Question

I am using Linux 2.6.26 kernel version and I am trying to change the interrupt descriptor table using a kernel module. I am only trying to change the page fault table entry here. So I make a copy of the original IDT and make changes to the page fault table entry only. The objective of the ISR is to print out information of the page fault before calling the original Page fault handler. But the kernel just crashes once I load it with insmod i.e it specifically crashed with the “loadIDTR” function. With further debugging, I found out that by not changing any entry if I load the IDTR it works fine. I am out of ideas.

I have pasted the code below

#include <linux/module.h>   // for init_module() 
#include <linux/init.h>
#include <linux/mm.h>       // for get_free_page()
#include <linux/sched.h>
#include <linux/spinlock.h>

#define SUCCESS 0
#define PGFAULT_INT 0x0E

static char modname[] = "pgfaults";
static unsigned short oldidtr[3], newidtr[3];
static unsigned long long *oldidt, *newidt;
static unsigned long isr_orig, kpage;
static char *why[]={ "sra", "srp", "swa", "swp", "ura", "urp", "uwa", "uwp" };

unsigned long long gate_desc_orig,gate_desc_orig1;

static void my_intrept( unsigned long *tos )
{
    // stack-layout:
    // es,ds,edi,esi,ebp,esp,ebx,edx,ecx,eax,err,eip,cs,efl
    //  0  1   2   3   4   5   6   7   8   9  10  11 12  13  
    volatile unsigned long  vaddr;
    struct task_struct *task = current;
    unsigned long   err = tos[ 10 ];    
    unsigned long   eip = tos[ 11 ];
    static int  count = 0;
    int     exe, len = 0;
    char        msg[80]="";

    // get the faulting virtual address from register CR2
    asm(" mov %%cr2, %%eax ; movl %%eax, %0 " : "=m" (vaddr) ); 

    // construct the diagnostic message
    len += sprintf( msg+len, "#%-6d ", ++count );
    len += sprintf( msg+len, "%16s  ", task->comm );
    len += sprintf( msg+len, "pid=%-5d  ", task->pid );
    len += sprintf( msg+len, "CR2=%08X  ", (unsigned int) vaddr );
    len += sprintf( msg+len, "EIP=%08X  ", (unsigned int) eip );
    len += sprintf( msg+len, "%s ", why[ err ] );
    // note if an instruction-fetch caused the page-fault
    if ( vaddr == eip ) exe = 'x'; else exe = ' ';
    len += sprintf( msg+len, "%c ", exe );
    // print this diagnostic message to the kernel log
    printk( "<1>  %s \n", msg );
}




//----------  NEW PAGE-FAULT EXCEPTION-HANDLER  ---------//
asmlinkage void isr0x0E( void );
asm("   .text                       ");
asm("   .type   isr0x0E, @function          ");
asm("isr0x0E:                       ");
asm("   pushal                      ");
asm("   pushl   %ds                 "); 
asm("   pushl   %es                 ");
//
asm("   movl    %ss, %eax               "); 
asm("   movl    %eax, %ds               ");
asm("   movl    %eax, %es               ");
//
asm("   pushl   %esp                    "); 
asm("   call    my_intrept              ");
asm("   addl    $4, %esp                ");
//
asm("   popl    %es                 "); 
asm("   popl    %ds                 ");
asm("   popal                       ");
asm("   jmp *isr_orig               ");
//-------------------------------------------------------//



static void load_IDTR( void *regimage )
{
    asm(" lidt %0 " : : "m" (*(unsigned short*)regimage) );
}



int pgfault_init( void )
{
    int         i;
    unsigned long long gate_desc,gate_desc1,gate_desc2;

    spinlock_t lock =SPIN_LOCK_UNLOCKED;
    unsigned long flags;
    unsigned short selector1;

    // allocate a mapped kernel page for our new IDT
    kpage =__get_free_page( GFP_KERNEL);
    if ( !kpage ) return -ENOMEM;


    // initialize our other global variables

    asm(" sidt oldidtr ; sidt newidtr ");

    memcpy( newidtr+1, &kpage, sizeof( kpage ) );
    oldidt = (unsigned long long *)(*(unsigned long*)(oldidtr+1));
    newidt = (unsigned long long *)(*(unsigned long*)(newidtr+1));

    // extract and save entry-point to original page-pault handler
    gate_desc_orig = oldidt[ PGFAULT_INT ];
    gate_desc =gate_desc_orig & 0xFFFF00000000FFFF;

    gate_desc |= ( gate_desc >> 32 );
    isr_orig = (unsigned long)gate_desc;
    // initialize our new Interrupt Descriptor Table
    memcpy( newidt, oldidt, 256*sizeof( unsigned long long ) );

    gate_desc_orig1 = (unsigned long)isr0x0E;
    gate_desc = gate_desc_orig1 & 0x00000000FFFFFFFF;

    gate_desc = gate_desc | ( gate_desc << 32 );
    gate_desc1= 0xFFFF0000;
    gate_desc1= gate_desc1 << 32;
    gate_desc1= gate_desc1 | 0x0000FFFF;
    gate_desc = gate_desc & gate_desc1;
    gate_desc2= 0x0000EF00;
    gate_desc2= gate_desc2 <<32;
    gate_desc2= gate_desc2 | 0x00100000;
    gate_desc = gate_desc | gate_desc2; // trap-gate
    //Part which is most likely creating a fault when loading the idtr
    newidt[ PGFAULT_INT ] = gate_desc;
    //**********************************************
    // activate the new IDT

    spin_lock_irqsave(&lock,flags);
    load_IDTR( newidtr );
    spin_unlock_irqrestore(&lock,flags);

//  smp_call_function( load_IDTR, oldidtr, 1, 1 );
    return  SUCCESS;
}



void pgfault_exit( void )
{

    // reactivate the old IDT
    unsigned long flags;
    spinlock_t lock =SPIN_LOCK_UNLOCKED;
    spin_lock_irqsave(&lock,flags);
    load_IDTR( oldidtr );
    spin_unlock_irqrestore(&lock,flags);
//  smp_call_function( load_IDTR, oldidtr, 1, 1 );

    // release allocated kernel page 
    if ( kpage ) free_page( kpage );
}
EXPORT_SYMBOL_GPL(my_intrept);
MODULE_LICENSE("GPL"); 
module_init( pgfault_init);
module_exit( pgfault_exit);

Answer 1

Your segment selector in your trap gate descriptor appears to be hardcoded to 0x0010, when it should be __KERNEL_CS (which is 0x0060 in the 2.6.26 kernel sources I have).

By the way, this is pretty baroque:

gate_desc_orig1 = (unsigned long)isr0x0E;
gate_desc = gate_desc_orig1 & 0x00000000FFFFFFFF;

gate_desc = gate_desc | ( gate_desc << 32 );
gate_desc1= 0xFFFF0000;
gate_desc1= gate_desc1 << 32;
gate_desc1= gate_desc1 | 0x0000FFFF;
gate_desc = gate_desc & gate_desc1;
gate_desc2= 0x0000EF00;
gate_desc2= gate_desc2 <<32;
gate_desc2= gate_desc2 | 0x00100000;
gate_desc = gate_desc | gate_desc2; // trap-gate

You could simplify that down to (with the __KERNEL_CS fix):

gate_desc = (unsigned long long)isr0x0E * 0x100000001ULL;
gate_desc &= 0xFFFF00000000FFFFULL;
gate_desc |= 0x0000EF0000000000ULL; // trap-gate
gate_desc |= (unsigned long long)__KERNEL_CS << 16;