Re: [PATCH kernel] KVM: PPC: Protect kvm_vcpu_read_guest with srcu locks

[Paul Mackerras <paulus@ozlabs.org>]

On Tue, Jun 09, 2020 at 12:12:29PM +1000, Alexey Kardashevskiy wrote:
> The kvm_vcpu_read_guest/kvm_vcpu_write_guest used for nested guests
> eventually call srcu_dereference_check to dereference a memslot and
> lockdep produces a warning as neither kvm->slots_lock nor
> kvm->srcu lock is held and kvm->users_count is above zero (>100 in fact).
> 
> This wraps mentioned VCPU read/write helpers in srcu read lock/unlock as
> it is done in other places. This uses vcpu->srcu_idx when possible.
> 
> These helpers are only used for nested KVM so this may explain why
> we did not see these before.
> 
> Here is an example of a warning:
> 
> =============================
> WARNING: suspicious RCU usage
> 5.7.0-rc3-le_dma-bypass.3.2_a+fstn1 #897 Not tainted
> -----------------------------
> include/linux/kvm_host.h:633 suspicious rcu_dereference_check() usage!
> 
> other info that might help us debug this:
> 
> rcu_scheduler_active = 2, debug_locks = 1
> 1 lock held by qemu-system-ppc/2752:
>  #0: c000200359016be0 (&vcpu->mutex){+.+.}-{3:3}, at: kvm_vcpu_ioctl+0x144/0xd80 [kvm]
> 
> stack backtrace:
> CPU: 80 PID: 2752 Comm: qemu-system-ppc Not tainted 5.7.0-rc3-le_dma-bypass.3.2_a+fstn1 #897
> Call Trace:
> [c0002003591ab240] [c000000000b23ab4] dump_stack+0x190/0x25c (unreliable)
> [c0002003591ab2b0] [c00000000023f954] lockdep_rcu_suspicious+0x140/0x164
> [c0002003591ab330] [c008000004a445f8] kvm_vcpu_gfn_to_memslot+0x4c0/0x510 [kvm]
> [c0002003591ab3a0] [c008000004a44c18] kvm_vcpu_read_guest+0xa0/0x180 [kvm]
> [c0002003591ab410] [c008000004ff9bd8] kvmhv_enter_nested_guest+0x90/0xb80 [kvm_hv]
> [c0002003591ab980] [c008000004fe07bc] kvmppc_pseries_do_hcall+0x7b4/0x1c30 [kvm_hv]
> [c0002003591aba10] [c008000004fe5d30] kvmppc_vcpu_run_hv+0x10a8/0x1a30 [kvm_hv]
> [c0002003591abae0] [c008000004a5d954] kvmppc_vcpu_run+0x4c/0x70 [kvm]
> [c0002003591abb10] [c008000004a56e54] kvm_arch_vcpu_ioctl_run+0x56c/0x7c0 [kvm]
> [c0002003591abba0] [c008000004a3ddc4] kvm_vcpu_ioctl+0x4ac/0xd80 [kvm]
> [c0002003591abd20] [c0000000006ebb58] ksys_ioctl+0x188/0x210
> [c0002003591abd70] [c0000000006ebc28] sys_ioctl+0x48/0xb0
> [c0002003591abdb0] [c000000000042764] system_call_exception+0x1d4/0x2e0
> [c0002003591abe20] [c00000000000cce8] system_call_common+0xe8/0x214
> 
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>

Thanks, patch applied to my kvm-ppc-next branch.

Paul.