KVM: x86/mmu: Check that root is valid/loaded when pre-faulting SPTEs

Error out if kvm_mmu_reload() fails when pre-faulting memory, as trying to
fault-in SPTEs will fail miserably due to root.hpa pointing at garbage.

Note, kvm_mmu_reload() can return -EIO and thus trigger the WARN on -EIO
in kvm_vcpu_pre_fault_memory(), but all such paths also WARN, i.e. the
WARN isn't user-triggerable and won't run afoul of warn-on-panic because
the kernel would already be panicking.

  BUG: unable to handle page fault for address: 000029ffffffffe8
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 0 P4D 0
  Oops: Oops: 0000 [#1] PREEMPT SMP
  CPU: 22 PID: 1069 Comm: pre_fault_memor Not tainted 6.10.0-rc7-332d2c1d713e-next-vm torvalds#548
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:is_page_fault_stale+0x3e/0xe0 [kvm]
  RSP: 0018:ffffc9000114bd48 EFLAGS: 00010206
  RAX: 00003fffffffffc0 RBX: ffff88810a07c080 RCX: ffffc9000114bd78
  RDX: ffff88810a07c080 RSI: ffffea0000000000 RDI: ffff88810a07c080
  RBP: ffffc9000114bd78 R08: 00007fa3c8c00000 R09: 8000000000000225
  R10: ffffea00043d7d80 R11: 0000000000000000 R12: ffff88810a07c080
  R13: 0000000100000000 R14: ffffc9000114be58 R15: 0000000000000000
  FS:  00007fa3c9da0740(0000) GS:ffff888277d80000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 000029ffffffffe8 CR3: 000000011d698000 CR4: 0000000000352eb0
  Call Trace:
   <TASK>
   kvm_tdp_page_fault+0xcc/0x160 [kvm]
   kvm_mmu_do_page_fault+0xfb/0x1f0 [kvm]
   kvm_arch_vcpu_pre_fault_memory+0xd0/0x1a0 [kvm]
   kvm_vcpu_ioctl+0x761/0x8c0 [kvm]
   __x64_sys_ioctl+0x82/0xb0
   do_syscall_64+0x5b/0x160
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
   </TASK>
  Modules linked in: kvm_intel kvm
  CR2: 000029ffffffffe8
  ---[ end trace 0000000000000000 ]---

Fixes: 6e01b76 ("KVM: x86: Implement kvm_arch_vcpu_pre_fault_memory()")
Reported-by: syzbot+23786faffb695f17edaa@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/0000000000002b84dc061dd73544@google.com
Reviewed-by: Kai Huang <kai.huang@intel.com>
Tested-by: xingwei lee <xrivendell7@gmail.com>
Tested-by: yuxin wang <wang1315768607@163.com>
Link: https://lore.kernel.org/r/20240723000211.3352304-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/mmu/mmu.c

@@ -4750,7 +4750,9 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
 	 * reload is efficient when called repeatedly, so we can do it on
 	 * every iteration.
 	 */
-	kvm_mmu_reload(vcpu);
+	r = kvm_mmu_reload(vcpu);
+	if (r)
+		return r;
 
 	if (kvm_arch_has_private_mem(vcpu->kvm) &&
 	    kvm_mem_is_private(vcpu->kvm, gpa_to_gfn(range->gpa)))