[PATCH v2] KVM: x86: Check for rmaps allocation

[Nikunj A Dadhania <nikunj@amd.com>]

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

Return early when rmaps are not present.

Reported-by: Vasant Hegde <vasant.hegde@amd.com>
Tested-by: Vasant Hegde <vasant.hegde@amd.com>
Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
---

v1: https://lore.kernel.org/kvm/20220104092814.11553-1-nikunj@amd.com/T/#u

Check the rmaps inside kvm_mmu_rmaps_stat_show() as the rmaps can be
allocated dynamically (Peter Xu)

 arch/x86/kvm/debugfs.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/debugfs.c b/arch/x86/kvm/debugfs.c
index 543a8c04025c..9240b3b7f8dd 100644
--- a/arch/x86/kvm/debugfs.c
+++ b/arch/x86/kvm/debugfs.c
@@ -95,6 +95,9 @@ static int kvm_mmu_rmaps_stat_show(struct seq_file *m, void *v)
 	unsigned int *log[KVM_NR_PAGE_SIZES], *cur;
 	int i, j, k, l, ret;
 
+	if (!kvm_memslots_have_rmaps(kvm))
+		return 0;
+
 	ret = -ENOMEM;
 	memset(log, 0, sizeof(log));
 	for (i = 0; i < KVM_NR_PAGE_SIZES; i++) {
-- 
2.32.0