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SubjectRe: [PATCH] KVM: VMX: Fix rflags cache during vCPU reset
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On 20.11.2017 10:17, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@hotmail.com>
>
> Reported by syzkaller:
>
> *** Guest State ***
> CR0: actual=0x0000000080010031, shadow=0x0000000060000010, gh_mask=fffffffffffffff7
> CR4: actual=0x0000000000002061, shadow=0x0000000000000000, gh_mask=ffffffffffffe8f1
> CR3 = 0x000000002081e000
> RSP = 0x000000000000fffa RIP = 0x0000000000000000
> RFLAGS=0x00023000 DR7 = 0x0000000000000002
> ..........................
> ------------[ cut here ]------------
> WARNING: CPU: 6 PID: 24431 at /home/kernel/linux/arch/x86/kvm//x86.c:7302 kvm_arch_vcpu_ioctl_run+0x651/0x2ea0 [kvm]
> CPU: 6 PID: 24431 Comm: reprotest Tainted: G W OE 4.14.0+ #26
> RIP: 0010:kvm_arch_vcpu_ioctl_run+0x651/0x2ea0 [kvm]
> RSP: 0018:ffff880291d179e0 EFLAGS: 00010202
> Call Trace:
> ? synchronize_rcu.part.59+0x9b/0xf0
> ? synchronize_rcu_expedited+0xa0/0xa0
> ? kfree_call_rcu+0x20/0x20
> ? trace_raw_output_rcu_utilization+0x70/0x70
> ? kvm_arch_vcpu_runnable+0x220/0x220 [kvm]
> ? wait_for_completion+0x222/0x2a0
> ? __wait_rcu_gp+0x1d3/0x230
> ? kvm_vcpu_ioctl+0x479/0x880 [kvm]
> kvm_vcpu_ioctl+0x479/0x880 [kvm]
> ? kvm_dev_ioctl+0xba0/0xba0 [kvm]
> ? pick_next_task_fair+0x629/0xce0
> ? sched_clock+0x1b/0x20
> ? check_chain_key+0x131/0x1e0
> ? lock_acquire+0x2c0/0x2c0
> ? check_flags.part.44+0x62/0x250
> ? check_chain_key+0x131/0x1e0
> ? __lock_acquire+0xd2e/0x1cb0
> do_vfs_ioctl+0x142/0x9a0
> ? ioctl_preallocate+0x150/0x150
> ? lock_acquire+0x2c0/0x2c0
> ? lock_acquire+0x118/0x2c0
> ? __lock_is_held+0x4b/0xe0
> ? __fget+0x16a/0x250
> SyS_ioctl+0x74/0x80
> entry_SYSCALL_64_fastpath+0x23/0x9a
>
> The syzkaller testcase tries to setup the processor specific debug registers
> and configure vCPU for handling guest debug events through KVM_SET_GUEST_DEBUG.
> The KVM_SET_GUEST_DEBUG ioctl will get and set rflags in order to set TF bit
> if single step is needed. All regs' caches are reset to avail and GUEST_RFLAGS
> vmcs field is reset to 0x2 during vCPU reset. However, the cache of rflags is
> not reset during vCPU reset. The function vmx_get_rflags() returns an unreset
> rflags cache value since the cache is marked avail, it is 0 after boot. Vmentry
> fails if the rflags reserved bit 1 is 0.
>
> This patch fixes it by resetting both the GUEST_RFLAGS vmcs field and its cache
> to 0x2 during vCPU reset.
>
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Nadav Amit <nadav.amit@gmail.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
> ---
> arch/x86/kvm/vmx.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index b348920..131fa1c 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -5590,6 +5590,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> vmcs_write64(GUEST_IA32_DEBUGCTL, 0);
> }

Indeed, looks like the right think to do when looking at vmx_set_rflags().

Reviewed-by: David Hildenbrand <david@redhat.com>

>
> + vmx->rflags = 0x02;
> vmcs_writel(GUEST_RFLAGS, 0x02);
> kvm_rip_write(vcpu, 0xfff0);
>
>


--

Thanks,

David / dhildenb

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