linux-kernel - [PATCH 4.15 043/202] [Variant 3/Meltdown] arm64: mm: Permit transitioning from Global to Non-Global without BBM

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Date:   Thu, 15 Feb 2018 16:15:43 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>
Subject: [PATCH 4.15 043/202] [Variant 3/Meltdown] arm64: mm: Permit transitioning from Global to Non-Global without BBM

4.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Will Deacon <will.deacon@....com>


Commit 4e6020565596 upstream.

Break-before-make is not needed when transitioning from Global to
Non-Global mappings, provided that the contiguous hint is not being used.

Signed-off-by: Will Deacon <will.deacon@....com>
Signed-off-by: Catalin Marinas <catalin.marinas@....com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/arm64/mm/mmu.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -117,6 +117,10 @@ static bool pgattr_change_is_safe(u64 ol
 	if ((old | new) & PTE_CONT)
 		return false;
 
+	/* Transitioning from Global to Non-Global is safe */
+	if (((old ^ new) == PTE_NG) && (new & PTE_NG))
+		return true;
+
 	return ((old ^ new) & ~mask) == 0;
 }