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Subject[22/82] x86, vm86: Fix preemption bug for int1 debug and int3 breakpoint handlers.
2.6.35-stable review patch.  If anyone has any objections, please let us know.


From: Bart Oldeman <>

commit 6554287b1de0448f1e02e200d02b43914e997d15 upstream.

Impact: fix kernel bug such as:
BUG: scheduling while atomic: dosemu.bin/19680/0x00000004
See also Ubuntu bug 455067 at

Commits 4915a35e35a037254550a2ba9f367a812bc37d40
("Use preempt_conditional_sti/cli in do_int3, like on x86_64.")
and 3d2a71a596bd9c761c8487a2178e95f8a61da083
("x86, traps: converge do_debug handlers")
started disabling preemption in int1 and int3 handlers on i386.
The problem with vm86 is that the call to handle_vm86_trap() may jump
straight to entry_32.S and never returns so preempt is never enabled
again, and there is an imbalance in the preempt count.

Commit be716615fe596ee117292dc615e95f707fb67fd1 ("x86, vm86:
fix preemption bug"), which was later (accidentally?) reverted by commit
08d68323d1f0c34452e614263b212ca556dae47f ("hw-breakpoints: modifying
generic debug exception to use thread-specific debug registers")
fixed the problem for debug exceptions but not for breakpoints.

There are three solutions to this problem.

1. Reenable preemption before calling handle_vm86_trap(). This
was the approach that was later reverted.

2. Do not disable preemption for i386 in breakpoint and debug handlers.
This was the situation before October 2008. As far as I understand
preemption only needs to be disabled on x86_64 because a seperate stack is
used, but it's nice to have things work the same way on
i386 and x86_64.

3. Let handle_vm86_trap() return instead of jumping to assembly code.
By setting a flag in _TIF_WORK_MASK, either TIF_IRET or TIF_NOTIFY_RESUME,
the code in entry_32.S is instructed to return to 32 bit mode from
V86 mode. The logic in entry_32.S was already present to handle signals.
(I chose TIF_IRET because it's slightly more efficient in
do_notify_resume() in signal.c, but in fact TIF_IRET can probably be
replaced by TIF_NOTIFY_RESUME everywhere.)

I'm submitting approach 3, because I believe it is the most elegant
and prevents future confusion. Still, an obvious
preempt_conditional_cli(regs); is necessary in traps.c to correct the

[ hpa: This is technically a regression, but because:
1. the regression is so old,
2. the patch seems relatively high risk, justifying more testing, and
3. we're late in the 2.6.36-rc cycle,

I'm queuing it up for the 2.6.37 merge window. It might, however,
justify as a -stable backport at a latter time, hence Cc: stable. ]

Signed-off-by: Bart Oldeman <>
LKML-Reference: <alpine.DEB.2.00.1009231312330.4732@localhost.localdomain>
Cc: Frederic Weisbecker <>
Cc: K.Prasad <>
Cc: Alan Stern <>
Cc: Alexander van Heukelum <>
Signed-off-by: H. Peter Anvin <>
Signed-off-by: Greg Kroah-Hartman <>

arch/x86/kernel/traps.c | 1 +
arch/x86/kernel/vm86_32.c | 10 ++++++++--
2 files changed, 9 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -568,6 +568,7 @@ dotraplinkage void __kprobes do_debug(st
if (regs->flags & X86_VM_MASK) {
handle_vm86_trap((struct kernel_vm86_regs *) regs,
error_code, 1);
+ preempt_conditional_cli(regs);

--- a/arch/x86/kernel/vm86_32.c
+++ b/arch/x86/kernel/vm86_32.c
@@ -551,8 +551,14 @@ 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))
- return_to_32bit(regs, VM86_TRAP + (trapno << 8));
+ if ((trapno == 3) || (trapno == 1)) {
+ KVM86->regs32->ax = VM86_TRAP + (trapno << 8);
+ /* setting this flag forces the code in entry_32.S to
+ call save_v86_state() and change the stack pointer
+ to KVM86->regs32 */
+ set_thread_flag(TIF_IRET);
+ return 0;
+ }
do_int(regs, trapno, (unsigned char __user *) (regs-> << 4), SP(regs));
return 0;

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