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    Subject[PATCH 4.19 021/187] x86/hyperv: Fix kernel panic when kexec on HyperV
    Date
    4.19-stable review patch.  If anyone has any objections, please let me know.

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

    [ 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@redhat.com>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Cc: "K. Y. Srinivasan" <kys@microsoft.com>
    Cc: Haiyang Zhang <haiyangz@microsoft.com>
    Cc: Stephen Hemminger <sthemmin@microsoft.com>
    Cc: Sasha Levin <sashal@kernel.org>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: "H. Peter Anvin" <hpa@zytor.com>
    Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
    Cc: Dave Young <dyoung@redhat.com>
    Cc: devel@linuxdriverproject.org
    Link: https://lkml.kernel.org/r/20190306111827.14131-1-kasong@redhat.com
    Signed-off-by: Sasha Levin <sashal@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 20c876c7c5bf..87abd5145cc9 100644
    --- a/arch/x86/hyperv/hv_init.c
    +++ b/arch/x86/hyperv/hv_init.c
    @@ -387,6 +387,13 @@ 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


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