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    Subject[PATCH 4.16 201/279] x86/xen: Reset VCPU0 info pointer after shared_info remap
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    4.16-stable review patch.  If anyone has any objections, please let me know.

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

    From: "van der Linden, Frank" <fllinden@amazon.com>

    [ Upstream commit d1ecfa9d1f402366b1776fbf84e635678a51414f ]

    This patch fixes crashes during boot for HVM guests on older (pre HVM
    vector callback) Xen versions. Without this, current kernels will always
    fail to boot on those Xen versions.

    Sample stack trace:

    BUG: unable to handle kernel paging request at ffffffffff200000
    IP: __xen_evtchn_do_upcall+0x1e/0x80
    PGD 1e0e067 P4D 1e0e067 PUD 1e10067 PMD 235c067 PTE 0
    Oops: 0002 [#1] SMP PTI
    Modules linked in:
    CPU: 0 PID: 512 Comm: kworker/u2:0 Not tainted 4.14.33-52.13.amzn1.x86_64 #1
    Hardware name: Xen HVM domU, BIOS 3.4.3.amazon 11/11/2016
    task: ffff88002531d700 task.stack: ffffc90000480000
    RIP: 0010:__xen_evtchn_do_upcall+0x1e/0x80
    RSP: 0000:ffff880025403ef0 EFLAGS: 00010046
    RAX: ffffffff813cc760 RBX: ffffffffff200000 RCX: ffffc90000483ef0
    RDX: ffff880020540a00 RSI: ffff880023c78000 RDI: 000000000000001c
    RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000000
    R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
    R13: ffff880025403f5c R14: 0000000000000000 R15: 0000000000000000
    FS: 0000000000000000(0000) GS:ffff880025400000(0000) knlGS:0000000000000000
    CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: ffffffffff200000 CR3: 0000000001e0a000 CR4: 00000000000006f0
    Call Trace:
    <IRQ>
    do_hvm_evtchn_intr+0xa/0x10
    __handle_irq_event_percpu+0x43/0x1a0
    handle_irq_event_percpu+0x20/0x50
    handle_irq_event+0x39/0x60
    handle_fasteoi_irq+0x80/0x140
    handle_irq+0xaf/0x120
    do_IRQ+0x41/0xd0
    common_interrupt+0x7d/0x7d
    </IRQ>

    During boot, the HYPERVISOR_shared_info page gets remapped to make it work
    with KASLR. This means that any pointer derived from it needs to be
    adjusted.

    The only value that this applies to is the vcpu_info pointer for VCPU 0.
    For PV and HVM with the callback vector feature, this gets done via the
    smp_ops prepare_boot_cpu callback. Older Xen versions do not support the
    HVM callback vector, so there is no Xen-specific smp_ops set up in that
    scenario. So, the vcpu_info pointer for VCPU 0 never gets set to the proper
    value, and the first reference of it will be bad. Fix this by resetting it
    immediately after the remap.

    Signed-off-by: Frank van der Linden <fllinden@amazon.com>
    Reviewed-by: Eduardo Valentin <eduval@amazon.com>
    Reviewed-by: Alakesh Haloi <alakeshh@amazon.com>
    Reviewed-by: Vallish Vaidyeshwara <vallish@amazon.com>
    Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Cc: Juergen Gross <jgross@suse.com>
    Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Cc: xen-devel@lists.xenproject.org
    Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/x86/xen/enlighten_hvm.c | 13 +++++++++++++
    1 file changed, 13 insertions(+)

    --- a/arch/x86/xen/enlighten_hvm.c
    +++ b/arch/x86/xen/enlighten_hvm.c
    @@ -65,6 +65,19 @@ static void __init xen_hvm_init_mem_mapp
    {
    early_memunmap(HYPERVISOR_shared_info, PAGE_SIZE);
    HYPERVISOR_shared_info = __va(PFN_PHYS(shared_info_pfn));
    +
    + /*
    + * The virtual address of the shared_info page has changed, so
    + * the vcpu_info pointer for VCPU 0 is now stale.
    + *
    + * The prepare_boot_cpu callback will re-initialize it via
    + * xen_vcpu_setup, but we can't rely on that to be called for
    + * old Xen versions (xen_have_vector_callback == 0).
    + *
    + * It is, in any case, bad to have a stale vcpu_info pointer
    + * so reset it now.
    + */
    + xen_vcpu_info_reset(0);
    }

    static void __init init_hvm_pv_info(void)

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