Re: [PATCH] x86, vm86: fix preemption bug for int3 breakpointhandlers.

[Bart Oldeman]

Thomas,

On Mon, 25 May 2009, Thomas Gleixner wrote:
> On Sun, 24 May 2009, Bart Oldeman wrote:
> > --- a/arch/x86/kernel/vm86_32.c
> > +++ b/arch/x86/kernel/vm86_32.c
> > @@ -551,8 +551,12 @@ cannot_handle:
> >  int handle_vm86_trap(struct kernel_vm86_regs *regs, long error_code, int
> > trapno)
> >  {
> >  	if (VMPI.is_vm86pus) {
> > -		if ((trapno == 3) || (trapno == 1))
> > +		if ((trapno == 3) || (trapno == 1)) {
> > +			/* re-enable preemption: return_to_32bit()
> > +			   jumps straight to entry_32.S */
> > +			dec_preempt_count();
>
>  This will break other callers of handle_vm86_trap().

sorry for the late reply, I was just pointed out by someone else about the
existence of this bug again. You are right of course. Below is a new 
patch.

Note that on the linux-2.6-x86.git tree, commit 
08d68323d1f0c34452e614263b212ca556dae47f ("hw-breakpoints: modifying
generic debug exception to use thread-specific debug registers") broke 
vm86 debug exceptions as well again. The trouble is that 
handle_vm86_trap() may jump and change the stack to let the kernel return 
to 32 bit user space, so the handle_vm86_trap() call itself may not 
return.

--
Impact: fix kernel bug such as:
May 22 16:47:47 localhost kernel: note: dosemu.bin[5281] exited with preempt_count 1

Commit be716615fe596ee117292dc615e95f707fb67fd1 ("x86, vm86:
fix preemption bug"), fixed the problem for debug exceptions but not for 
breakpoints. This change also fixes breakpoints.

Cc: stable@xxxxxxxxxx
Signed-off-by: Bart Oldeman <bartoldeman@xxxxxxxxxxxxxxxxxxxxx>
---
 arch/x86/kernel/traps.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -181,9 +181,14 @@ kernel_trap:

 #ifdef CONFIG_X86_32
 vm86_trap:
+	/* reenable preemption: handle_vm86_trap() might sleep */
+	dec_preempt_count();
 	if (handle_vm86_trap((struct kernel_vm86_regs *) regs,
-						error_code, trapnr))
+						error_code, trapnr)) {
+		inc_preempt_count();
 		goto trap_signal;
+	}
+	inc_preempt_count();
 	return;
 #endif
 }
--
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