From: Claudio Jeker <cjeker@diehard.n-r-g.com>
Subject: Re: snmpd closefrom
To: Alexander Bluhm <bluhm@openbsd.org>
Cc: tech@openbsd.org
Date: Tue, 9 Apr 2024 14:20:19 +0200

On Tue, Apr 09, 2024 at 02:13:30PM +0200, Alexander Bluhm wrote:
> Hi,
> 
> fstat output shows that snmpd_metrics uses file descriptors 0, 1,
> 2 for regular communication.  This should not happen as any output
> to stderr would interfere with other data.
> 
> stdin, stdout, stderr are reserverd.  They should point to a terminal
> or /dev/null.  Redirects to other files is also fine.  But closing
> and then opening some files or sockets to 0, 1, 2 is not allowed.
> 
> The closefrom(1) in snmpd is the culprit.  With closefrom(4)
> descriptors 0, 1, 2 are /dev/null, 3 is a socketpair shared with
> the parent, and higher numbers are used for other files.
> 
> ok?

Why call closefrom() in the first place? If the code used O_CLOEXEC etc
there would be no need for that. snmpd_backend() is called once from
main() in the setup code so it feels strange to need closefrom() there.
 
> bluhm
> 
> Index: usr.sbin/snmpd/snmpd.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/usr.sbin/snmpd/snmpd.c,v
> diff -u -p -r1.51 snmpd.c
> --- usr.sbin/snmpd/snmpd.c	8 Apr 2024 13:18:54 -0000	1.51
> +++ usr.sbin/snmpd/snmpd.c	9 Apr 2024 11:56:08 -0000
> @@ -358,14 +358,10 @@ snmpd_backend(struct snmpd *env)
>  	}
>  	if (env->sc_flags & SNMPD_F_VERBOSE)
>  		argv[i++] = "-vv";
> -	if (env->sc_flags & SNMPD_F_DEBUG) {
> +	if (env->sc_flags & SNMPD_F_DEBUG)
>  		argv[i++] = "-d";
> -		argv[i++] = "-x";
> -		argv[i++] = "3";
> -	} else {
> -		argv[i++] = "-x";
> -		argv[i++] = "0";
> -	}
> +	argv[i++] = "-x";
> +	argv[i++] = "3";
>  	argv[i] = NULL;
>  	while ((file = readdir(dir)) != NULL) {
>  		if (file->d_name[0] == '.')
> @@ -377,10 +373,9 @@ snmpd_backend(struct snmpd *env)
>  			fatal("fork");
>  		case 0:
>  			close(pair[1]);
> -			if (dup2(pair[0],
> -			    env->sc_flags & SNMPD_F_DEBUG ? 3 : 0) == -1)
> +			if (dup2(pair[0], 3) == -1)
>  				fatal("dup2");
> -			if (closefrom(env->sc_flags & SNMPD_F_DEBUG ? 4 : 1) == -1)
> +			if (closefrom(4) == -1)
>  				fatal("closefrom");
>  			(void)snprintf(execpath, sizeof(execpath), "%s/%s",
>  			    SNMPD_BACKEND, file->d_name);
> 

-- 
:wq Claudio