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Date: Tue, 11 Sep 2018 15:30:57 +0800
From: Baoquan He <bhe@...hat.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: tglx@...utronix.de, hpa@...or.com, thgarnie@...gle.com,
kirill.shutemov@...ux.intel.com, x86@...nel.org,
linux-kernel@...r.kernel.org,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2 2/3] x86/mm/KASLR: Calculate the actual size of
vmemmap region
On 09/10/18 at 08:11am, Ingo Molnar wrote:
>
> * Baoquan He <bhe@...hat.com> wrote:
>
> > @@ -108,6 +109,14 @@ void __init kernel_randomize_memory(void)
> > if (memory_tb < kaslr_regions[0].size_tb)
> > kaslr_regions[0].size_tb = memory_tb;
> >
> > + /*
> > + * Calculate how many TB vmemmap region needs, and align to
> > + * 1TB boundary.
> > + * */
>
> Yeah, so that's not the standard comment style ...
Sorry for this, Will change. Thanks.
About clean up you suggested, I have made two patches and paste them at
below. Please help check if it's OK. Thanks a lot.
> So I get the part where the 'base' pointer is essentially pointers to various global variables
> used by the MM to get the virtual base address of the kernel, vmalloc and vmemmap areas from,
> which base addresses can thus be modified by the very early KASLR code to dynamically shape the
> virtual memory layout of these kernel memory areas on a per bootup basis.
>
> (BTW., that would be a great piece of information to add for the uninitiated. It's not like
> it's obvious!)
>
> But what does 'size_tb' do? Nothing explains it and your patch doesn't make it clearer either.
> Also, get_padding() looks like an unnecessary layer of obfuscation:
Yes, I agree. Have made an patch according to your suggestion, paste
it here:
>From c74b1e49eaa0e00335adbbb7e2d5df6d60518d4f Mon Sep 17 00:00:00 2001
From: Baoquan He <bhe@...hat.com>
Date: Tue, 11 Sep 2018 14:34:26 +0800
Subject: [PATCH] x86/mm/KASLR: Add code comments to explain fields of struct
kaslr_memory_region
Signed-off-by: Baoquan He <bhe@...hat.com>
Suggested-by: Ingo Molnar <mingo@...nel.org>
---
arch/x86/mm/kaslr.c | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)
diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
index 91053cee7648..402984d8d729 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -41,9 +41,21 @@
static const unsigned long vaddr_end = CPU_ENTRY_AREA_BASE;
/*
- * Memory regions randomized by KASLR (except modules that use a separate logic
- * earlier during boot). The list is ordered based on virtual addresses. This
- * order is kept after randomization.
+ * Memory regions randomized by KASLR (except modules that use a separate
+ * logic earlier during boot). Currently they are the physical memory
+ * mapping, vmalloc and vmemmap regions, are ordered based on virtual
+ * addresses. The order is kept after randomization.
+ *
+ * @base: points to various global variables used by the MM to get the
+ * virtual base address of the above regions, which base addresses can
+ * thus be modified by the very early KASLR code to dynamically shape
+ * the virtual memory layout of these kernel memory regions on a per
+ * bootup basis.
+ *
+ * @size_tb: size in TB of each memory region. Thereinto, the size of
+ * the physical memory mapping region is variable, calculated according
+ * to the actual size of system RAM in order to save more space for
+ * randomization. The rest are fixed values related to paging mode.
*/
static __initdata struct kaslr_memory_region {
unsigned long *base;
> /* Get size in bytes used by the memory region */
> static inline unsigned long get_padding(struct kaslr_memory_region *region)
> {
> return (region->size_tb << TB_SHIFT);
> }
Yes, we can open code get_padding() as the following patch.
>From e4cababd630af06085cb79a0bae9c00acd5272c0 Mon Sep 17 00:00:00 2001
From: Baoquan He <bhe@...hat.com>
Date: Tue, 11 Sep 2018 14:39:38 +0800
Subject: [PATCH] x86/mm/KASLR: Open code unnecessary function get_padding
It's used only twice and we do bit shifts in the parent function
anyway so it's not like it's hiding some uninteresting detail.
Signed-off-by: Baoquan He <bhe@...hat.com>
Suggested-by: Ingo Molnar <mingo@...nel.org>
---
arch/x86/mm/kaslr.c | 10 ++--------
1 file changed, 2 insertions(+), 8 deletions(-)
diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
index 9774b6e39f63..91053cee7648 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -54,12 +54,6 @@ static __initdata struct kaslr_memory_region {
{ &vmemmap_base, 0 },
};
-/* Get size in bytes used by the memory region */
-static inline unsigned long get_padding(struct kaslr_memory_region *region)
-{
- return (region->size_tb << TB_SHIFT);
-}
-
/*
* Apply no randomization if KASLR was disabled at boot or if KASAN
* is enabled. KASAN shadow mappings rely on regions being PGD aligned.
@@ -120,7 +114,7 @@ void __init kernel_randomize_memory(void)
/* Calculate entropy available between regions */
remain_entropy = vaddr_end - vaddr_start;
for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
- remain_entropy -= get_padding(&kaslr_regions[i]);
+ remain_entropy -= kaslr_regions[i].size_tb << TB_SHIFT;
prandom_seed_state(&rand_state, kaslr_get_random_long("Memory"));
@@ -144,7 +138,7 @@ void __init kernel_randomize_memory(void)
* Jump the region and add a minimum padding based on
* randomization alignment.
*/
- vaddr += get_padding(&kaslr_regions[i]);
+ vaddr += kaslr_regions[i].size_tb << TB_SHIFT;
if (pgtable_l5_enabled())
vaddr = round_up(vaddr + 1, P4D_SIZE);
else
--
2.13.6
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