Re: [PATCH] KVM: x86: fix access memslots w/o hold srcu read lock

["Chen, Tiejun" <tiejun.chen@intel.com>]

On 2014/10/31 12:33, Wanpeng Li wrote:
> The srcu read lock must be held while accessing memslots (e.g.
> when using gfn_to_* functions), however, commit c24ae0dcd3e8
> ("kvm: x86: Unpin and remove kvm_arch->apic_access_page") call
> gfn_to_page() in kvm_vcpu_reload_apic_access_page() w/o hold it
> which leads to suspicious rcu_dereference_check() usage warning.
> This patch fix it by holding srcu read lock when call gfn_to_page()
> in kvm_vcpu_reload_apic_access_page() function.
>
>
> [ INFO: suspicious RCU usage. ]
> 3.18.0-rc2-test2+ #70 Not tainted
> -------------------------------
> include/linux/kvm_host.h:474 suspicious rcu_dereference_check() usage!
>
> other info that might help us debug this:
>
> rcu_scheduler_active = 1, debug_locks = 0
> 1 lock held by qemu-system-x86/2371:
>   #0:  (&vcpu->mutex){+.+...}, at: [<ffffffffa037d800>] vcpu_load+0x20/0xd0 [kvm]
>
> stack backtrace:
> CPU: 4 PID: 2371 Comm: qemu-system-x86 Not tainted 3.18.0-rc2-test2+ #70
> Hardware name: Dell Inc. OptiPlex 9010/0M9KCM, BIOS A12 01/10/2013
>   0000000000000001 ffff880209983ca8 ffffffff816f514f 0000000000000000
>   ffff8802099b8990 ffff880209983cd8 ffffffff810bd687 00000000000fee00
>   ffff880208a2c000 ffff880208a10000 ffff88020ef50040 ffff880209983d08
> Call Trace:
>   [<ffffffff816f514f>] dump_stack+0x4e/0x71
>   [<ffffffff810bd687>] lockdep_rcu_suspicious+0xe7/0x120
>   [<ffffffffa037d055>] gfn_to_memslot+0xd5/0xe0 [kvm]
>   [<ffffffffa03807d3>] __gfn_to_pfn+0x33/0x60 [kvm]
>   [<ffffffffa0380885>] gfn_to_page+0x25/0x90 [kvm]
>   [<ffffffffa038aeec>] kvm_vcpu_reload_apic_access_page+0x3c/0x80 [kvm]
>   [<ffffffffa08f0a9c>] vmx_vcpu_reset+0x20c/0x460 [kvm_intel]
>   [<ffffffffa039ab8e>] kvm_vcpu_reset+0x15e/0x1b0 [kvm]
>   [<ffffffffa039ac0c>] kvm_arch_vcpu_setup+0x2c/0x50 [kvm]
>   [<ffffffffa037f7e0>] kvm_vm_ioctl+0x1d0/0x780 [kvm]
>   [<ffffffff810bc664>] ? __lock_is_held+0x54/0x80
>   [<ffffffff812231f0>] do_vfs_ioctl+0x300/0x520
>   [<ffffffff8122ee45>] ? __fget+0x5/0x250
>   [<ffffffff8122f0fa>] ? __fget_light+0x2a/0xe0
>   [<ffffffff81223491>] SyS_ioctl+0x81/0xa0
>   [<ffffffff816fed6d>] system_call_fastpath+0x16/0x1b
>
> Reported-by: Takashi Iwai <tiwai@suse.de>
> Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
> ---
>   arch/x86/kvm/x86.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0033df3..2d97329 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6059,6 +6059,7 @@ static void kvm_vcpu_flush_tlb(struct kvm_vcpu *vcpu)
>   void kvm_vcpu_reload_apic_access_page(struct kvm_vcpu *vcpu)
>   {
>   	struct page *page = NULL;
> +	int idx;
>
>   	if (!irqchip_in_kernel(vcpu->kvm))
>   		return;
> @@ -6066,7 +6067,9 @@ void kvm_vcpu_reload_apic_access_page(struct kvm_vcpu *vcpu)
>   	if (!kvm_x86_ops->set_apic_access_page_addr)
>   		return;
>
> +	idx = srcu_read_lock(&vcpu->kvm->srcu);

There's another scenario that we already hold srcu before call 
kvm_vcpu_reload_apic_access_page(),

__vcpu_run()
     |
     + vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
     + r = vcpu_enter_guest(vcpu);
         |
         + kvm_vcpu_reload_apic_access_page(vcpu);

So according to backtrace I think we should fix as follows:

kvm: x86: vmx: hold kvm->srcu while reload apic access page

kvm_vcpu_reload_apic_access_page() needs to access memslots via
gfn_to_page(), so its necessary to hold kvm->srcu.

Signed-off-by: Tiejun Chen <tiejun.chen@intel.com>
---
  arch/x86/kvm/vmx.c | 3 +++
  1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index b25a588..9fa1f46 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -4442,6 +4442,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu)
  {
         struct vcpu_vmx *vmx = to_vmx(vcpu);
         struct msr_data apic_base_msr;
+       int idx;

         vmx->rmode.vm86_active = 0;

@@ -4509,7 +4510,9 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu)
                 vmcs_write32(TPR_THRESHOLD, 0);
         }

+       idx = srcu_read_lock(&vcpu->kvm->srcu);
         kvm_vcpu_reload_apic_access_page(vcpu);
+       srcu_read_unlock(&vcpu->kvm->srcu, idx);

         if (vmx_vm_has_apicv(vcpu->kvm))
                 memset(&vmx->pi_desc, 0, sizeof(struct pi_desc));
-- 
1.9.1

Thanks
Tiejun
>   	page = gfn_to_page(vcpu->kvm, APIC_DEFAULT_PHYS_BASE >> PAGE_SHIFT);
> +	srcu_read_unlock(&vcpu->kvm->srcu, idx);
>   	kvm_x86_ops->set_apic_access_page_addr(vcpu, page_to_phys(page));
>
>   	/*
>