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

[Paul Moore]

On Wed, Nov 16, 2022 at 2:04 PM KP Singh <kpsingh at kernel.org> wrote:
> On Wed, Nov 16, 2022 at 6:55 PM Alexei Starovoitov
> <alexei.starovoitov at gmail.com> wrote:
> >
> > 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 ?
>
> +1

I'm not sure we want to refactor the LSM hooks right now, we've got
too much stuff in-progress which I consider higher value/priority.
While I'm generally in favor of improving the sanity of interfaces,
I'd much rather we resolve the IMA/EVM special cases and land the
stacking changes before we start playing with refactoring the hooks.
I know this is a bummer for the BPF folks, but the IMA/EVM and
stacking patches affect everybody.

-- 
paul-moore.com