[PATCH V3] arm64/mm: Validate hotplug range before creating linear mapping

[Anshuman Khandual]

During memory hotplug process, the linear mapping should not be created for
a given memory range if that would fall outside the maximum allowed linear
range. Else it might cause memory corruption in the kernel virtual space.

Maximum linear mapping region is [PAGE_OFFSET..(PAGE_END -1)] accommodating
both its ends but excluding PAGE_END. Max physical range that can be mapped
inside this linear mapping range, must also be derived from its end points.

This ensures that arch_add_memory() validates memory hot add range for its
potential linear mapping requirements, before creating it with
__create_pgd_mapping().

Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Will Deacon <will at kernel.org>
Cc: Mark Rutland <mark.rutland at arm.com>
Cc: Ard Biesheuvel <ardb at kernel.org>
Cc: Steven Price <steven.price at arm.com>
Cc: Robin Murphy <robin.murphy at arm.com>
Cc: David Hildenbrand <david at redhat.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: linux-arm-kernel at lists.infradead.org
Cc: linux-kernel at vger.kernel.org
Fixes: 4ab215061554 ("arm64: Add memory hotplug support")
Signed-off-by: Anshuman Khandual <anshuman.khandual at arm.com>
---
This applies on 5.10-rc3

Changes in V3:

- Dropped comment paragraphs from code and commit message per Ard
- Dropped parenthesis brackets in the comparison per Ard

Changes in V2: (https://lore.kernel.org/linux-arm-kernel/1605236574-14636-1-git-send-email-anshuman.khandual@arm.com/)

- Replaced (start + size) with (start + size - 1) in the comparison per Ard
- Dropped parenthesis brackets in the comparison per Ard

Changes in V1: (https://lore.kernel.org/linux-arm-kernel/1600332402-30123-1-git-send-email-anshuman.khandual@arm.com/)

 arch/arm64/mm/mmu.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 1c0f3e02f731..ca692a815731 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1444,11 +1444,28 @@ static void __remove_pgd_mapping(pgd_t *pgdir, unsigned long start, u64 size)
 	free_empty_tables(start, end, PAGE_OFFSET, PAGE_END);
 }
 
+static bool inside_linear_region(u64 start, u64 size)
+{
+	/*
+	 * Linear mapping region is the range [PAGE_OFFSET..(PAGE_END - 1)]
+	 * accommodating both its ends but excluding PAGE_END. Max physical
+	 * range which can be mapped inside this linear mapping range, must
+	 * also be derived from its end points.
+	 */
+	return start >= __pa(_PAGE_OFFSET(vabits_actual)) &&
+	       (start + size - 1) <= __pa(PAGE_END - 1);
+}
+
 int arch_add_memory(int nid, u64 start, u64 size,
 		    struct mhp_params *params)
 {
 	int ret, flags = 0;
 
+	if (!inside_linear_region(start, size)) {
+		pr_err("[%llx %llx] is outside linear mapping region\n", start, start + size);
+		return -EINVAL;
+	}
+
 	if (rodata_full || debug_pagealloc_enabled())
 		flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
 
-- 
2.20.1