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

Casey Schaufler
2021-10-12T14:28:01Z

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.

>
> 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;
>> }
>>
>> int security_sb_alloc(struct super_block *sb)
>>
>>