Re: [PATCH v2] x86, mm: set NX across entire PMD at boot

From: Yinghai Lu
Date: Sun Nov 16 2014 - 23:00:24 EST

On Sun, Nov 16, 2014 at 3:44 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> _brk_end is adjusted at boot time via extend_brk() up to __brk_limit,
> which is the same as _end. We usually do not use all of that space. So
> it's expected that _brk_end < _end.
>> Is this correct? It sounded like tglx wanted the pmd split, like this:
> Yes, I wanted to get rid of the high mapping for anything between
> _brk_end and _end, and I brought you on the wrong track with my
> suggestion to call free_init_pages(). Sorry about that.


> That happened because I missed the completely non obvious fact, that
> only the effective brk section is reserved for the kernel via
> reserve_brk(). So the area between _brk_end and _end is already
> reusable. Though that reuse works only by chance and not by design and
> is completely undocumented as everything else in that area.

where is info for everything else?

> So the initial patch to get rid of the X mapping is of course to just
> extend the area to the PMD. A little bit different to your initial
> patch, but essentially the same.
> - unsigned long all_end = PFN_ALIGN(&_end);
> + unsigned long all_end = roundup((unsigned long) &_end, PMD_SIZE);
> I'm going to apply your V1 patch with the above roundup()
> simplification. If a page of that area gets used later on then we are
> going to split up the PMD anyway.

should use _brk_end instead of &_end for the roundup?

> But still we want to get rid of that highmap between _brk_end and
> _end, but there is absolutely no reason to come up with extra silly
> functions for that.
> So the obvious solution is to let setup_arch() reserve the memory up
> to _end instead of _bss_stop, get rid of the extra reservation in
> reserve_brk() and then let free_initmem() release the area between
> _brk_end and _end. No extra hackery, no side effects, just works.

So where get the highmap to be removed?
free_init_pages via free_initmem()?

with the code change like you suggested,

arch/x86/kernel/setup.c | 6 +-----
arch/x86/mm/init.c | 6 ++++++
arch/x86/mm/init_64.c | 2 +-
3 files changed, 8 insertions(+), 6 deletions(-)

Index: linux-2.6/arch/x86/kernel/setup.c
--- linux-2.6.orig/arch/x86/kernel/setup.c
+++ linux-2.6/arch/x86/kernel/setup.c
@@ -286,10 +286,6 @@ static void __init cleanup_highmap(void)

static void __init reserve_brk(void)
- if (_brk_end > _brk_start)
- memblock_reserve(__pa_symbol(_brk_start),
- _brk_end - _brk_start);
/* Mark brk area as locked down and no longer taking any
new allocations */
_brk_start = 0;
@@ -857,7 +853,7 @@ dump_kernel_offset(struct notifier_block
void __init setup_arch(char **cmdline_p)
- (unsigned long)__bss_stop - (unsigned long)_text);
+ (unsigned long)_end - (unsigned long)_text);


Index: linux-2.6/arch/x86/mm/init_64.c
--- linux-2.6.orig/arch/x86/mm/init_64.c
+++ linux-2.6/arch/x86/mm/init_64.c
@@ -1122,7 +1122,7 @@ void mark_rodata_ro(void)
unsigned long end = (unsigned long) &__end_rodata_hpage_align;
unsigned long text_end = PFN_ALIGN(&__stop___ex_table);
unsigned long rodata_end = PFN_ALIGN(&__end_rodata);
- unsigned long all_end = PFN_ALIGN(&_end);
+ unsigned long all_end = roundup((unsigned long)&_end, PMD_SIZE);

printk(KERN_INFO "Write protecting the kernel read-only data: %luk\n",
(end - start) >> 10);
Index: linux-2.6/arch/x86/mm/init.c
--- linux-2.6.orig/arch/x86/mm/init.c
+++ linux-2.6/arch/x86/mm/init.c
@@ -637,9 +637,15 @@ void free_init_pages(char *what, unsigne

void free_initmem(void)
+ unsigned long all_end = roundup((unsigned long)&_end, PMD_SIZE);
free_init_pages("unused kernel",
(unsigned long)(&__init_begin),
(unsigned long)(&__init_end));
+#ifdef CONFIG_X86_64
+ free_init_pages("unused brk", PFN_ALIGN(_brk_end), all_end);


[ 7.942636] Freeing unused brk memory: 500K (ffffffff84383000 -

---[ High Kernel Mapping ]---
0xffffffff80000000-0xffffffff81000000 16M pmd
0xffffffff81000000-0xffffffff82200000 18M ro PSE GLB x pmd
0xffffffff82200000-0xffffffff82c00000 10M ro PSE GLB NX pmd
0xffffffff82c00000-0xffffffff82e00000 2M RW PSE GLB NX pmd
0xffffffff82e00000-0xffffffff83000000 2M RW GLB NX pte
0xffffffff83000000-0xffffffff83200000 2M RW PSE GLB NX pmd
0xffffffff83200000-0xffffffff83400000 2M RW GLB NX pte
0xffffffff83400000-0xffffffff84200000 14M RW PSE GLB NX pmd
0xffffffff84200000-0xffffffff84400000 2M RW GLB NX pte
0xffffffff84400000-0xffffffffa0000000 444M pmd

the _brk_end to &_end highmap is still there

Also if you want to remove highmap between _brk_end and _end,
Do you want highmap for
[text_end, rodata_start)
[rodata_end, _sdata),
as mark_rodata_ro is calling

free_init_pages("unused kernel",
(unsigned long) __va(__pa_symbol(text_end)),
(unsigned long) __va(__pa_symbol(rodata_start)));
free_init_pages("unused kernel",
(unsigned long) __va(__pa_symbol(rodata_end)),
(unsigned long) __va(__pa_symbol(_sdata)));


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