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Re: local root on linux 2.2.15
From: mrwizard () PSU EDU (Joseph Gooch)
Date: Thu, 15 Jun 2000 16:41:27 -0400

-----Original Message-----
From: Bugtraq List [mailto:BUGTRAQ () SECURITYFOCUS COM]On
Behalf Of Peter
da Silva
Sent: Thursday, June 15, 2000 11:44 AM
Subject: Re: local root on linux 2.2.15

In article <87bt184i7z.fsf () arabella intern opera no> you write:
Always check the return value of system calls.  Always.  Always.
  cap_user_header_t header;
  cap_user_data_t data;
  header = malloc(8);
  data = malloc(12);
  header->pid = 0;
  header->version = _LINUX_CAPABILITY_VERSION;
  data->inheritable = data->effective = data->permitted = 0;

Two bugs here:

      1. If sizeof(cap_user_header_t) or sizeof(cap_user_data_t)
         increases, you'll get a buffer overflow in the malloc()ed
         data. This isn't as bad as a buffer overflow on stack,
         because it's almost impossible to exploit for anything but
         a DOS attack, but it's easy to avoid:

         header = malloc(sizeof (cap_user_header_t) );
         data = malloc(sizeof (cap_user_data_t) );

      2. Ironically, you're not checking the return value of a system
         call, namely brk() or sbrk() (or maybe mmap(), depending on
         how they're implementing malloc() in Lunix these
days). Before
         using header or data, check that malloc() succeeded.

         if(! (header = malloc(sizeof (cap_user_header_t) ) ) ) {
              perror("malloc: header");
              return or exit();
         if(! (data = malloc(sizeof (cap_user_data_t) ) ) ) {
              perror("malloc: data");
              return or exit();

  capset(header, data);

I don't have a recent Linux box to check, but isn't this a
system call?
If this fails, what happens? In the sample code, nothing
bad...  but if
you don't get in the habit of automatically writing robust code you're
going to be reading one of these alerts some day with your
name on it...
as the victim.

The proper way to do this would be to use libcap :)  I.e. the following

cap_t cap_d;

cap_d = cap_init();
if (!cap_d) {perror("Malloc failed"); exit(-1); }
if (cap_set_proc(cap_d)) {perror("capset failed"); exit(-1); }

Of course either code is acceptible, the libcap code is just more robust.
The proper checks could be added to his model just as easily.

Joe Gooch

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