From a3a7115884633035878742aa0089ab01fc90d196 Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Mon, 29 Jan 2018 11:59:58 +0000 Subject: [PATCH] arm64: entry: Reword comment about post_ttbr_update_workaround 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 Signed-off-by: Catalin Marinas Signed-off-by: Ard Biesheuvel Signed-off-by: Greg Kroah-Hartman (cherry picked from commit 8703f27d7c5db70ffd793a813160f28e810cc1bc) CVE-2017-5753 CVE-2017-5715 CVE-2017-5754 Signed-off-by: Paolo Pisati Acked-by: Brad Figg Acked-by: Kleber Sacilotto de Souza Signed-off-by: Khalid Elmously --- arch/arm64/kernel/entry.S | 13 +++---------- 1 file changed, 3 insertions(+), 10 deletions(-) diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S index b2e5b78a5e3c..b5063036629d 100644 --- a/arch/arm64/kernel/entry.S +++ b/arch/arm64/kernel/entry.S @@ -964,16 +964,9 @@ 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 -- 2.39.2