KVM: PPC: Protect kvm_vcpu_read_guest with srcu locks

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 <[e-mail]>
Signed-off-by: Paul Mackerras <[e-mail]>

arch/powerpc/kvm/book3s_64_mmu_radix.c

···
 			return -EINVAL;
 		/* Read the entry from guest memory */
 		addr = base + (index * sizeof(rpte));
+		vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
 		ret = kvm_read_guest(kvm, addr, &rpte, sizeof(rpte));
+		srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
 		if (ret) {
 			if (pte_ret_p)
 				*pte_ret_p = addr;
···
 
 	/* Read the table to find the root of the radix tree */
 	ptbl = (table & PRTB_MASK) + (table_index * sizeof(entry));
+	vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
 	ret = kvm_read_guest(kvm, ptbl, &entry, sizeof(entry));
+	srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
 	if (ret)
 		return ret;
 

arch/powerpc/kvm/book3s_hv_nested.c

···
 
 	/* copy parameters in */
 	hv_ptr = kvmppc_get_gpr(vcpu, 4);
+	regs_ptr = kvmppc_get_gpr(vcpu, 5);
+	vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
 	err = kvm_vcpu_read_guest(vcpu, hv_ptr, &l2_hv,
-				  sizeof(struct hv_guest_state));
+				  sizeof(struct hv_guest_state)) ||
+		kvm_vcpu_read_guest(vcpu, regs_ptr, &l2_regs,
+				    sizeof(struct pt_regs));
+	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
 	if (err)
 		return H_PARAMETER;
+
 	if (kvmppc_need_byteswap(vcpu))
 		byteswap_hv_regs(&l2_hv);
 	if (l2_hv.version != HV_GUEST_STATE_VERSION)
 		return H_P2;
 
-	regs_ptr = kvmppc_get_gpr(vcpu, 5);
-	err = kvm_vcpu_read_guest(vcpu, regs_ptr, &l2_regs,
-				  sizeof(struct pt_regs));
-	if (err)
-		return H_PARAMETER;
 	if (kvmppc_need_byteswap(vcpu))
 		byteswap_pt_regs(&l2_regs);
 	if (l2_hv.vcpu_token >= NR_CPUS)
···
 		byteswap_hv_regs(&l2_hv);
 		byteswap_pt_regs(&l2_regs);
 	}
+	vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
 	err = kvm_vcpu_write_guest(vcpu, hv_ptr, &l2_hv,
-				   sizeof(struct hv_guest_state));
-	if (err)
-		return H_AUTHORITY;
-	err = kvm_vcpu_write_guest(vcpu, regs_ptr, &l2_regs,
+				   sizeof(struct hv_guest_state)) ||
+		kvm_vcpu_write_guest(vcpu, regs_ptr, &l2_regs,
 				   sizeof(struct pt_regs));
+	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
 	if (err)
 		return H_AUTHORITY;
 
···
 			goto not_found;
 
 		/* Write what was loaded into our buffer back to the L1 guest */
+		vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
 		rc = kvm_vcpu_write_guest(vcpu, gp_to, buf, n);
+		srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
 		if (rc)
 			goto not_found;
 	} else {
 		/* Load the data to be stored from the L1 guest into our buf */
+		vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
 		rc = kvm_vcpu_read_guest(vcpu, gp_from, buf, n);
+		srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
 		if (rc)
 			goto not_found;
 
···
 
 	ret = -EFAULT;
 	ptbl_addr = (kvm->arch.l1_ptcr & PRTB_MASK) + (gp->l1_lpid << 4);
-	if (gp->l1_lpid < (1ul << ((kvm->arch.l1_ptcr & PRTS_MASK) + 8)))
+	if (gp->l1_lpid < (1ul << ((kvm->arch.l1_ptcr & PRTS_MASK) + 8))) {
+		int srcu_idx = srcu_read_lock(&kvm->srcu);
 		ret = kvm_read_guest(kvm, ptbl_addr,
 				     &ptbl_entry, sizeof(ptbl_entry));
+		srcu_read_unlock(&kvm->srcu, srcu_idx);
+	}
 	if (ret) {
 		gp->l1_gr_to_hr = 0;
 		gp->process_table = 0;

arch/powerpc/kvm/book3s_rtas.c

···
 	 */
 	args_phys = kvmppc_get_gpr(vcpu, 4) & KVM_PAM;
 
+	vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
 	rc = kvm_read_guest(vcpu->kvm, args_phys, &args, sizeof(args));
+	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
 	if (rc)
 		goto fail;
 

arch/powerpc/kvm/powerpc.c

···
 		return EMULATE_DONE;
 	}
 
-	if (kvm_read_guest(vcpu->kvm, pte.raddr, ptr, size))
+	vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
+	rc = kvm_read_guest(vcpu->kvm, pte.raddr, ptr, size);
+	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
+	if (rc)
 		return EMULATE_DO_MMIO;
 
 	return EMULATE_DONE;