Oops in 2.2.20, 100% reproduceable and fully tracked

Stas Sergeev (stssppnn@yahoo.com)
Tue, 11 Dec 2001 20:41:45 +0300


Hello.

When I am trying to start one weird program under dosemu, I always get an Oops.
I have tracked this Oops down to a line of a source code in the kernel, see
below.

At first, the Oops itself:

---
ksymoops 2.4.3 on i686 2.2.20.  Options used
     -V (default)
     -k /proc/ksyms (default)
     -l /proc/modules (default)
     -o /lib/modules/2.2.20/ (default)
     -m /boot/System.map (specified)

Dec 2 21:04:05 localhost kernel: Unable to handle kernel paging request at virtual address 000078c3 Dec 2 21:04:05 localhost kernel: current->tss.cr3 = 0ad2f000, %cr3 = 0ad2f000 Dec 2 21:04:05 localhost kernel: *pde = 0b802067 Dec 2 21:04:05 localhost kernel: Oops: 0003 Dec 2 21:04:05 localhost kernel: CPU: 0 Dec 2 21:04:05 localhost kernel: EIP: 0010:[handle_vm86_fault+1149/2212] Dec 2 21:04:05 localhost kernel: EFLAGS: 00010206 Dec 2 21:04:05 localhost kernel: eax: 00000009 ebx: 00003013 ecx: 00000020 edx: 00000021 Dec 2 21:04:05 localhost kernel: esi: 000000f3 edi: 000077d0 ebp: 000c39e0 esp: cad31ea8 Dec 2 21:04:05 localhost kernel: ds: 0018 es: 0018 ss: 0018 Dec 2 21:04:05 localhost kernel: Process xdos (pid: 872, process nr: 57, stackpage=cad31000) Dec 2 21:04:05 localhost kernel: Stack: 000077d0 000000f4 00003013 000c39e0 00002827 cad31f00 00000000 c010a8cc Dec 2 21:04:05 localhost kernel: 00009fff 00000000 00000000 cad30000 cad30000 00002827 c010a90f cad31f00 Dec 2 21:04:05 localhost kernel: 00000000 cad30000 00000000 c010a05d cad31f00 00000000 00000001 00000005 Dec 2 21:04:05 localhost kernel: Call Trace: [do_general_protection+0/116] [do_general_protection+67/116] [error_code+53/64] [system_call+52/56] Dec 2 21:04:06 localhost kernel: Code: 88 7c 37 00 66 4e 88 5c 37 00 68 9d eb 1b c0 e8 3f 8b 00 00 Using defaults from ksymoops -t elf32-i386 -a i386

Code; 00000000 Before first symbol 00000000 <_EIP>: Code; 00000000 Before first symbol 0: 88 7c 37 00 mov %bh,0x0(%edi,%esi,1) Code; 00000004 Before first symbol 4: 66 4e dec %si Code; 00000006 Before first symbol 6: 88 5c 37 00 mov %bl,0x0(%edi,%esi,1) Code; 0000000a Before first symbol a: 68 9d eb 1b c0 push $0xc01beb9d Code; 0000000e Before first symbol f: e8 3f 8b 00 00 call 8b53 <_EIP+0x8b53> 00008b52 Before first symbol

---

The faulting code is here: arch/i386/kernel/vm86.c:341:

---
#define pushw(base, ptr, val) \
__asm__ __volatile__( \
	"decw %w0\n\t" \
	"movb %h2,0(%1,%0)\n\t" \  <-- fault is HERE!
	"decw %w0\n\t" \
	"movb %b2,0(%1,%0)" \
	: "=r" (ptr) \
	: "r" (base), "q" (val), "0" (ptr))
---

It is executed here, when it GPFs: vm86.c:509:

---
	case 0x9c:
		SP(regs) -= 2;
		IP(regs)++;
		pushw(ssp, sp, get_vflags(regs)); <--handle_vm86_fault+1149/2212
 		VM86_FAULT_RETURN;
---

The registers: ebx contains 0x3013. This is the value returned by get_vflags(regs). edi=0x77d0 and esi=0xf3 contains dosemu's ss and sp. So virtual flags are going to be moved to ss:sp to simulate pushf. This all seems OK to me and I don't understand why the code mov %bh,0x0(%edi,%esi,1) can raise GPF with such values in regs.

So, I have collected all the info I ever could from this Oops, but I still don't see a bug. I hope someone can help me. Any ideas such as how this code (atleast theoretically) can raise GPF? Any suggestions such as how can I collect more info on this subject?

I am pretty shure I can reproduce this Oops at any kernel, atleast at any 2.2.x, but currently I am using 2.2.20.

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