kvm: properly check debugfs dentry before using it

debugfs can now report an error code if something went wrong instead of
just NULL.  So if the return value is to be used as a "real" dentry, it
needs to be checked if it is an error before dereferencing it.

This is now happening because of ff9fb72bc077 ("debugfs: return error
values, not NULL").  syzbot has found a way to trigger multiple debugfs
files attempting to be created, which fails, and then the error code
gets passed to dentry_path_raw() which obviously does not like it.

Reported-by: Eric Biggers <ebiggers@kernel.org>
Reported-and-tested-by: syzbot+7857962b4d45e602b8ad@syzkaller.appspotmail.com
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: kvm@vger.kernel.org
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 585845203db89e5d2bc90722fd75bfcf595671d9..076bc38963bf68d36733e4674a1f433a974de047 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4044,7 +4044,7 @@ static void kvm_uevent_notify_change(unsigned int type, struct kvm *kvm)
        }
        add_uevent_var(env, "PID=%d", kvm->userspace_pid);
 
-       if (kvm->debugfs_dentry) {
+       if (!IS_ERR_OR_NULL(kvm->debugfs_dentry)) {
                char *tmp, *p = kmalloc(PATH_MAX, GFP_KERNEL);
 
                if (p) {