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Subject[PATCH] x86: prevent unconditional writes to DebugCtl MSR
Otherwise, enabling (or better, subsequent disabling) of single
stepping would cause a kernel oops on CPUs not having this MSR.

The patch could have been added a conditional to the MSR write in
user_disable_single_step(), but centralizing the updates seems safer
and (looking forward) better manageable.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Markus Metzger <markus.t.metzger@intel.com>

---
arch/x86/kernel/kprobes.c | 4 ++--
arch/x86/kernel/process_32.c | 4 ++--
arch/x86/kernel/process_64.c | 4 ++--
arch/x86/kernel/step.c | 2 +-
include/asm-x86/processor.h | 9 +++++++++
5 files changed, 16 insertions(+), 7 deletions(-)

--- linux-2.6.25-rc5/arch/x86/kernel/kprobes.c 2008-03-10 13:24:10.000000000 +0100
+++ 2.6.25-rc5-x86-debugctlmsr/arch/x86/kernel/kprobes.c 2008-03-06 11:02:51.000000000 +0100
@@ -410,13 +410,13 @@ static void __kprobes set_current_kprobe
static void __kprobes clear_btf(void)
{
if (test_thread_flag(TIF_DEBUGCTLMSR))
- wrmsrl(MSR_IA32_DEBUGCTLMSR, 0);
+ update_debugctlmsr(0);
}

static void __kprobes restore_btf(void)
{
if (test_thread_flag(TIF_DEBUGCTLMSR))
- wrmsrl(MSR_IA32_DEBUGCTLMSR, current->thread.debugctlmsr);
+ update_debugctlmsr(current->thread.debugctlmsr);
}

static void __kprobes prepare_singlestep(struct kprobe *p, struct pt_regs *regs)
--- linux-2.6.25-rc5/arch/x86/kernel/process_32.c 2008-03-10 13:24:10.000000000 +0100
+++ 2.6.25-rc5-x86-debugctlmsr/arch/x86/kernel/process_32.c 2008-03-06 11:02:51.000000000 +0100
@@ -575,12 +575,12 @@ __switch_to_xtra(struct task_struct *pre
/* we clear debugctl to make sure DS
* is not in use when we change it */
debugctl = 0;
- wrmsrl(MSR_IA32_DEBUGCTLMSR, 0);
+ update_debugctlmsr(0);
wrmsr(MSR_IA32_DS_AREA, next->ds_area_msr, 0);
}

if (next->debugctlmsr != debugctl)
- wrmsr(MSR_IA32_DEBUGCTLMSR, next->debugctlmsr, 0);
+ update_debugctlmsr(next->debugctlmsr);

if (test_tsk_thread_flag(next_p, TIF_DEBUG)) {
set_debugreg(next->debugreg0, 0);
--- linux-2.6.25-rc5/arch/x86/kernel/process_64.c 2008-03-10 13:24:10.000000000 +0100
+++ 2.6.25-rc5-x86-debugctlmsr/arch/x86/kernel/process_64.c 2008-03-06 11:02:51.000000000 +0100
@@ -573,12 +573,12 @@ static inline void __switch_to_xtra(stru
/* we clear debugctl to make sure DS
* is not in use when we change it */
debugctl = 0;
- wrmsrl(MSR_IA32_DEBUGCTLMSR, 0);
+ update_debugctlmsr(0);
wrmsrl(MSR_IA32_DS_AREA, next->ds_area_msr);
}

if (next->debugctlmsr != debugctl)
- wrmsrl(MSR_IA32_DEBUGCTLMSR, next->debugctlmsr);
+ update_debugctlmsr(next->debugctlmsr);

if (test_tsk_thread_flag(next_p, TIF_DEBUG)) {
loaddebug(next, 0);
--- linux-2.6.25-rc5/arch/x86/kernel/step.c 2008-03-10 13:24:10.000000000 +0100
+++ 2.6.25-rc5-x86-debugctlmsr/arch/x86/kernel/step.c 2008-03-06 11:02:51.000000000 +0100
@@ -145,7 +145,7 @@ static void write_debugctlmsr(struct tas
if (child != current)
return;

- wrmsrl(MSR_IA32_DEBUGCTLMSR, val);
+ update_debugctlmsr(val);
}

/*
--- linux-2.6.25-rc5/include/asm-x86/processor.h 2008-03-10 13:24:33.000000000 +0100
+++ 2.6.25-rc5-x86-debugctlmsr/include/asm-x86/processor.h 2008-03-06 11:02:51.000000000 +0100
@@ -662,6 +662,15 @@ extern void switch_to_new_gdt(void);
extern void cpu_init(void);
extern void init_gdt(int cpu);

+static inline void update_debugctlmsr(unsigned long debugctlmsr)
+{
+#ifndef CONFIG_X86_DEBUGCTLMSR
+ if (boot_cpu_data.x86 < 6)
+ return;
+#endif
+ wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctlmsr);
+}
+
/* from system description table in BIOS. Mostly for MCA use, but
* others may find it useful. */
extern unsigned int machine_id;




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