if ((ret < 0 && !(ret_flags & LSM_RET_NEG)) ||
> > > +           (ret == 0 && !(ret_flags & LSM_RET_ZERO)) ||
> > > +           (ret == 1 && !(ret_flags & LSM_RET_ONE)) ||
> > > +           (ret > 1 && !(ret_flags & LSM_RET_GT_ONE)))
> > > +               return false;
> > > +
> > > +       return true;
> > > +}
> > > +
> > >  /* For every LSM hook that allows attachment of BPF programs, declare a nop
> > >   * function where a BPF program can be attached.
> > >   */
> > > @@ -30,6 +41,15 @@ noinline RET bpf_lsm_##NAME(__VA_ARGS__)     \
> > >  #include <linux/lsm_hook_defs.h>
> > >  #undef LSM_HOOK
> > >
> > > +#define LSM_HOOK(RET, DEFAULT, RET_FLAGS, NAME, ...)   \
> > > +noinline RET bpf_lsm_##NAME##_ret(int ret)     \
> > > +{                                              \
> > > +       return is_ret_value_allowed(ret, RET_FLAGS) ? ret : DEFAULT; \
> > > +}
> > > +
> > > +#include <linux/lsm_hook_defs.h>
> > > +#undef LSM_HOOK
> > > +
> >
> > because lsm hooks is mess of undocumented return values your
> > "solution" is to add hundreds of noninline functions
> > and hack the call into them in JITs ?!
>
> I revisited the documentation and checked each LSM hook one by one.
> Hopefully, I completed it correctly, but I would review again (others
> are also welcome to do it).
>
> Not sure if there is a more efficient way. Do you have any idea?
> Maybe we find a way to use only one check function (by reusing the
> address of the attachment point?).
>
> Regarding the JIT approach, I didn't find a reliable solution for using
> just the verifier. As I wrote to you, there could be the case where the
> range can include positive values, despite the possible return values
> are zero and -EACCES.

Didn't you find that there are only 12 or so odd return cases.
Maybe refactor some of them to something that the verifier can enforce
and denylist the rest ?

Also denylist those that Casey mentioned like security_secid_to_secctx ?