[PATCH 5.15 126/230] KVM: Initialize debugfs_dentry when a VM is created to avoid NULL deref

Greg Kroah-Hartman
2022-07-11T09:52:05Z

From: Sean Christopherson <seanjc@google.com>

[ Upstream commit 5c697c367a66307a5d943c3449421aff2aa3ca4a ]

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>
Reviewed-by: Oliver Upton <oupton@google.com>
Message-Id: <20220415004622.2207751-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 9134ae252d7c..9eac68ae291e 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -934,12 +934,6 @@ 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;

@@ -1055,6 +1049,12 @@ 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))
--
2.35.1