Re: [PATCH v2] LSM: general protection fault in legacy_parse_param

James Morris
2022-01-27T17:56:40Z

On Thu, 27 Jan 2022, 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.
>
> The SELinux hook incorrectly returns 1 on success. There was a time
> when this was correct, however the current expectation is that it
> return 0 on success. This is repaired.
>
> Reported-by: syzbot+d1e3b1d92d25abf97943@syzkaller.appspotmail.com
> Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

Acked-by: James Morris <jamorris@linux.microsoft.com>

> ---
> security/security.c | 17 +++++++++++++++--
> security/selinux/hooks.c | 5 ++---
> 2 files changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/security/security.c b/security/security.c
> index 3d4eb474f35b..e649c8691be2 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -884,9 +884,22 @@ int security_fs_context_dup(struct fs_context *fc, struct
> fs_context *src_fc)
> return call_int_hook(fs_context_dup, 0, fc, src_fc);
> }
>
> -int security_fs_context_parse_param(struct fs_context *fc, struct
> fs_parameter *param)
> +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)
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 5b6895e4fc29..371f67a37f9a 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2860,10 +2860,9 @@ static int selinux_fs_context_parse_param(struct
> fs_context *fc,
> return opt;
>
> rc = selinux_add_opt(opt, param->string, &fc->security);
> - if (!rc) {
> + if (!rc)
> param->string = NULL;
> - rc = 1;
> - }
> +
> return rc;
> }
>
>
>

--
James Morris
<jmorris@namei.org>