Re: [PATCH v2] KEYS: trusted: Remove redundant static calls usage

[Jarkko Sakkinen]

On Tue, 2023-10-10 at 16:19 +0200, Ahmad Fatoum wrote:
> Hello Jarkko,
> 
> On 10.10.23 15:49, Jarkko Sakkinen wrote:
> > On Tue, 2023-10-10 at 18:44 +0530, Sumit Garg wrote:
> > > On Tue, 10 Oct 2023 at 18:03, Jarkko Sakkinen <jarkko@xxxxxxxxxx> wrote:
> > > > 
> > > > On Fri, 2023-10-06 at 10:48 +0530, Sumit Garg wrote:
> > > > > Static calls invocations aren't well supported from module __init and
> > > > > __exit functions. Especially the static call from cleanup_trusted() led
> > > > > to a crash on x86 kernel with CONFIG_DEBUG_VIRTUAL=y.
> > > > > 
> > > > > However, the usage of static call invocations for trusted_key_init()
> > > > > and trusted_key_exit() don't add any value from either a performance or
> > > > > security perspective. Hence switch to use indirect function calls instead.
> > > > > 
> > > > > Note here that although it will fix the current crash report, ultimately
> > > > > the static call infrastructure should be fixed to either support its
> > > > > future usage from module __init and __exit functions or not.
> > > > > 
> > > > > Reported-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> > > > > Link: https://lore.kernel.org/lkml/ZRhKq6e5nF%2F4ZIV1@fedora/#t
> > > > > Fixes: 5d0682be3189 ("KEYS: trusted: Add generic trusted keys framework")
> > > > > Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
> > > > > ---
> > > > > 
> > > > > Changes in v2:
> > > > > - Polish commit message as per comments from Mimi
> > > > > 
> > > > >  security/keys/trusted-keys/trusted_core.c | 13 +++++--------
> > > > >  1 file changed, 5 insertions(+), 8 deletions(-)
> > > > > 
> > > > > diff --git a/security/keys/trusted-keys/trusted_core.c b/security/keys/trusted-keys/trusted_core.c
> > > > > index c6fc50d67214..85fb5c22529a 100644
> > > > > --- a/security/keys/trusted-keys/trusted_core.c
> > > > > +++ b/security/keys/trusted-keys/trusted_core.c
> > > > > @@ -44,13 +44,12 @@ static const struct trusted_key_source trusted_key_sources[] = {
> > > > >  #endif
> > > > >  };
> > > > > 
> > > > > -DEFINE_STATIC_CALL_NULL(trusted_key_init, *trusted_key_sources[0].ops->init);
> > > > >  DEFINE_STATIC_CALL_NULL(trusted_key_seal, *trusted_key_sources[0].ops->seal);
> > > > >  DEFINE_STATIC_CALL_NULL(trusted_key_unseal,
> > > > >                         *trusted_key_sources[0].ops->unseal);
> > > > >  DEFINE_STATIC_CALL_NULL(trusted_key_get_random,
> > > > >                         *trusted_key_sources[0].ops->get_random);
> > > > > -DEFINE_STATIC_CALL_NULL(trusted_key_exit, *trusted_key_sources[0].ops->exit);
> > > > > +static void (*trusted_key_exit)(void);
> > > > >  static unsigned char migratable;
> > > > > 
> > > > >  enum {
> > > > > @@ -359,19 +358,16 @@ static int __init init_trusted(void)
> > > > >                 if (!get_random)
> > > > >                         get_random = kernel_get_random;
> > > > > 
> > > > > -               static_call_update(trusted_key_init,
> > > > > -                                  trusted_key_sources[i].ops->init);
> > > > >                 static_call_update(trusted_key_seal,
> > > > >                                    trusted_key_sources[i].ops->seal);
> > > > >                 static_call_update(trusted_key_unseal,
> > > > >                                    trusted_key_sources[i].ops->unseal);
> > > > >                 static_call_update(trusted_key_get_random,
> > > > >                                    get_random);
> > > > > -               static_call_update(trusted_key_exit,
> > > > > -                                  trusted_key_sources[i].ops->exit);
> > > > > +               trusted_key_exit = trusted_key_sources[i].ops->exit;
> > > > >                 migratable = trusted_key_sources[i].ops->migratable;
> > > > > 
> > > > > -               ret = static_call(trusted_key_init)();
> > > > > +               ret = trusted_key_sources[i].ops->init();
> > > > >                 if (!ret)
> > > > >                         break;
> > > > >         }
> > > > > @@ -388,7 +384,8 @@ static int __init init_trusted(void)
> > > > > 
> > > > >  static void __exit cleanup_trusted(void)
> > > > >  {
> > > > > -       static_call_cond(trusted_key_exit)();
> > > > > +       if (trusted_key_exit)
> > > > > +               (*trusted_key_exit)();
> > > > >  }
> > > > > 
> > > > >  late_initcall(init_trusted);
> > > > 
> > > > Would it be less confusing to require trusted_key_exit from each?
> > > > 
> > > 
> > > It is already required for each trust source to provide exit callback
> > > but this NULL check was added via this fix [1] in case there isn't any
> > > trust source present.
> > > 
> > > [1] https://lkml.kernel.org/stable/20220126184155.220814-1-dave.kleikamp@xxxxxxxxxx/
> > 
> > I'd considering creating a placeholder trusted_key_default_exit() with
> > perhaps pr_debug() statement acknowledging it getting called.
> > 
> > Hmm.. if we had that I wonder if we could get away with __weak... Then
> > you would not need to assign anything. This is not through-out analyzed.
> > Tbh I'm not sure how module loader handles this type of scenario but
> > at least the placeholder function would make sense in any case.
> 
> If you define a default exit function as __weak and expect trusted key sources
> to override it, you can only have one trust source at most in the compiled
> kernel and no boot-time selection would be possible.

Right, got it, thank you.

So, I still would consider trusted_key_default_exit() and assign that in the
declaration to trusted_exit.

BR, Jarkko