linux-kernel - Re: [PATCH RFC v2 1/2] selinux: Fix selinux_sb_mnt_opts

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Date:   Tue, 25 Jan 2022 13:51:51 -0500
From:   Scott Mayhew <smayhew@...hat.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     selinux@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v2 1/2] selinux: Fix selinux_sb_mnt_opts_compat()

On Tue, 25 Jan 2022, Paul Moore wrote:

> On Tue, Jan 25, 2022 at 12:31 PM Scott Mayhew <smayhew@...hat.com> wrote:
> > On Mon, 24 Jan 2022, Paul Moore wrote:
> > > On Thu, Jan 20, 2022 at 4:50 PM Scott Mayhew <smayhew@...hat.com> wrote:
> > > >
> > > > selinux_sb_mnt_opts_compat() is called under the sb_lock spinlock and
> > > > shouldn't be performing any memory allocations.  Fix this by parsing the
> > > > sids at the same time we're chopping up the security mount options
> > > > string and then using the pre-parsed sids when doing the comparison.
> > > >
> > > > Fixes: cc274ae7763d ("selinux: fix sleeping function called from invalid context")
> > > > Fixes: 69c4a42d72eb ("lsm,selinux: add new hook to compare new mount to an existing mount")
> > > > Signed-off-by: Scott Mayhew <smayhew@...hat.com>
> > > > ---
> > > >  security/selinux/hooks.c | 112 ++++++++++++++++++++++++++-------------
> > > >  1 file changed, 76 insertions(+), 36 deletions(-)
> 
> ...
> 
> > > >         switch (token) {
> > > >         case Opt_context:
> > > >                 if (opts->context || opts->defcontext)
> > > >                         goto err;
> > > >                 opts->context = s;
> > > > +               if (preparse_sid) {
> > > > +                       rc = parse_sid(NULL, s, &sid);
> > > > +                       if (rc == 0) {
> > > > +                               opts->context_sid = sid;
> > > > +                               opts->preparsed |= CONTEXT_MNT;
> > > > +                       }
> > > > +               }
> > >
> > > Is there a reason why we need a dedicated sid variable as opposed to
> > > passing opt->context_sid as the parameter?  For example:
> > >
> > >   rc = parse_sid(NULL, s, &opts->context_sid);
> >
> > We don't need a dedicated sid variable.  Should I make similar changes
> > in the second patch (get rid of the local sid variable in
> > selinux_sb_remount() and the *context_sid variables in
> > selinux_set_mnt_opts())?
> 
> Yes please, I should have explicitly mentioned that.

Actually, delayed_superblock_init() calls selinux_set_mnt_opts() with
mnt_opts == NULL, so there would have to be a lot of checks like

        if (opts && opts->fscontext_sid) {

in the later parts of that function, which is kind of clunky.  I can
still do it if you want though.

-Scott

> 
> Thanks.
> 
> -- 
> paul moore
> paul-moore.com
>