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

Casey Schaufler
2022-01-25T23:30:57Z

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.

>
>>> 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;
>>>> }