Re: [9front] git: use new /dev/drivers for privdrop

[unobe@cpan.org]

In general, how does one restrict computation/process limitations?  I
am admittedly ignorant, so if anyone has docs for me to read instead
of replying that what I want to do is already solved or is a waste of
time I'd appreciate it.

I see proc(3) describes each process's status file as containing the
user, amount of memory, and scheduling priority.  So for unnecessary
churn and limiting fork bombs, is the idea to have a monitor that
watches the interested values (e.g.  memory or number of processes)
for a user, and kill if a certain amount has been reached?

For example, say I want a sandboxing area for people to "try 9front".
With moody's recent work, it removes a big attack vector by
restricting certain drivers.  But isn't it still possible to fork-bomb
a server, or to just cause unnecessary churn (i.e., computation), or
just open too many files, or fill a disk?

To address the "fill a disk" scenario, I assume the best thing is to
create a temporary disk that is used by the process which is provided
to the test user--they'll only have so much space to work with.  If
there's no space to allocate for the new temporary disk, it's easy
enough to stop the process from starting.  Disk-resizing would be
neat, but I don't care about that to start.

Quoth ori@eigenstate.org:
> This needs to be held off for a bit, but it seems
> like a good idea: we shouldn't need any devices at
> all for this code, so let's not keep them.
> 
> ddiff 01a6de812c2fd38c9a28036300e3c2c7dc8441f5 uncommitted
> --- a/sys/src/cmd/git/serve.c
> +++ b/sys/src/cmd/git/serve.c
> @@ -5,7 +5,7 @@
>  
>  #include "git.h"
>  
> -char	*pathpfx = nil;
> +char	*pathpfx = "/";
>  int	allowwrite;
>  
>  int
> @@ -469,6 +469,22 @@
>  	return 0;
>  }
>  
> +void
> +privdrop(void)
> +{
> +	int fd;
> +
> +	if(rfork(RFNAMEG) == -1)
> +		sysfatal("rfork: %r");
> +	if((fd = open("#c/drivers", OWRITE)) == -1)
> +		sysfatal("open drivers: %r");
> +	if(bind(pathpfx, "/", MREPL) == -1)
> +		sysfatal("bind: %r");
> +	if(write(fd, "chdev &", strlen("chdev &")) == -1)
> +		sysfatal("drop permissions: %r");
> +	close(fd);
> +}
> +
>  char*
>  parsecmd(char *buf, char *cmd, int ncmd)
>  {
> @@ -518,15 +534,8 @@
>  	}ARGEND;
>  
>  	gitinit();
> +	privdrop();
>  	interactive = 0;
> -	if(rfork(RFNAMEG) == -1)
> -		sysfatal("rfork: %r");
> -	if(pathpfx != nil){
> -		if(bind(pathpfx, "/", MREPL) == -1)
> -			sysfatal("bind: %r");
> -	}
> -	if(rfork(RFNOMNT) == -1)
> -		sysfatal("rfork: %r");
>  
>  	initconn(&c, 0, 1);
>  	if(readpkt(&c, buf, sizeof(buf)) == -1)
>