Re: [PATCH] sysctl: fix proc_dobool() usability

[Ondrej Mosnacek <omosnace@redhat.com>]

On Tue, Feb 21, 2023 at 11:04 PM Luis Chamberlain <mcgrof@kernel.org> wrote:
> On Tue, Feb 21, 2023 at 09:34:49AM -0800, Kees Cook wrote:
> > On Fri, Feb 10, 2023 at 03:58:23PM +0100, Ondrej Mosnacek wrote:
> > > Currently proc_dobool expects a (bool *) in table->data, but sizeof(int)
> > > in table->maxsize, because it uses do_proc_dointvec() directly.
> > >
> > > This is unsafe for at least two reasons:
> > > 1. A sysctl table definition may use { .data = &variable, .maxsize =
> > >    sizeof(variable) }, not realizing that this makes the sysctl unusable
> > >    (see the Fixes: tag) and that they need to use the completely
> > >    counterintuitive sizeof(int) instead.
> > > 2. proc_dobool() will currently try to parse an array of values if given
> > >    .maxsize >= 2*sizeof(int), but will try to write values of type bool
> > >    by offsets of sizeof(int), so it will not work correctly with neither
> > >    an (int *) nor a (bool *). There is no .maxsize validation to prevent
> > >    this.
> > >
> > > Fix this by:
> > > 1. Constraining proc_dobool() to allow only one value and .maxsize ==
> > >    sizeof(bool).
> > > 2. Wrapping the original struct ctl_table in a temporary one with .data
> > >    pointing to a local int variable and .maxsize set to sizeof(int) and
> > >    passing this one to proc_dointvec(), converting the value to/from
> > >    bool as needed (using proc_dou8vec_minmax() as an example).
> > > 3. Extending sysctl_check_table() to enforce proc_dobool() expectations.
> > > 4. Fixing the proc_dobool() docstring (it was just copy-pasted from
> > >    proc_douintvec, apparently...).
> > > 5. Converting all existing proc_dobool() users to set .maxsize to
> > >    sizeof(bool) instead of sizeof(int).
> > >
> > > Fixes: 83efeeeb3d04 ("tty: Allow TIOCSTI to be disabled")
> > > Fixes: a2071573d634 ("sysctl: introduce new proc handler proc_dobool")
> > > Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
> >
> > Ah nice, thanks for tracking this down.
> >
> > Acked-by: Kees Cook <keescook@chromium.org>
>
> Queued onto sysctl-next, will send to Linus as this is a fix too.

Thanks, Luis!

-- 
Ondrej Mosnacek
Senior Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.