kvm: properly check debugfs dentry before using it

BugLink: https://bugs.launchpad.net/bugs/1855787
[ Upstream commit 8ed0579c12b2fe56a1fac2f712f58fc26c1dc49b ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 79ed514e32bdf0c9488b4062cd1312946e6c1ecb..50a9eb1b62f449a34b1298d5ac1a0d26f7be6220 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3986,7 +3986,7 @@ 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) {