2018-01-15 14:26:15 +03:00
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From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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2018-01-06 17:13:39 +03:00
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From: Andy Lutomirski <luto@kernel.org>
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Date: Wed, 6 Sep 2017 19:54:54 -0700
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2018-01-15 14:26:15 +03:00
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Subject: [PATCH] x86/mm: Document how CR4.PCIDE restore works
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2018-01-06 17:13:39 +03:00
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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CVE-2017-5754
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While debugging a problem, I thought that using
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cr4_set_bits_and_update_boot() to restore CR4.PCIDE would be
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helpful. It turns out to be counterproductive.
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Add a comment documenting how this works.
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Signed-off-by: Andy Lutomirski <luto@kernel.org>
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Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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(cherry picked from commit 1c9fe4409ce3e9c78b1ed96ee8ed699d4f03bf33)
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Signed-off-by: Andy Whitcroft <apw@canonical.com>
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Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
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(cherry picked from commit 0d69e4c4a2db42a9bac6609a3df15bd91163f8b9)
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Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
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---
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arch/x86/kernel/cpu/common.c | 13 +++++++++++++
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1 file changed, 13 insertions(+)
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diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
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index b95cd94ca97b..0b80ed14ff52 100644
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--- a/arch/x86/kernel/cpu/common.c
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+++ b/arch/x86/kernel/cpu/common.c
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@@ -333,6 +333,19 @@ static void setup_pcid(struct cpuinfo_x86 *c)
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{
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if (cpu_has(c, X86_FEATURE_PCID)) {
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if (cpu_has(c, X86_FEATURE_PGE)) {
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+ /*
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+ * We'd like to use cr4_set_bits_and_update_boot(),
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+ * but we can't. CR4.PCIDE is special and can only
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+ * be set in long mode, and the early CPU init code
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+ * doesn't know this and would try to restore CR4.PCIDE
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+ * prior to entering long mode.
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+ *
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+ * Instead, we rely on the fact that hotplug, resume,
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+ * etc all fully restore CR4 before they write anything
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+ * that could have nonzero PCID bits to CR3. CR4.PCIDE
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+ * has no effect on the page tables themselves, so we
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+ * don't need it to be restored early.
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+ */
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cr4_set_bits(X86_CR4_PCIDE);
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} else {
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/*
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--
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2.14.2
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