[SSSD] [PATCH] krb5_child: Fix use after free

Jakub Hrozek jhrozek at redhat.com
Mon Apr 7 18:30:46 UTC 2014


On Mon, Apr 07, 2014 at 08:03:32PM +0200, Lukas Slebodnik wrote:
> On (07/04/14 18:53), Jakub Hrozek wrote:
> >On Fri, Apr 04, 2014 at 03:41:38PM +0200, Lukas Slebodnik wrote:
> >> On (04/04/14 15:18), Jakub Hrozek wrote:
> >> >On Thu, Apr 03, 2014 at 07:11:37PM +0200, Jakub Hrozek wrote:
> >> >> On Thu, Mar 20, 2014 at 05:53:31PM +0100, Lukas Slebodnik wrote:
> >> >> > On (20/03/14 17:21), Jakub Hrozek wrote:
> >> >> > >On Thu, Mar 20, 2014 at 05:00:00PM +0100, Sumit Bose wrote:
> >> >> > >> On Thu, Mar 20, 2014 at 04:20:59PM +0100, Lukas Slebodnik wrote:
> >> >> > >> > ehlo,
> >> >> > >> > 
> >> >> > >> > debug_prg_name is used in debug_fn and it was allocated under
> >> >> > >> > talloc context "kr". The variable "kr" was removed before the last debug
> >> >> > >> > messages in function main. It is very little change that it will be overridden.
> >> >> > >> > 
> >> >> > >> > It is possible to see this issue with exported environment variable
> >> >> > >> > TALLOC_FREE_FILL=255
> >> >> > >> 
> >> >> > >> I'm pretty sure the patch works as expected and fixes the isssue. But I
> >> >> > >> wonder if a different approach might be better? I think it does not make
> >> >> > >> sense to allocate debug_prg_name on a given talloc context but that it
> >> >> > >> would be better to just allocate it on NULL. This is e.g. done in the
> >> >> > >> ldap_child (here a talloc_free() is missing on exit but this would be a
> >> >> > >> different story).  Then debug_prg_name can even be allocate before kr
> >> >> > >> and the debug messages for a failed allocation of kr can use the right
> >> >> > >> program name and not "sssd".
> >> >> > >
> >> >> > >I agree, because also given that krb5_child is a short lived process,
> >> >> > >we don't care too much about possible leaks.
> >> >> > No problem.
> >> >> > 
> >> >> > New version attached.
> >> >> > 
> >> >> > LS
> >> >> 
> >> >> This version works for me, do you want to also add a talloc_free() on
> >> >> exit to be clean or do you also consider this not needed for short-lived
> >> >> processes?
> >> >
> >> >Thinking again, it would be nicer to explicitly free the string on child
> >> >exit. Yes, the leak it's not a big deal for a short-lived process but it
> >> >would read better.
> >> 
> >> With the first version of patch string was freed, because it was alocated
> >> under "kr" context.
> >> Now, you should decide which version do you want to push :-)
> >> 
> >> LS
> >
> >I think it's easier to explain with a patch :-)
> 
> You didn't test your patch :-)

I left that to you :-)

See another proposal below:

> 
> diff --git a/src/providers/krb5/krb5_child.c b/src/providers/krb5/krb5_child.c
> index 764f6ac7bf57b1f7d882961b7c6fa518818aaf23..aec0d9389dd4f3ae005b73ff12ca8293cee7560f 100644
> --- a/src/providers/krb5/krb5_child.c
> +++ b/src/providers/krb5/krb5_child.c
> @@ -2013,6 +2013,7 @@ int main(int argc, const char *argv[])
>          DEBUG(SSSDBG_CRIT_FAILURE, "talloc failed.\n");
>          exit(-1);
>      }
> +    talloc_steal(kr, debug_prg_name);
> 
>      if (debug_fd != -1) {
>          ret = set_debug_file_from_fd(debug_fd);
> 
> // snip
> done:
>     krb5_cleanup(kr);
>     talloc_free(kr);
>     ^^^^^^^^^^^^^^^
> //debug_prg_name is freed
> 
>     if (ret == EOK) {
>         DEBUG(SSSDBG_TRACE_FUNC, "krb5_child completed successfully\n");
>         ^^^^^^
> // use after free
          talloc_free(tmp_str);
>         exit(0);
>     } else {
>         DEBUG(SSSDBG_CRIT_FAILURE, "krb5_child failed!\n");
>         ^^^^^^
> // use after free
          talloc_free(tmp_str);
>         exit(-1);
>     }
> 

Or just:
 done:
     krb5_cleanup(kr);
     talloc_free(kr);
     if (ret == EOK) {
         DEBUG(SSSDBG_TRACE_FUNC, "krb5_child completed successfully\n");
         talloc_free(tmp_str);
         rv = 0;
     } else {
         DEBUG(SSSDBG_CRIT_FAILURE, "krb5_child failed!\n");
         talloc_free(tmp_str);
         rv = -1;
     }
     exit(rv);



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