linux-kernel - [PATCH 4.14 054/195] [Variant 3/Meltdown] arm64: entry: Reword comment about post_ttbr_update

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Date:   Thu, 15 Feb 2018 16:15:45 +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>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: [PATCH 4.14 054/195] [Variant 3/Meltdown] arm64: entry: Reword comment about post_ttbr_update_workaround

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

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

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


Commit f167211a93ac upstream.

We don't fully understand the Cavium ThunderX erratum, but it appears
that mapping the kernel as nG can lead to horrible consequences such as
attempting to execute userspace from kernel context. Since kpti isn't
enabled for these CPUs anyway, simplify the comment justifying the lack
of post_ttbr_update_workaround in the exception trampoline.

Signed-off-by: Will Deacon <will.deacon@....com>
Signed-off-by: Catalin Marinas <catalin.marinas@....com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/arm64/kernel/entry.S |   13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -962,16 +962,9 @@ alternative_else_nop_endif
 	orr	\tmp, \tmp, #USER_ASID_FLAG
 	msr	ttbr1_el1, \tmp
 	/*
-	 * We avoid running the post_ttbr_update_workaround here because the
-	 * user and kernel ASIDs don't have conflicting mappings, so any
-	 * "blessing" as described in:
-	 *
-	 *   http://lkml.kernel.org/r/56BB848A.6060603@caviumnetworks.com
-	 *
-	 * will not hurt correctness. Whilst this may partially defeat the
-	 * point of using split ASIDs in the first place, it avoids
-	 * the hit of invalidating the entire I-cache on every return to
-	 * userspace.
+	 * We avoid running the post_ttbr_update_workaround here because
+	 * it's only needed by Cavium ThunderX, which requires KPTI to be
+	 * disabled.
 	 */
 	.endm