Re: [PATCH] LSM: general protection fault in legacy_parse_param

Paul Moore
2022-01-25T23:36:30Z

On Tue, Jan 25, 2022 at 6:30 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
>
> On 1/25/2022 2:18 PM, Paul Moore wrote:
> > On Tue, Oct 12, 2021 at 10:27 AM Casey Schaufler <casey@schaufler-ca.com> wrote:
> >> On 10/12/2021 3:32 AM, Christian Brauner wrote:
> >>> On Mon, Oct 11, 2021 at 03:40:22PM -0700, Casey Schaufler wrote:
> >>>> The usual LSM hook "bail on fail" scheme doesn't work for cases where
> >>>> a security module may return an error code indicating that it does not
> >>>> recognize an input. In this particular case Smack sees a mount option
> >>>> that it recognizes, and returns 0. A call to a BPF hook follows, which
> >>>> returns -ENOPARAM, which confuses the caller because Smack has processed
> >>>> its data.
> >>>>
> >>>> Reported-by: syzbot+d1e3b1d92d25abf97943@syzkaller.appspotmail.com
> >>>> Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
> >>>> ---
> >>> Thanks!
> >>> Note, I think that we still have the SELinux issue we discussed in the
> >>> other thread:
> >>>
> >>> rc = selinux_add_opt(opt, param->string, &fc->security);
> >>> if (!rc) {
> >>> param->string = NULL;
> >>> rc = 1;
> >>> }
> >>>
> >>> SELinux returns 1 not the expected 0. Not sure if that got fixed or is
> >>> queued-up for -next. In any case, this here seems correct independent of
> >>> that:
> >> The aforementioned SELinux change depends on this patch. As the SELinux
> >> code is today it blocks the problem seen with Smack, but introduces a
> >> different issue. It prevents the BPF hook from being called.
> >>
> >> So the question becomes whether the SELinux change should be included
> >> here, or done separately. Without the security_fs_context_parse_param()
> >> change the selinux_fs_context_parse_param() change results in messy
> >> failures for SELinux mounts.
> > FWIW, this patch looks good to me, so:
> >
> > Acked-by: Paul Moore <paul@paul-moore.com>
> >
> > ... and with respect to the SELinux hook implementation returning 1 on
> > success, I don't have a good answer and looking through my inbox I see
> > David Howells hasn't responded either. I see nothing in the original
> > commit explaining why, so I'm going to say let's just change it to
> > zero and be done with it; the good news is that if we do it now we've
> > got almost a full cycle in linux-next to see what falls apart. As far
> > as the question of one vs two patches, it might be good to put both
> > changes into a single patch just so that folks who do backports don't
> > accidentally skip one and create a bad kernel build. Casey, did you
> > want to respin this patch or would you prefer me to submit another
> > version?
>
> I can create a single patch. I tried the combination on Fedora
> and it worked just fine. I'll rebase and resend.

Great, thank you.

> >>> Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
> >>>
> >>>> security/security.c | 14 +++++++++++++-
> >>>> 1 file changed, 13 insertions(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/security/security.c b/security/security.c
> >>>> index 09533cbb7221..3cf0faaf1c5b 100644
> >>>> --- a/security/security.c
> >>>> +++ b/security/security.c
> >>>> @@ -885,7 +885,19 @@ int security_fs_context_dup(struct fs_context *fc, struct fs_context *src_fc)
> >>>>
> >>>> int security_fs_context_parse_param(struct fs_context *fc, struct fs_parameter *param)
> >>>> {
> >>>> - return call_int_hook(fs_context_parse_param, -ENOPARAM, fc, param);
> >>>> + struct security_hook_list *hp;
> >>>> + int trc;
> >>>> + int rc = -ENOPARAM;
> >>>> +
> >>>> + hlist_for_each_entry(hp, &security_hook_heads.fs_context_parse_param,
> >>>> + list) {
> >>>> + trc = hp->hook.fs_context_parse_param(fc, param);
> >>>> + if (trc == 0)
> >>>> + rc = 0;
> >>>> + else if (trc != -ENOPARAM)
> >>>> + return trc;
> >>>> + }
> >>>> + return rc;
> >>>> }

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