[PATCH 3/3] KVM: PPC: Book3s HV: Take SRCU read lock around kvmppc_h_page_init() call

[Gautam Menghani]

Since there is no srcu read lock around kvmppc_h_page_init(), a
"suspicious rcu_dereference_check() usage" can be triggered by running
host linux with CONFIG_PROVE_RCU=y and the following code in the guest:

plpar_hcall_norets(H_PAGE_INIT, H_ZERO_PAGE, phys_addr, 0);

 =============================
 WARNING: suspicious RCU usage
 7.0.0-dirty #227 Not tainted
 -----------------------------
./include/linux/kvm_host.h:1074 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/2093:
 #0: c000000354197ab0 (&vcpu->mutex){+.+.}-{3:3}, at: kvm_vcpu_ioctl+0x10c/0xaf0

stack backtrace:
CPU: 30 UID: 0 PID: 2093 Comm: qemu-system-ppc Not tainted 7.0.0-dirty #227 PREEMPT(full)
Hardware name: IBM,9080-HEX POWER10 (architected) 0x800200 0xf000006 of:IBM,FW1060.60 (NH1060_158) hv:phyp pSeries
Call Trace:
[c00000033e1bf430] [c000000001b3dbd4] dump_stack_lvl+0xc8/0x130 (unreliable)
[c00000033e1bf470] [c0000000003da964] lockdep_rcu_suspicious+0x1f4/0x290
[c00000033e1bf520] [c000000000231424] gfn_to_memslot+0x1b4/0x1c0
[c00000033e1bf560] [c00000000023637c] kvm_clear_guest+0x9c/0x110
[c00000033e1bf5c0] [c0000000002714a4] kvmppc_h_page_init+0x160/0x1a0
[c00000033e1bf610] [c00000000026c040] kvmppc_pseries_do_hcall+0x1930/0x1940
[c00000033e1bf6d0] [c00000000026fb18] kvmppc_vcpu_run_hv+0x268/0x800
[c00000033e1bf7a0] [c000000000246b30] kvmppc_vcpu_run+0x30/0x50
[c00000033e1bf7c0] [c000000000241ed4] kvm_arch_vcpu_ioctl_run+0x364/0x530
[c00000033e1bf860] [c00000000022b870] kvm_vcpu_ioctl+0x1b0/0xaf0
[c00000033e1bfa50] [c0000000009b4514] sys_ioctl+0x144/0x190
[c00000033e1bfab0] [c000000000034330] system_call_exception+0x170/0x370
[c00000033e1bfe50] [c00000000000d05c] system_call_vectored_common+0x15c/0x2ec

Fix this by taking a srcu read lock around kvmppc_h_page_init().

Fixes: 2d34d1c3bbfd ("KVM: PPC: Book3S HV: Implement virtual mode H_PAGE_INIT handler")
Cc: stable@xxxxxxxxxxxxxxx # 5.2+
Signed-off-by: Gautam Menghani <gautam@xxxxxxxxxxxxx>
---
 arch/powerpc/kvm/book3s_hv.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index dbac3573b2c8..8678a93bac51 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -1386,9 +1386,11 @@ int kvmppc_pseries_do_hcall(struct kvm_vcpu *vcpu)
 			ret = kvmhv_copy_tofrom_guest_nested(vcpu);
 		break;
 	case H_PAGE_INIT:
+		kvm_vcpu_srcu_read_lock(vcpu);
 		ret = kvmppc_h_page_init(vcpu, kvmppc_get_gpr(vcpu, 4),
 					 kvmppc_get_gpr(vcpu, 5),
 					 kvmppc_get_gpr(vcpu, 6));
+		kvm_vcpu_srcu_read_unlock(vcpu);
 		break;
 	case H_SVM_PAGE_IN:
 		ret = H_UNSUPPORTED;
-- 
2.52.0