Initialize debugfs_entry to its semi-magical -ENOENT value when the VM
is created.  KVM's teardown when VM creation fails is kludgy and calls
kvm_uevent_notify_change() and kvm_destroy_vm_debugfs() even if KVM never
attempted kvm_create_vm_debugfs().  Because debugfs_entry is zero
initialized, the IS_ERR() checks pass and KVM derefs a NULL pointer.

  BUG: kernel NULL pointer dereference, address: 0000000000000018
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 1068b1067 P4D 1068b1067 PUD 1068b0067 PMD 0
  Oops: 0000 [#1] SMP
  CPU: 0 PID: 871 Comm: repro Not tainted 5.18.0-rc1+ #825
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:__dentry_path+0x7b/0x130
  Call Trace:
   <TASK>
   dentry_path_raw+0x42/0x70
   kvm_uevent_notify_change.part.0+0x10c/0x200 [kvm]
   kvm_put_kvm+0x63/0x2b0 [kvm]
   kvm_dev_ioctl+0x43a/0x920 [kvm]
   __x64_sys_ioctl+0x83/0xb0
   do_syscall_64+0x31/0x50
   entry_SYSCALL_64_after_hwframe+0x44/0xae
   </TASK>
  Modules linked in: kvm_intel kvm irqbypass

Fixes: a44a4cc1c969 ("KVM: Don't create VM debugfs files outside of the VM directory")
Cc: stable@vger.kernel.org
Cc: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oupton@google.com>
Reported-by: syzbot+df6fbbd2ee39f21289ef@syzkaller.appspotmail.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 virt/kvm/kvm_main.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index dfb7dabdbc63..d292c4397579 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -955,12 +955,6 @@ static int kvm_create_vm_debugfs(struct kvm *kvm, int fd)
 	int kvm_debugfs_num_entries = kvm_vm_stats_header.num_desc +
 				      kvm_vcpu_stats_header.num_desc;
 
-	/*
-	 * Force subsequent debugfs file creations to fail if the VM directory
-	 * is not created.
-	 */
-	kvm->debugfs_dentry = ERR_PTR(-ENOENT);
-
 	if (!debugfs_initialized())
 		return 0;
 
@@ -1081,6 +1075,12 @@ static struct kvm *kvm_create_vm(unsigned long type)
 
 	BUILD_BUG_ON(KVM_MEM_SLOTS_NUM > SHRT_MAX);
 
+	/*
+	 * Force subsequent debugfs file creations to fail if the VM directory
+	 * is not created (by kvm_create_vm_debugfs()).
+	 */
+	kvm->debugfs_dentry = ERR_PTR(-ENOENT);
+
 	if (init_srcu_struct(&kvm->srcu))
 		goto out_err_no_srcu;
 	if (init_srcu_struct(&kvm->irq_srcu))

base-commit: 150866cd0ec871c765181d145aa0912628289c8a
-- 
2.36.0.rc0.470.gd361397f0d-goog

Queued, thanks.

Paolo

Hi Sean,

On Thu, Apr 14, 2022 at 5:46 PM Sean Christopherson <seanjc@google.com> wrote:
>
> Initialize debugfs_entry to its semi-magical -ENOENT value when the VM
> is created.  KVM's teardown when VM creation fails is kludgy and calls
> kvm_uevent_notify_change() and kvm_destroy_vm_debugfs() even if KVM never
> attempted kvm_create_vm_debugfs().  Because debugfs_entry is zero

Boo! I've got a few patches to bring kvm_create_vm_debugfs() in line
with the rest of the VM initialization. Kinda gross it is done in the
ioctl body.

> initialized, the IS_ERR() checks pass and KVM derefs a NULL pointer.
>
>   BUG: kernel NULL pointer dereference, address: 0000000000000018
>   #PF: supervisor read access in kernel mode
>   #PF: error_code(0x0000) - not-present page
>   PGD 1068b1067 P4D 1068b1067 PUD 1068b0067 PMD 0
>   Oops: 0000 [#1] SMP
>   CPU: 0 PID: 871 Comm: repro Not tainted 5.18.0-rc1+ #825
>   Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
>   RIP: 0010:__dentry_path+0x7b/0x130
>   Call Trace:
>    <TASK>
>    dentry_path_raw+0x42/0x70
>    kvm_uevent_notify_change.part.0+0x10c/0x200 [kvm]
>    kvm_put_kvm+0x63/0x2b0 [kvm]
>    kvm_dev_ioctl+0x43a/0x920 [kvm]
>    __x64_sys_ioctl+0x83/0xb0
>    do_syscall_64+0x31/0x50
>    entry_SYSCALL_64_after_hwframe+0x44/0xae
>    </TASK>
>   Modules linked in: kvm_intel kvm irqbypass
>
> Fixes: a44a4cc1c969 ("KVM: Don't create VM debugfs files outside of the VM directory")
> Cc: stable@vger.kernel.org
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Oliver Upton <oupton@google.com>
> Reported-by: syzbot+df6fbbd2ee39f21289ef@syzkaller.appspotmail.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Looks good, grats to the bots for finding my dirty laundry.

Reviewed-by: Oliver Upton <oupton@google.com>