linux-kernel - Re: [PATCH AUTOSEL 4.19 40/57] kvm: properly check debugfs dentry before using it

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Date:   Sat, 30 Mar 2019 06:43:50 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH AUTOSEL 4.19 40/57] kvm: properly check debugfs dentry
 before using it

On Fri, Mar 29, 2019 at 09:28:33PM -0400, Sasha Levin wrote:
> 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.
> 
> Reported-by: Eric Biggers <ebiggers@...nel.org>
> Reported-and-tested-by: syzbot+7857962b4d45e602b8ad@...kaller.appspotmail.com
> Cc: "Radim Krčmář" <rkrcmar@...hat.com>
> Cc: kvm@...r.kernel.org
> Acked-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  virt/kvm/kvm_main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index c436d95fd7aa..fb74f994cb63 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3912,7 +3912,7 @@ 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) {
> -- 
> 2.19.1
> 

Same thing here, this was a 5.0-only thing, no need for it to be
backported, please drop it from all trees.

thanks,

greg k-h