Re: [RFC PATCH 6/6] sysctl: introduce /proc/sys/kernel/modprobe_sysctl_alias

[Mauricio Faria de Oliveira <mfo@canonical.com>]

On Tue, Jul 26, 2022 at 6:24 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> On Fri, Jul 22, 2022 at 11:24 AM Mauricio Faria de Oliveira
> <mfo@canonical.com> wrote:
> >
> > The goal of the earlier patches is to let sysctl userspace tools
> > load the kernel module with a sysctl entry that is not available
> > yet in /proc/sys/ when the tool runs (so it can become available).
> >
> > Let's expose this file for userspace for two reasons:
> >
> > 1) Allow such tools to identify that the running kernel has the
> >    code which produces sysctl module aliases, so they could run
> >    'modprobe sysctl:<entry>' only when it may actually help.
> >
> > 2) Allow an administrator to hint such tools not to do that, if
> >    that is desired for some reason (e.g., rather have the tools
> >    fail if something is misconfigured in a critical deployment).
>
> This flag is just a hint.
> User-space tools are still able to ignore it.
>
> Perhaps, such administrator's choice might be specified in
> tools' configuration file.
>
> For example,
>
> /etc/modprobe.d/forbid-sysctl-alias.conf
>
> may specify
>
>     blacklist:  sysctl:*
>
> if they want to forbid sysctl aliasing.
> (but I do not know if this works or not).

Yes, it's just a hint. I considered this isn't strong enough, but
didn't think more into it.

Now, your idea with modprobe.d is strong enough. We have to change it a bit, as
only 'alias' supports wildcards per modprobe.d(5), then add 'install'
to make sure.

# cat /etc/modprobe.d/disable-sysctl-alias.conf
alias sysctl:* sysctl_alias_off
install sysctl_alias_off /bin/false
# or /bin/true, per the sysadmin.

# modprobe sysctl:nf_conntrack_max
modprobe: ERROR: ../libkmod/libkmod-module.c:990 command_do() Error
running install command '/bin/false' for module sysctl_alias_off:
retcode 1
modprobe: ERROR: could not insert 'sysctl_alias_off': Invalid argument

I'll document this in the commit message for now.

P.S.: Since the flag is a hint to userspace tools in sense 1) as well
(so they know not to run modprobe if sysctl aliases aren't expected),
the idea or the file itself seems worth keeping -- but maybe differently.

Thanks,


>
>
>
>
>
>
>
>
>
>
>
>
>
>
> > Also add a module parameter for that (proc.modprobe_sysctl_alias),
> > for another method that doesn't depend on sysctl tools to be set
> > (that wouldn't fail them to try and set it if it's not there yet).
> >
> > Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
> > ---
> >  fs/proc/proc_sysctl.c  | 8 ++++++++
> >  include/linux/module.h | 1 +
> >  kernel/sysctl.c        | 9 +++++++++
> >  3 files changed, 18 insertions(+)
> >
> > diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> > index ebbf8702387e..1e63819fcda8 100644
> > --- a/fs/proc/proc_sysctl.c
> > +++ b/fs/proc/proc_sysctl.c
> > @@ -33,6 +33,14 @@ static void check_struct_sysctl_device_id(void)
> >         BUILD_BUG_ON(offsetof(struct sysctl_device_id, procname)
> >                         != offsetof(struct ctl_table, procname));
> >  }
> > +
> > +/*
> > + * Hint sysctl userspace tools whether or not to run modprobe with sysctl alias
> > + * ('modprobe sysctl:entry') if they cannot find the file '/proc/sys/.../entry'
> > + */
> > +int modprobe_sysctl_alias = 1;
> > +module_param(modprobe_sysctl_alias, int, 0644);
> > +
> >  #else
> >  static void check_struct_sysctl_device_id(void) {}
> >  #endif
> > diff --git a/include/linux/module.h b/include/linux/module.h
> > index 3010f687df19..5f565491c596 100644
> > --- a/include/linux/module.h
> > +++ b/include/linux/module.h
> > @@ -304,6 +304,7 @@ struct notifier_block;
> >  #ifdef CONFIG_MODULES
> >
> >  extern int modules_disabled; /* for sysctl */
> > +extern int modprobe_sysctl_alias; /* for proc sysctl */
> >  /* Get/put a kernel symbol (calls must be symmetric) */
> >  void *__symbol_get(const char *symbol);
> >  void *__symbol_get_gpl(const char *symbol);
> > diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> > index 15073621cfa8..b396cfcb55fc 100644
> > --- a/kernel/sysctl.c
> > +++ b/kernel/sysctl.c
> > @@ -1763,6 +1763,15 @@ static struct ctl_table kern_table[] = {
> >                 .mode           = 0644,
> >                 .proc_handler   = proc_dostring,
> >         },
> > +#ifdef CONFIG_PROC_SYSCTL
> > +       {
> > +               .procname       = "modprobe_sysctl_alias",
> > +               .data           = &modprobe_sysctl_alias,
> > +               .maxlen         = sizeof(modprobe_sysctl_alias),
> > +               .mode           = 0644,
> > +               .proc_handler   = proc_dointvec,
> > +       },
> > +#endif
> >         {
> >                 .procname       = "modules_disabled",
> >                 .data           = &modules_disabled,
> > --
> > 2.25.1
> >
>
>
> --
> Best Regards
> Masahiro Yamada



--
Mauricio Faria de Oliveira