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

Christian Brauner
2021-10-12T10:32:50Z

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:

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)
>
>