linux-kernel - [PATCH] arm64: mm: correct the start of physical address in linear map

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Date:   Fri, 12 Feb 2021 20:23:16 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     pasha.tatashin@...een.com, tyhicks@...ux.microsoft.com,
        jmorris@...ei.org, catalin.marinas@....com, will@...nel.org,
        akpm@...ux-foundation.org, anshuman.khandual@....com,
        rppt@...nel.org, logang@...tatee.com, ardb@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: [PATCH] arm64: mm: correct the start of physical address in linear map

Memory hotplug may fail on systems with CONFIG_RANDOMIZE_BASE because the
linear map range is not checked correctly.

The start physical address that linear map covers can be actually at the
end of the range because of randmomization. Check that and if so reduce it
to 0.

This can be verified on QEMU with setting kaslr-seed to ~0ul:

memstart_offset_seed = 0xffff
START: __pa(_PAGE_OFFSET(vabits_actual)) = ffff9000c0000000
END:   __pa(PAGE_END - 1) =  1000bfffffff

Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
Fixes: 58284a901b42 ("arm64/mm: Validate hotplug range before creating linear mapping")
---
 arch/arm64/mm/mmu.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index ae0c3d023824..6057ecaea897 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1444,14 +1444,25 @@ static void __remove_pgd_mapping(pgd_t *pgdir, unsigned long start, u64 size)
 
 static bool inside_linear_region(u64 start, u64 size)
 {
+	u64 start_linear_pa = __pa(_PAGE_OFFSET(vabits_actual));
+	u64 end_linear_pa = __pa(PAGE_END - 1);
+
+	/*
+	 * Check for a wrap, it is possible because of randomized linear mapping
+	 * the start physical address is actually bigger than the end physical
+	 * address. In this case set start to zero because [0, end_linear_pa]
+	 * range must still be able to cover all addressable physical addresses.
+	 */
+	if (start_linear_pa > end_linear_pa)
+		start_linear_pa = 0;
+
 	/*
 	 * 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);
+	return start >= start_linear_pa && (start + size - 1) <= end_linear_pa;
 }
 
 int arch_add_memory(int nid, u64 start, u64 size,
-- 
2.25.1