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[v5 11/15] arm64/kasan: explicitly zero kasan shadow memory

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  [v5 11/15] arm64/kasan: explicitly zero kasan shadow memory Pavel Tatashin <pasha.tatashin@oracle.com> - 2017-08-03 23:30 +0200
    Re: [v5 11/15] arm64/kasan: explicitly zero kasan shadow memory Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-08-04 02:20 +0200
      Re: [v5 11/15] arm64/kasan: explicitly zero kasan shadow memory Pasha Tatashin <pasha.tatashin@oracle.com> - 2017-08-04 16:10 +0200

#1703425 — [v5 11/15] arm64/kasan: explicitly zero kasan shadow memory

To optimize the performance of struct page initialization,
vmemmap_populate() will no longer zero memory.

We must explicitly zero the memory that is allocated by vmemmap_populate()
for kasan, as this memory does not go through struct page initialization
path.

Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Reviewed-by: Steven Sistare <steven.sistare@oracle.com>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Reviewed-by: Bob Picco <bob.picco@oracle.com>
---
 arch/arm64/mm/kasan_init.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
index 81f03959a4ab..a57104bc54b8 100644
--- a/arch/arm64/mm/kasan_init.c
+++ b/arch/arm64/mm/kasan_init.c
@@ -135,6 +135,31 @@ static void __init clear_pgds(unsigned long start,
 		set_pgd(pgd_offset_k(start), __pgd(0));
 }
 
+/*
+ * Memory that was allocated by vmemmap_populate is not zeroed, so we must
+ * zero it here explicitly.
+ */
+static void
+zero_vemmap_populated_memory(void)
+{
+	struct memblock_region *reg;
+	u64 start, end;
+
+	for_each_memblock(memory, reg) {
+		start = __phys_to_virt(reg->base);
+		end = __phys_to_virt(reg->base + reg->size);
+
+		if (start >= end)
+			break;
+
+		memset((void *)start, 0, end - start);
+	}
+
+	start = (u64)kasan_mem_to_shadow(_stext);
+	end = (u64)kasan_mem_to_shadow(_end);
+	memset((void *)start, 0, end - start);
+}
+
 void __init kasan_init(void)
 {
 	u64 kimg_shadow_start, kimg_shadow_end;
@@ -205,6 +230,13 @@ void __init kasan_init(void)
 			pfn_pte(sym_to_pfn(kasan_zero_page), PAGE_KERNEL_RO));
 
 	memset(kasan_zero_page, 0, PAGE_SIZE);
+
+	/*
+	 * vmemmap_populate does not zero the memory, so we need to zero it
+	 * explicitly
+	 */
+	zero_vemmap_populated_memory();
+
 	cpu_replace_ttbr1(lm_alias(swapper_pg_dir));
 
 	/* At this point kasan is fully initialized. Enable error messages */
-- 
2.13.4

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#1703563

(+ arm64 maintainers)

Hi Pavel,

On 3 August 2017 at 22:23, Pavel Tatashin <pasha.tatashin@oracle.com> wrote:
> To optimize the performance of struct page initialization,
> vmemmap_populate() will no longer zero memory.
>
> We must explicitly zero the memory that is allocated by vmemmap_populate()
> for kasan, as this memory does not go through struct page initialization
> path.
>
> Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
> Reviewed-by: Steven Sistare <steven.sistare@oracle.com>
> Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
> Reviewed-by: Bob Picco <bob.picco@oracle.com>
> ---
>  arch/arm64/mm/kasan_init.c | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
>
> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
> index 81f03959a4ab..a57104bc54b8 100644
> --- a/arch/arm64/mm/kasan_init.c
> +++ b/arch/arm64/mm/kasan_init.c
> @@ -135,6 +135,31 @@ static void __init clear_pgds(unsigned long start,
>                 set_pgd(pgd_offset_k(start), __pgd(0));
>  }
>
> +/*
> + * Memory that was allocated by vmemmap_populate is not zeroed, so we must
> + * zero it here explicitly.
> + */
> +static void
> +zero_vemmap_populated_memory(void)

Typo here: vemmap -> vmemmap

> +{
> +       struct memblock_region *reg;
> +       u64 start, end;
> +
> +       for_each_memblock(memory, reg) {
> +               start = __phys_to_virt(reg->base);
> +               end = __phys_to_virt(reg->base + reg->size);
> +
> +               if (start >= end)

How would this ever be true? And why is it a stop condition?

> +                       break;
> +

Are you missing a couple of kasan_mem_to_shadow() calls here? I can't
believe your intention is to wipe all of DRAM.

> +               memset((void *)start, 0, end - start);
> +       }
> +
> +       start = (u64)kasan_mem_to_shadow(_stext);
> +       end = (u64)kasan_mem_to_shadow(_end);
> +       memset((void *)start, 0, end - start);
> +}
> +
>  void __init kasan_init(void)
>  {
>         u64 kimg_shadow_start, kimg_shadow_end;
> @@ -205,6 +230,13 @@ void __init kasan_init(void)
>                         pfn_pte(sym_to_pfn(kasan_zero_page), PAGE_KERNEL_RO));
>
>         memset(kasan_zero_page, 0, PAGE_SIZE);
> +
> +       /*
> +        * vmemmap_populate does not zero the memory, so we need to zero it
> +        * explicitly
> +        */
> +       zero_vemmap_populated_memory();
> +
>         cpu_replace_ttbr1(lm_alias(swapper_pg_dir));
>
>         /* At this point kasan is fully initialized. Enable error messages */
> --
> 2.13.4
>

KASAN uses vmemmap_populate as a convenience: kasan has nothing to do
with vmemmap, but the function already existed and happened to do what
KASAN requires.

Given that that will no longer be the case, it would be far better to
stop using vmemmap_populate altogether, and clone it into a KASAN
specific version (with an appropriate name) with the zeroing folded
into it.

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#1704014

Hi Ard,

Thank you very much for reviewing this. I will fix the bug you found in 
the next iteration.
>> +zero_vemmap_populated_memory(void)
> 
> Typo here: vemmap -> vmemmap

Yeap, will rename here, and in Intel variant.

> 
>> +{
>> +       struct memblock_region *reg;
>> +       u64 start, end;
>> +
>> +       for_each_memblock(memory, reg) {
>> +               start = __phys_to_virt(reg->base);
>> +               end = __phys_to_virt(reg->base + reg->size);
>> +
>> +               if (start >= end)
> How would this ever be true? And why is it a stop condition?

Yes this is a stop condition. Also look at the way kasan allocates its 
shadow memory in this file kasan_init():

187  	for_each_memblock(memory, reg) {
188  		void *start = (void *)__phys_to_virt(reg->base);
189  		void *end = (void *)__phys_to_virt(reg->base + reg->size);
190
191  		if (start >= end)
192  			break;
...
200  		vmemmap_populate(...)

>> +
> 
> Are you missing a couple of kasan_mem_to_shadow() calls here? I can't
> believe your intention is to wipe all of DRAM.

True. Thank you for catching this bug. I have not really tested on arm, 
only compiled for sanity checking. Need to figure out how to configure 
qemu to run most generic arm code. I tested on x86 and sparc both real 
and qemu hardware.

> 
> KASAN uses vmemmap_populate as a convenience: kasan has nothing to do
> with vmemmap, but the function already existed and happened to do what
> KASAN requires.
> 
> Given that that will no longer be the case, it would be far better to
> stop using vmemmap_populate altogether, and clone it into a KASAN
> specific version (with an appropriate name) with the zeroing folded
> into it.

I agree, but this would be outside of the scope of this project.

Pasha

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