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

Paul Moore
2022-01-25T22:18:16Z

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?

> > 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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