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

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Date:   Thu, 5 Dec 2019 23:29:28 +0100
From:   Pavel Machek <pavel@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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

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...

Best regards,
								Pavel

> +++ b/virt/kvm/kvm_main.c
> @@ -3990,7 +3990,7 @@ 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) {

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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