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From: Stas Sergeev <stssppnn@yahoo.com>
Reply-To: stas.orel@mailcity.com
To: linux-kernel@vger.kernel.org
Subject: Oops in 2.2.20, 100% reproduceable and fully tracked
Date: Tue, 11 Dec 2001 20:41:45 +0300
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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.

Thanks for any advice.
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