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A VMEnter that VMFails (as opposed to VMExits) does not touch host state beyond registers that are explicitly noted in the VMFail path, e.g. EFLAGS. Host state does not need to be loaded because VMFail is only signaled for consistency checks that occur before the CPU starts to load guest state, i.e. there is no need to restore any state as nothing has been modified. But in the case where a VMFail is detected by hardware and not by KVM (due to deferring consistency checks to hardware), KVM has already loaded some amount of guest state. Luckily, "loaded" only means loaded to KVM's software model, i.e. vmcs01 has not been modified. So, unwind our software model to the pre-VMEntry host state. Not restoring host state in this VMFail path leads to a variety of failures because we end up with stale data in vcpu->arch, e.g. CR0, CR4, EFER, etc... will all be out of sync relative to vmcs01. Any significant delta in the stale data is all but guaranteed to crash L1, e.g. emulation of SMEP, SMAP, UMIP, WP, etc... will be wrong. An alternative to this "soft" reload would be to load host state from vmcs12 as if we triggered a VMExit (as opposed to VMFail), but that is wildly inconsistent with respect to the VMX architecture, e.g. an L1 VMM with separate VMExit and VMFail paths would explode. Note that this approach does not mean KVM is 100% accurate with respect to VMX hardware behavior, even at an architectural level (the exact order of consistency checks is microarchitecture specific). But 100% emulation accuracy isn't the goal (with this patch), rather the goal is to be consistent in the information delivered to L1, e.g. a VMExit should not fall-through VMENTER, and a VMFail should not jump to HOST_RIP. This technically reverts commit "5af4157388ad (KVM: nVMX: Fix mmu context after VMLAUNCH/VMRESUME failure)", but retains the core aspects of that patch, just in an open coded form due to the need to pull state from vmcs01 instead of vmcs12. Restoring host state resolves a variety of issues introduced by commit "4f350c6dbcb9 (kvm: nVMX: Handle deferred early VMLAUNCH/VMRESUME failure properly)", which remedied the incorrect behavior of treating VMFail like VMExit but in doing so neglected to restore arch state that had been modified prior to attempting nested VMEnter. A sample failure that occurs due to stale vcpu.arch state is a fault of some form while emulating an LGDT (due to emulated UMIP) from L1 after a failed VMEntry to L3, in this case when running the KVM unit test test_tpr_threshold_values in L1. L0 also hits a WARN in this case due to a stale arch.cr4.UMIP. L1: BUG: unable to handle kernel paging request at ffffc90000663b9e PGD 276512067 P4D 276512067 PUD 276513067 PMD 274efa067 PTE 8000000271de2163 Oops: 0009 [#1] SMP CPU: 5 PID: 12495 Comm: qemu-system-x86 Tainted: G W 4.18.0-rc2+ #2 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 RIP: 0010:native_load_gdt+0x0/0x10 ... Call Trace: load_fixmap_gdt+0x22/0x30 __vmx_load_host_state+0x10e/0x1c0 [kvm_intel] vmx_switch_vmcs+0x2d/0x50 [kvm_intel] nested_vmx_vmexit+0x222/0x9c0 [kvm_intel] vmx_handle_exit+0x246/0x15a0 [kvm_intel] kvm_arch_vcpu_ioctl_run+0x850/0x1830 [kvm] kvm_vcpu_ioctl+0x3a1/0x5c0 [kvm] do_vfs_ioctl+0x9f/0x600 ksys_ioctl+0x66/0x70 __x64_sys_ioctl+0x16/0x20 do_syscall_64+0x4f/0x100 entry_SYSCALL_64_after_hwframe+0x44/0xa9 L0: WARNING: CPU: 2 PID: 3529 at arch/x86/kvm/vmx.c:6618 handle_desc+0x28/0x30 [kvm_intel] ... CPU: 2 PID: 3529 Comm: qemu-system-x86 Not tainted 4.17.2-coffee+ #76 Hardware name: Intel Corporation Kabylake Client platform/KBL S RIP: 0010:handle_desc+0x28/0x30 [kvm_intel] ... Call Trace: kvm_arch_vcpu_ioctl_run+0x863/0x1840 [kvm] kvm_vcpu_ioctl+0x3a1/0x5c0 [kvm] do_vfs_ioctl+0x9f/0x5e0 ksys_ioctl+0x66/0x70 __x64_sys_ioctl+0x16/0x20 do_syscall_64+0x49/0xf0 entry_SYSCALL_64_after_hwframe+0x44/0xa9 Fixes: |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.