use of Safe module

iaw4 has asked for the wisdom of the Perl Monks concerning the following question:

Dear perlmonks---this is an indirect followup question to my need to write a safe mini interpreter. I want to figure out how module Safe works. the problem is that I want web user A to enter questions that web user B should answer. I need to avoid web user A doing mischief. This is easier to explain with an example:


#!/usr/bin/perl -w
use strict;
use Safe;

my $secret="my-password-should-never-be-shown\n";

## the following are lines that a web user could try to pass to my pro
+gram
my $_ = <<HEREDOC;
sub round { return sprintf("%.\$_[1]f", \$_[0]); } ; my \$in=rand(); m
+y \$answer=(\$in+log(10)); print "what is \$in+log(10)?\\n"; round(\$
+answer,3)  ## this should evaluate just fine
print \$secret;                                          ## yikes, sho
+uld be an error
system("ls")                                             ## yikes, sho
+uld be an error
`ls`                                                     ## yikes, sho
+uld be an error
open(FIN, ">", "really-bad-to-write-to-fs")              ## yikes, sho
+uld be an error
HEREDOC

foreach (split(/\n/, $_)) {
  print "\n----------------\nExecuting '$_'\n\n";

  # this executes everything blindly and is a really bad idea
  my $result=eval($_);
  print "UNSAFE '$_'\n\t\t-> ".($result||"undef")."\n\n\n";

  # this does not do what I had hoped it to do
  # I want the first line be executed, and all other lines to be trapp
+ed.
  my $compartment= new Safe;
  $result= $compartment->reval("$_");

  print "SAFE '$_'\n\t\t-> ".($result||"undef")."\n\n\n";
}
[download]

Unfortunately, my reval does not execute anything. is there a set of ops that are what I should open up to allow exactly and only what I used in the first line?

Advice appreciated.

Comment on use of Safe module Download Code

Replies are listed 'Best First'.
Re: use of Safe module by Anonymous Monk on Jan 02, 2012 at 20:47 UTC
Advice appreciated. Um, you never permit anything, did you see the synopsis ? #!/usr/bin/perl -- use strict; use warnings; use Safe; my $secret = "my-password-should-never-be-shown\n"; local $ENV{secret} = $secret ; my @commands = ( q{sub round { return sprintf( "%.$_[1]f", $_[0] ); } my $in = rand(); my $answer = ( $in + log(10) ); print "what is $in+log(10)?\n"; round( $answer, 3 ); ## this should evaluate just fine }, q{ print $secret; ## yikes, should be an error }, q{ $secret }, q{ $ENV{secret} }, q{ $$ }, q{ system("ls") ## yikes, should be an error }, q{ `ls` ## yikes, should be an error }, q{ open( FIN, ">", "really-bad-to-write-to-fs" ) ## yikes, shou +ld be an error }, ); for my $command ( @commands ){ print "\n#### code start \n$command\n#### code end\n"; { my $compartment = new Safe; #~ $compartment->permit(qw/ :base_core :base_mem :base_io /); + #no #~ $compartment->permit(qw/ :base_math sprintf print /); # yes $compartment->permit(qw/ :base_math :base_core :base_mem :base +_io /); # ues my $result = $compartment->reval( $command ); print " $compartment => ".($result\|\|"undef")."\n############\n +\n"; } } __END__ #### code start sub round { return sprintf( "%.$_[1]f", $_[0] ); } my $in = rand(); my $answer = ( $in + log(10) ); print "what is $in+log(10)?\n"; round( $answer, 3 ); ## this should evaluate just fine #### code end what is 0.5277099609375+log(10)? Safe=HASH(0x99a4ac) => 2.830 ############ #### code start print $secret; ## yikes, should be an error #### code end Safe=HASH(0x99a48c) => 1 ############ #### code start $secret #### code end Safe=HASH(0xa2ad64) => undef ############ #### code start $ENV{secret} #### code end Safe=HASH(0xb10754) => undef ############ #### code start $$ #### code end Safe=HASH(0xb16374) => undef ############ #### code start system("ls") ## yikes, should be an error #### code end Safe=HASH(0xb105f4) => undef ############ #### code start `ls` ## yikes, should be an error #### code end Safe=HASH(0xb10114) => undef ############ #### code start open( FIN, ">", "really-bad-to-write-to-fs" ) ## yikes, should be +an error #### code end Safe=HASH(0xa628f4) => undef ############ [download]	[reply] [d/l]
Re^2: use of Safe module by iaw4 (Monk) on Jan 02, 2012 at 21:50 UTC
thank you. this is exactly what I needed. I suspected something like this. the problem I had was that I did not see a list with explanation that showed and recommended what the set of ':permit'-table items would be/should be. your example solution is great. /iaw	[reply]
Re^3: use of Safe module by Anonymous Monk on Jan 02, 2012 at 23:51 UTC
Safe, Opcode#Predefined Opcode Tags	[reply]


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