Fwd from bugtraq: Linux kernels DoSable by file-max limit

hi all,

a couple of days ago Paul Starzetz pointed on bugtraq a problem in the linux kernel regarding a possible DoS:

struct file * get_empty_filp(void)
{
static int old_max = 0;
struct file * f;

  file_list_lock();
  if (files_stat.nr_free_files > NR_RESERVED_FILES) {
  used_one:
      f = list_entry(free_list.next, struct file, f_list); 

[...]

/*
* Use a reserved one if we're the superuser */
[*] if (files_stat.nr_free_files && !current->euid)

      goto used_one; 

if free fds are == NR_RESERVED_FILES only root can open new files .. but suid binaries are euid == 0 so an attacker can eat off this reserved fds. for more information see
http://online.securityfocus.com/archive/1/281100/2002-07-07/2002-07-13/0
and the other posts.

i attached two possible patches:

@@ -51,9 +51,12 @@

		return f;
	}
	/*
-	 * Use a reserved one if we're the superuser
+	 * Use one of the first 16 reserved fds if we have euid == 0
+	 * and one of the second 16 reserved fds if we're the superuser
	 */

- if (files_stat.nr_free_files && !current->euid)
+ if (files_stat.nr_free_files > (NR_RESERVED_FILES/2) && !current->euid)
+ goto used_one;
+ else if (files_stat.nr_free_files <= (NR_RESERVED_FILES/2) && !current->uid)

		goto used_one;
	/*
	 * Allocate a new one if we're below the limit. 

@@ -65,7 +65,7 @@

extern int leases_enable, dir_notify_enable, lease_break_time;

#define NR_FILE 8192 /* this can well be larger on a larger system */
-#define NR_RESERVED_FILES 10 /* reserved for root */
+#define NR_RESERVED_FILES 32 /* first 16 for euid == 0 processes and second 16 only for root */ #define NR_SUPER 256

#define MAY_EXEC 1

excuse me if another patch was submitted but i didnt see it

cheers, elv