linux-kernel - Re: [PATCH 4.19 242/321] kvm: properly check debugfs dentry before using it

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Message-ID: <20191206130348.GC1399220@kroah.com>
Date:   Fri, 6 Dec 2019 14:03:48 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Pavel Machek <pavel@...x.de>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Eric Biggers <ebiggers@...nel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>,
        syzbot+7857962b4d45e602b8ad@...kaller.appspotmail.com
Subject: Re: [PATCH 4.19 242/321] kvm: properly check debugfs dentry before
 using it

On Thu, Dec 05, 2019 at 11:29:28PM +0100, Pavel Machek wrote:
> Hi!
> 
> > From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > 
> > [ 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.
> 
> 4.19-stable does not contain patch ff9fb72bc077, so is this still good
> idea? It should not break anything, as it still uses IS_ERR_OR_NULL,
> but...

Yes it should as just testing for NULL was incorrect in the first place.

thanks,

greg k-h