[PATCH v2] LSM: general protection fault in legacy_parse_param

[James Morris]

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 at syzkaller.appspotmail.com
> Signed-off-by: Casey Schaufler <casey at schaufler-ca.com>


Acked-by: James Morris <jamorris at 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 at namei.org>