linux-kernel - [PATCH] KVM: x86: Do not create mmu_rmaps

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Date:   Tue, 4 Jan 2022 14:58:14 +0530
From:   Nikunj A Dadhania <nikunj@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>
CC:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Peter Xu <peterx@...hat.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <nikunj@....com>,
        <vasant.hegde@....com>, <brijesh.singh@....com>
Subject: [PATCH] KVM: x86: Do not create mmu_rmaps_stat for TDP MMU

With TDP MMU being the default now, access to mmu_rmaps_stat debugfs
file causes following oops:

BUG: kernel NULL pointer dereference, address: 0000000000000000
PGD 0 P4D 0
Oops: 0000 [#1] PREEMPT SMP NOPTI
CPU: 7 PID: 3185 Comm: cat Not tainted 5.16.0-rc4+ #204
RIP: 0010:pte_list_count+0x6/0x40
 Call Trace:
  <TASK>
  ? kvm_mmu_rmaps_stat_show+0x15e/0x320
  seq_read_iter+0x126/0x4b0
  ? aa_file_perm+0x124/0x490
  seq_read+0xf5/0x140
  full_proxy_read+0x5c/0x80
  vfs_read+0x9f/0x1a0
  ksys_read+0x67/0xe0
  __x64_sys_read+0x19/0x20
  do_syscall_64+0x3b/0xc0
  entry_SYSCALL_64_after_hwframe+0x44/0xae
 RIP: 0033:0x7fca6fc13912

Create mmu_rmaps_stat debugfs file only when rmaps are created.

Reported-by: Vasant Hegde <vasant.hegde@....com>
Tested-by: Vasant Hegde <vasant.hegde@....com>
Signed-off-by: Nikunj A Dadhania <nikunj@....com>
---
 arch/x86/kvm/debugfs.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/debugfs.c b/arch/x86/kvm/debugfs.c
index 543a8c04025c..78bb09a3a7b7 100644
--- a/arch/x86/kvm/debugfs.c
+++ b/arch/x86/kvm/debugfs.c
@@ -180,7 +180,8 @@ static const struct file_operations mmu_rmaps_stat_fops = {
 
 int kvm_arch_create_vm_debugfs(struct kvm *kvm)
 {
-	debugfs_create_file("mmu_rmaps_stat", 0644, kvm->debugfs_dentry, kvm,
-			    &mmu_rmaps_stat_fops);
+	if (kvm_memslots_have_rmaps(kvm))
+		debugfs_create_file("mmu_rmaps_stat", 0644, kvm->debugfs_dentry, kvm,
+				    &mmu_rmaps_stat_fops);
 	return 0;
 }
-- 
2.32.0