linux-kernel - [PATCH] kvm: properly check debugfs dentry before using it

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Message-ID: <20190228150805.GA31904@kroah.com>
Date:   Thu, 28 Feb 2019 16:08:05 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Eric Biggers <ebiggers@...nel.org>
Cc:     kvm@...r.kernel.org,
        syzbot <syzbot+7857962b4d45e602b8ad@...kaller.appspotmail.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        penguin-kernel@...ove.sakura.ne.jp,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk
Subject: [PATCH] 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@...nel.org>
Reported-and-tested-by: syzbot+7857962b4d45e602b8ad@...kaller.appspotmail.com
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: "Radim Krčmář" <rkrcmar@...hat.com>
Cc: kvm@...r.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---

Paolo, this should be merged into 5.0-final, and if not there, then
5.1-rc1 and then backported to 5.0 through the stable tree.  If you
want me to send this to Linus, I will be glad to do so.

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 585845203db8..076bc38963bf 100644
--- 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) {