Re: [patch 43/60] x86/fixmap: Add debugstore entries to cpu_entry_area

[Borislav Petkov]

On Mon, Dec 04, 2017 at 03:07:49PM +0100, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 
> The intel PEBS/BTS debug store is a design trainwreck as is expects virtual
> addresses which must be visible in any execution context.

Sure, what can possibly go wrong?! :-\

> So it is required to make these mappings visible to user space when kernel
> page table isolation is active.
> 
> Provide enough room for the buffer mappings in the cpu_entry_area so the
> buffers are available in the user space visible fixmap.
> 
> At the point where the kernel side fixmap is populated there is no buffer
> available yet, but the kernel PMD must be populated. To achieve this set
> the fixmap entries for these buffers to non present.
> 
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  arch/x86/events/intel/ds.c      |    5 +++--
>  arch/x86/events/perf_event.h    |   21 ++-------------------
>  arch/x86/include/asm/fixmap.h   |   13 +++++++++++++
>  arch/x86/include/asm/intel_ds.h |   36 ++++++++++++++++++++++++++++++++++++
>  arch/x86/kernel/cpu/common.c    |   21 +++++++++++++++++++++
>  5 files changed, 75 insertions(+), 21 deletions(-)

...

> @@ -592,6 +603,16 @@ static void __init setup_cpu_entry_area(
>  	__set_fixmap(get_cpu_entry_area_index(cpu, entry_trampoline),
>  		     __pa_symbol(_entry_trampoline), PAGE_KERNEL_RX);
>  #endif
> +
> +#ifdef CONFIG_CPU_SUP_INTEL
> +	BUILD_BUG_ON(sizeof(struct debug_store) % PAGE_SIZE != 0);
> +	set_percpu_fixmap_pages(get_cpu_entry_area_index(cpu, cpu_debug_store),
> +				&per_cpu(cpu_debug_store, cpu),
> +				sizeof(struct debug_store) / PAGE_SIZE,
> +				PAGE_KERNEL);
> +	set_percpu_fixmap_ptes(get_cpu_entry_area_index(cpu, cpu_debug_buffers),
> +			       sizeof(struct debug_store_buffers) / PAGE_SIZE);
> +#endif

I guess we can do that additionally, so as not to setup the mappings on
distro kernels running !INTEL:

---
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 1364a8f378f8..5cfb68090a24 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -606,12 +606,16 @@ static void __init setup_cpu_entry_area(int cpu)
 
 #ifdef CONFIG_CPU_SUP_INTEL
 	BUILD_BUG_ON(sizeof(struct debug_store) % PAGE_SIZE != 0);
-	set_percpu_fixmap_pages(get_cpu_entry_area_index(cpu, cpu_debug_store),
-				&per_cpu(cpu_debug_store, cpu),
-				sizeof(struct debug_store) / PAGE_SIZE,
-				PAGE_KERNEL);
-	set_percpu_fixmap_ptes(get_cpu_entry_area_index(cpu, cpu_debug_buffers),
-			       sizeof(struct debug_store_buffers) / PAGE_SIZE);
+
+	if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL) {
+		set_percpu_fixmap_pages(get_cpu_entry_area_index(cpu, cpu_debug_store),
+					&per_cpu(cpu_debug_store, cpu),
+					sizeof(struct debug_store) / PAGE_SIZE,
+					PAGE_KERNEL);
+
+		set_percpu_fixmap_ptes(get_cpu_entry_area_index(cpu, cpu_debug_buffers),
+				       sizeof(struct debug_store_buffers) / PAGE_SIZE);
+	}
 #endif
 }

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix ImendÃrffer, Jane Smithard, Graham Norton, HRB 21284 (AG NÃrnberg)
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