linux-kernel - Re: Bug report about KASLR and ZONE

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Date:   Thu, 12 Jul 2018 09:19:54 +0800
From:   Chao Fan <fanc.fnst@...fujitsu.com>
To:     Baoquan He <bhe@...hat.com>
CC:     <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
        <yasu.isimatu@...il.com>, <keescook@...omium.org>,
        <indou.takao@...fujitsu.com>, <caoj.fnst@...fujitsu.com>,
        <douly.fnst@...fujitsu.com>, <mhocko@...e.com>, <vbabka@...e.cz>,
        <mgorman@...hsingularity.net>, <fanc.fnst@...fujitsu.com>
Subject: Re: Bug report about KASLR and ZONE_MOVABLE

On Wed, Jul 11, 2018 at 08:40:08PM +0800, Baoquan He wrote:
>Please try this v3 patch:
>
>>From 9850d3de9c02e570dc7572069a9749a8add4c4c7 Mon Sep 17 00:00:00 2001
>From: Baoquan He <bhe@...hat.com>
>Date: Wed, 11 Jul 2018 20:31:51 +0800
>Subject: [PATCH v3] mm, page_alloc: find movable zone after kernel text
>
>In find_zone_movable_pfns_for_nodes(), when try to find the starting
>PFN movable zone begins in each node, kernel text position is not
>considered. KASLR may put kernel after which movable zone begins.
>
>Fix it by finding movable zone after kernel text on that node.
>
>Signed-off-by: Baoquan He <bhe@...hat.com>
>---
> mm/page_alloc.c | 20 +++++++++++++++-----
> 1 file changed, 15 insertions(+), 5 deletions(-)
>
>diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>index 1521100..390eb35 100644
>--- a/mm/page_alloc.c
>+++ b/mm/page_alloc.c
>@@ -6547,7 +6547,7 @@ static unsigned long __init early_calculate_totalpages(void)
> static void __init find_zone_movable_pfns_for_nodes(void)
> {
> 	int i, nid;
>-	unsigned long usable_startpfn;
>+	unsigned long usable_startpfn, real_startpfn;
> 	unsigned long kernelcore_node, kernelcore_remaining;
> 	/* save the state before borrow the nodemask */
> 	nodemask_t saved_node_state = node_states[N_MEMORY];
>@@ -6681,10 +6681,20 @@ static void __init find_zone_movable_pfns_for_nodes(void)
> 			if (start_pfn >= end_pfn)
> 				continue;

Hi Baoquan,

Thanks for your quick reply and PATCH.
I think it can work well after reviewing the code. But I think the new
variable 'real_startpfn' is unnecessary. How about this:

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6d00f746c2fd..0fc9c4283947 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6492,6 +6492,10 @@ static void __init find_zone_movable_pfns_for_nodes(void)
                        if (start_pfn >= end_pfn)
                                continue;

+                       if (pfn_to_nid(PFN_UP(_etext)) == i)
+                               usable_startpfn = max(usable_startpfn,
+                                                 PFN_UP(_etext));
+
                        /* Account for what is only usable for kernelcore */
                        if (start_pfn < usable_startpfn) {
                                unsigned long kernel_pages;

I think the logic of these two method are the same, and this method
change less code. If I am wrong, please let me know.

Thanks,
Chao Fan

> 
>+			/*
>+			 * KASLR may put kernel near tail of node memory,
>+			 * start after kernel on that node to find PFN
>+			 * which zone begins.
>+			 */
>+			if (pfn_to_nid(PFN_UP(_etext)) == i)
>+				real_startpfn = max(usable_startpfn,
>+						PFN_UP(_etext))
>+			else
>+				real_startpfn = usable_startpfn;
> 			/* Account for what is only usable for kernelcore */
>-			if (start_pfn < usable_startpfn) {
>+			if (start_pfn < real_startpfn) {
> 				unsigned long kernel_pages;
>-				kernel_pages = min(end_pfn, usable_startpfn)
>+				kernel_pages = min(end_pfn, real_startpfn)
> 								- start_pfn;
> 
> 				kernelcore_remaining -= min(kernel_pages,
>@@ -6693,7 +6703,7 @@ static void __init find_zone_movable_pfns_for_nodes(void)
> 							required_kernelcore);
> 
> 				/* Continue if range is now fully accounted */
>-				if (end_pfn <= usable_startpfn) {
>+				if (end_pfn <= real_startpfn) {
> 
> 					/*
> 					 * Push zone_movable_pfn to the end so
>@@ -6704,7 +6714,7 @@ static void __init find_zone_movable_pfns_for_nodes(void)
> 					zone_movable_pfn[nid] = end_pfn;
> 					continue;
> 				}
>-				start_pfn = usable_startpfn;
>+				start_pfn = real_startpfn;
> 			}
> 
> 			/*
>-- 
>2.1.0
>
>
>