[PATCH AUTOSEL 5.0 028/262] x86/hyperv: Fix kernel panic when kexec on HyperV

[Sasha Levin]

From: Kairui Song <kasong at redhat.com>

[ Upstream commit 179fb36abb097976997f50733d5b122a29158cba ]

After commit 68bb7bfb7985 ("X86/Hyper-V: Enable IPI enlightenments"),
kexec fails with a kernel panic:

kexec_core: Starting new kernel
BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS Hyper-V UEFI Release v3.0 03/02/2018
RIP: 0010:0xffffc9000001d000

Call Trace:
 ? __send_ipi_mask+0x1c6/0x2d0
 ? hv_send_ipi_mask_allbutself+0x6d/0xb0
 ? mp_save_irq+0x70/0x70
 ? __ioapic_read_entry+0x32/0x50
 ? ioapic_read_entry+0x39/0x50
 ? clear_IO_APIC_pin+0xb8/0x110
 ? native_stop_other_cpus+0x6e/0x170
 ? native_machine_shutdown+0x22/0x40
 ? kernel_kexec+0x136/0x156

That happens if hypercall based IPIs are used because the hypercall page is
reset very early upon kexec reboot, but kexec sends IPIs to stop CPUs,
which invokes the hypercall and dereferences the unusable page.

To fix his, reset hv_hypercall_pg to NULL before the page is reset to avoid
any misuse, IPI sending will fall back to the non hypercall based
method. This only happens on kexec / kdump so just setting the pointer to
NULL is good enough.

Fixes: 68bb7bfb7985 ("X86/Hyper-V: Enable IPI enlightenments")
Signed-off-by: Kairui Song <kasong at redhat.com>
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Cc: "K. Y. Srinivasan" <kys at microsoft.com>
Cc: Haiyang Zhang <haiyangz at microsoft.com>
Cc: Stephen Hemminger <sthemmin at microsoft.com>
Cc: Sasha Levin <sashal at kernel.org>
Cc: Borislav Petkov <bp at alien8.de>
Cc: "H. Peter Anvin" <hpa at zytor.com>
Cc: Vitaly Kuznetsov <vkuznets at redhat.com>
Cc: Dave Young <dyoung at redhat.com>
Cc: devel at linuxdriverproject.org
Link: https://lkml.kernel.org/r/20190306111827.14131-1-kasong@redhat.com
Signed-off-by: Sasha Levin <sashal at kernel.org>
---
 arch/x86/hyperv/hv_init.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 7abb09e2eeb8..d3f42b6bbdac 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -406,6 +406,13 @@ void hyperv_cleanup(void)
 	/* Reset our OS id */
 	wrmsrl(HV_X64_MSR_GUEST_OS_ID, 0);
 
+	/*
+	 * Reset hypercall page reference before reset the page,
+	 * let hypercall operations fail safely rather than
+	 * panic the kernel for using invalid hypercall page
+	 */
+	hv_hypercall_pg = NULL;
+
 	/* Reset the hypercall page */
 	hypercall_msr.as_uint64 = 0;
 	wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
-- 
2.19.1