[patch 10/39] x86, vm86: fix preemption bug

[Greg KH]

2.6.28-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

commit be716615fe596ee117292dc615e95f707fb67fd1 upstream.

Commit 3d2a71a596bd9c761c8487a2178e95f8a61da083 ("x86, traps: converge
do_debug handlers") changed the preemption disable logic of do_debug()
so vm86_handle_trap() is called with preemption disabled resulting in:

 BUG: sleeping function called from invalid context at include/linux/kernel.h:155
 in_atomic(): 1, irqs_disabled(): 0, pid: 3005, name: dosemu.bin
 Pid: 3005, comm: dosemu.bin Tainted: G        W  2.6.29-rc1 #51
 Call Trace:
  [<c050d669>] copy_to_user+0x33/0x108
  [<c04181f4>] save_v86_state+0x65/0x149
  [<c0418531>] handle_vm86_trap+0x20/0x8f
  [<c064e345>] do_debug+0x15b/0x1a4
  [<c064df1f>] debug_stack_correct+0x27/0x2c
  [<c040365b>] sysenter_do_call+0x12/0x2f
 BUG: scheduling while atomic: dosemu.bin/3005/0x10000001

Restore the original calling convention and reenable preemption before
calling handle_vm86_trap().

Reported-by: Michal Suchanek <hramrach@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 arch/x86/kernel/traps.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -104,6 +104,12 @@ static inline void preempt_conditional_s
 		local_irq_enable();
 }
 
+static inline void conditional_cli(struct pt_regs *regs)
+{
+	if (regs->flags & X86_EFLAGS_IF)
+		local_irq_disable();
+}
+
 static inline void preempt_conditional_cli(struct pt_regs *regs)
 {
 	if (regs->flags & X86_EFLAGS_IF)
@@ -629,8 +635,10 @@ clear_dr7:
 
 #ifdef CONFIG_X86_32
 debug_vm86:
+	/* reenable preemption: handle_vm86_trap() might sleep */
+	dec_preempt_count();
 	handle_vm86_trap((struct kernel_vm86_regs *) regs, error_code, 1);
-	preempt_conditional_cli(regs);
+	conditional_cli(regs);
 	return;
 #endif
 

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