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- From 20e07f035810f1b2bb3d816e49f48f6b6a37bf64 Mon Sep 17 00:00:00 2001
- From: Andy Lutomirski <[email protected]>
- Date: Wed, 6 Sep 2017 19:54:54 -0700
- Subject: [PATCH 017/242] x86/mm: Document how CR4.PCIDE restore works
- MIME-Version: 1.0
- Content-Type: text/plain; charset=UTF-8
- Content-Transfer-Encoding: 8bit
- CVE-2017-5754
- While debugging a problem, I thought that using
- cr4_set_bits_and_update_boot() to restore CR4.PCIDE would be
- helpful. It turns out to be counterproductive.
- Add a comment documenting how this works.
- Signed-off-by: Andy Lutomirski <[email protected]>
- Signed-off-by: Linus Torvalds <[email protected]>
- (cherry picked from commit 1c9fe4409ce3e9c78b1ed96ee8ed699d4f03bf33)
- Signed-off-by: Andy Whitcroft <[email protected]>
- Signed-off-by: Kleber Sacilotto de Souza <[email protected]>
- (cherry picked from commit 0d69e4c4a2db42a9bac6609a3df15bd91163f8b9)
- Signed-off-by: Fabian Grünbichler <[email protected]>
- ---
- arch/x86/kernel/cpu/common.c | 13 +++++++++++++
- 1 file changed, 13 insertions(+)
- diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
- index b95cd94ca97b..0b80ed14ff52 100644
- --- a/arch/x86/kernel/cpu/common.c
- +++ b/arch/x86/kernel/cpu/common.c
- @@ -333,6 +333,19 @@ static void setup_pcid(struct cpuinfo_x86 *c)
- {
- if (cpu_has(c, X86_FEATURE_PCID)) {
- if (cpu_has(c, X86_FEATURE_PGE)) {
- + /*
- + * We'd like to use cr4_set_bits_and_update_boot(),
- + * but we can't. CR4.PCIDE is special and can only
- + * be set in long mode, and the early CPU init code
- + * doesn't know this and would try to restore CR4.PCIDE
- + * prior to entering long mode.
- + *
- + * Instead, we rely on the fact that hotplug, resume,
- + * etc all fully restore CR4 before they write anything
- + * that could have nonzero PCID bits to CR3. CR4.PCIDE
- + * has no effect on the page tables themselves, so we
- + * don't need it to be restored early.
- + */
- cr4_set_bits(X86_CR4_PCIDE);
- } else {
- /*
- --
- 2.14.2
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