linux-kernel - [tip: x86/urgent] x86/apic: Include the LDR when clearing out APIC registers

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Date:   Mon, 26 Aug 2019 18:09:10 -0000
From:   tip-bot2 for Bandan Das <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Bandan Das <bsd@...hat.com>, Thomas Gleixner <tglx@...utronix.de>,
        stable@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/urgent] x86/apic: Include the LDR when clearing out APIC registers

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     558682b5291937a70748d36fd9ba757fb25b99ae
Gitweb:        https://git.kernel.org/tip/558682b5291937a70748d36fd9ba757fb25b99ae
Author:        Bandan Das <bsd@...hat.com>
AuthorDate:    Mon, 26 Aug 2019 06:15:13 -04:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Mon, 26 Aug 2019 20:00:57 +02:00

x86/apic: Include the LDR when clearing out APIC registers

Although APIC initialization will typically clear out the LDR before
setting it, the APIC cleanup code should reset the LDR.

This was discovered with a 32-bit KVM guest jumping into a kdump
kernel. The stale bits in the LDR triggered a bug in the KVM APIC
implementation which caused the destination mapping for VCPUs to be
corrupted.

Note that this isn't intended to paper over the KVM APIC bug. The kernel
has to clear the LDR when resetting the APIC registers except when X2APIC
is enabled.

This lacks a Fixes tag because missing to clear LDR goes way back into pre
git history.

[ tglx: Made x2apic_enabled a function call as required ]

Signed-off-by: Bandan Das <bsd@...hat.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: stable@...r.kernel.org
Link: https://lkml.kernel.org/r/20190826101513.5080-3-bsd@redhat.com

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

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index aa5495d..dba2828 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1179,6 +1179,10 @@ void clear_local_APIC(void)
 	apic_write(APIC_LVT0, v | APIC_LVT_MASKED);
 	v = apic_read(APIC_LVT1);
 	apic_write(APIC_LVT1, v | APIC_LVT_MASKED);
+	if (!x2apic_enabled()) {
+		v = apic_read(APIC_LDR) & ~APIC_LDR_MASK;
+		apic_write(APIC_LDR, v);
+	}
 	if (maxlvt >= 4) {
 		v = apic_read(APIC_LVTPC);
 		apic_write(APIC_LVTPC, v | APIC_LVT_MASKED);