linux-kernel - [PATCH V3 2/4] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump

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Message-Id: <1557824407-19092-3-git-send-email-anshuman.khandual@arm.com>
Date:   Tue, 14 May 2019 14:30:05 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        akpm@...ux-foundation.org, catalin.marinas@....com,
        will.deacon@....com
Cc:     mhocko@...e.com, mgorman@...hsingularity.net, james.morse@....com,
        mark.rutland@....com, robin.murphy@....com, cpandya@...eaurora.org,
        arunks@...eaurora.org, dan.j.williams@...el.com, osalvador@...e.de,
        david@...hat.com, cai@....pw, logang@...tatee.com,
        ira.weiny@...el.com
Subject: [PATCH V3 2/4] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump

The arm64 pagetable dump code can race with concurrent modification of the
kernel page tables. When a leaf entries are modified concurrently, the dump
code may log stale or inconsistent information for a VA range, but this is
otherwise not harmful.

When intermediate levels of table are freed, the dump code will continue to
use memory which has been freed and potentially reallocated for another
purpose. In such cases, the dump code may dereference bogus addressses,
leading to a number of potential problems.

Intermediate levels of table may by freed during memory hot-remove, or when
installing a huge mapping in the vmalloc region. To avoid racing with these
cases, take the memory hotplug lock when walking the kernel page table.

Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
---
 arch/arm64/mm/ptdump_debugfs.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/arm64/mm/ptdump_debugfs.c b/arch/arm64/mm/ptdump_debugfs.c
index 064163f..80171d1 100644
--- a/arch/arm64/mm/ptdump_debugfs.c
+++ b/arch/arm64/mm/ptdump_debugfs.c
@@ -7,7 +7,10 @@
 static int ptdump_show(struct seq_file *m, void *v)
 {
 	struct ptdump_info *info = m->private;
+
+	get_online_mems();
 	ptdump_walk_pgd(m, info);
+	put_online_mems();
 	return 0;
 }
 DEFINE_SHOW_ATTRIBUTE(ptdump);
-- 
2.7.4