linux-kernel - [PATCH 4.9 22/75] x86/mm/64: Fix reboot interaction with CR4.PCIDE

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Date:   Mon,  1 Jan 2018 15:31:59 +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, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Hugh Dickins <hughd@...gle.com>
Subject: [PATCH 4.9 22/75] x86/mm/64: Fix reboot interaction with CR4.PCIDE

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

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

From: Andy Lutomirski <luto@...nel.org>

commit 924c6b900cfdf376b07bccfd80e62b21914f8a5a upstream.

Trying to reboot via real mode fails with PCID on: long mode cannot
be exited while CR4.PCIDE is set.  (No, I have no idea why, but the
SDM and actual CPUs are in agreement here.)  The result is a GPF and
a hang instead of a reboot.

I didn't catch this in testing because neither my computer nor my VM
reboots this way.  I can trigger it with reboot=bios, though.

Fixes: 660da7c9228f ("x86/mm: Enable CR4.PCIDE on supported systems")
Reported-and-tested-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Borislav Petkov <bp@...en8.de>
Link: https://lkml.kernel.org/r/f1e7d965998018450a7a70c2823873686a8b21c0.1507524746.git.luto@kernel.org
Cc: Hugh Dickins <hughd@...gle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/x86/kernel/reboot.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -106,6 +106,10 @@ void __noreturn machine_real_restart(uns
 	load_cr3(initial_page_table);
 #else
 	write_cr3(real_mode_header->trampoline_pgd);
+
+	/* Exiting long mode will fail if CR4.PCIDE is set. */
+	if (static_cpu_has(X86_FEATURE_PCID))
+		cr4_clear_bits(X86_CR4_PCIDE);
 #endif
 
 	/* Jump to the identity-mapped low memory code */