Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:
> After commit 63d0434 ("KVM: x86: move kvm_create_vcpu_debugfs after
> last failure point") we are creating the pre-vCPU debugfs files
> after the creation of the vCPU file descriptor. This makes it
> possible for userspace to reach kvm_vcpu_release before
> kvm_create_vcpu_debugfs has finished. The vcpu->debugfs_dentry
> then does not have any associated inode anymore, and this causes
> a NULL-pointer dereference in debugfs_create_file.
>
> The solution is simply to avoid removing the files; they are
> cleaned up when the VM file descriptor is closed (and that must be
> after KVM_CREATE_VCPU returns). We can stop storing the dentry
> in struct kvm_vcpu too, because it is not needed anywhere after
> kvm_create_vcpu_debugfs returns.
>
> Reported-by: syzbot+705f4401d5a93a59b87d@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 63d04348371b ("KVM: x86: move kvm_create_vcpu_debugfs after last failure point")
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
> arch/arm64/kvm/arm.c | 5 -----
> arch/x86/kvm/debugfs.c | 10 +++++-----
> include/linux/kvm_host.h | 3 +--
> virt/kvm/kvm_main.c | 8 ++++----
> 4 files changed, 10 insertions(+), 16 deletions(-)
>
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 7a57381c05e8..45276ed50dd6 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -144,11 +144,6 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
> return ret;
> }
>
> -int kvm_arch_create_vcpu_debugfs(struct kvm_vcpu *vcpu)
> -{
> - return 0;
> -}
> -
> vm_fault_t kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
> {
> return VM_FAULT_SIGBUS;
> diff --git a/arch/x86/kvm/debugfs.c b/arch/x86/kvm/debugfs.c
> index 018aebce33ff..7e818d64bb4d 100644
> --- a/arch/x86/kvm/debugfs.c
> +++ b/arch/x86/kvm/debugfs.c
> @@ -43,22 +43,22 @@ static int vcpu_get_tsc_scaling_frac_bits(void *data, u64 *val)
>
> DEFINE_SIMPLE_ATTRIBUTE(vcpu_tsc_scaling_frac_fops, vcpu_get_tsc_scaling_frac_bits, NULL, "%llu\n");
>
> -void kvm_arch_create_vcpu_debugfs(struct kvm_vcpu *vcpu)
> +void kvm_arch_create_vcpu_debugfs(struct kvm_vcpu *vcpu, struct dentry *debugfs_dentry)
> {
> - debugfs_create_file("tsc-offset", 0444, vcpu->debugfs_dentry, vcpu,
> + debugfs_create_file("tsc-offset", 0444, debugfs_dentry, vcpu,
> &vcpu_tsc_offset_fops);
>
> if (lapic_in_kernel(vcpu))
> debugfs_create_file("lapic_timer_advance_ns", 0444,
> - vcpu->debugfs_dentry, vcpu,
> + debugfs_dentry, vcpu,
> &vcpu_timer_advance_ns_fops);
>
> if (kvm_has_tsc_control) {
> debugfs_create_file("tsc-scaling-ratio", 0444,
> - vcpu->debugfs_dentry, vcpu,
> + debugfs_dentry, vcpu,
> &vcpu_tsc_scaling_fops);
> debugfs_create_file("tsc-scaling-ratio-frac-bits", 0444,
> - vcpu->debugfs_dentry, vcpu,
> + debugfs_dentry, vcpu,
> &vcpu_tsc_scaling_frac_fops);
> }
> }
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index f43b59b1294c..d38d6b9c24be 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -318,7 +318,6 @@ struct kvm_vcpu {
> bool preempted;
> bool ready;
> struct kvm_vcpu_arch arch;
> - struct dentry *debugfs_dentry;
> };
>
> static inline int kvm_vcpu_exiting_guest_mode(struct kvm_vcpu *vcpu)
> @@ -888,7 +887,7 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu);
> void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu);
>
> #ifdef __KVM_HAVE_ARCH_VCPU_DEBUGFS
> -void kvm_arch_create_vcpu_debugfs(struct kvm_vcpu *vcpu);
> +void kvm_arch_create_vcpu_debugfs(struct kvm_vcpu *vcpu, struct dentry *debugfs_dentry);
> #endif
>
> int kvm_arch_hardware_enable(void);
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 7fa1e38e1659..3577eb84eac0 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2973,7 +2973,6 @@ static int kvm_vcpu_release(struct inode *inode, struct file *filp)
> {
> struct kvm_vcpu *vcpu = filp->private_data;
>
> - debugfs_remove_recursive(vcpu->debugfs_dentry);
> kvm_put_kvm(vcpu->kvm);
> return 0;
> }
> @@ -3000,16 +2999,17 @@ static int create_vcpu_fd(struct kvm_vcpu *vcpu)
> static void kvm_create_vcpu_debugfs(struct kvm_vcpu *vcpu)
> {
> #ifdef __KVM_HAVE_ARCH_VCPU_DEBUGFS
> + struct dentry *debugfs_dentry;
> char dir_name[ITOA_MAX_LEN * 2];
>
> if (!debugfs_initialized())
> return;
>
> snprintf(dir_name, sizeof(dir_name), "vcpu%d", vcpu->vcpu_id);
> - vcpu->debugfs_dentry = debugfs_create_dir(dir_name,
> - vcpu->kvm->debugfs_dentry);
> + debugfs_dentry = debugfs_create_dir(dir_name,
> + vcpu->kvm->debugfs_dentry);
>
> - kvm_arch_create_vcpu_debugfs(vcpu);
> + kvm_arch_create_vcpu_debugfs(vcpu, debugfs_dentry);
> #endif
> }
FWIW,
Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Thanks!
--
Vitaly
