linux-hardening - Re: [PATCH] KVM: PPC: Replace zero-length array with flexible array member

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Message-ID: <bb4faf3a-9fe9-280b-cb4c-e4904b0b2a8f@embeddedor.com>
Date:   Mon, 20 Sep 2021 19:05:07 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Len Baker <len.baker@....com>, Paul Mackerras <paulus@...abs.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Kees Cook <keescook@...omium.org>, kvm-ppc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-hardening@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: PPC: Replace zero-length array with flexible array
 member



On 9/18/21 09:21, Len Baker wrote:
> There is a regular need in the kernel to provide a way to declare having
> a dynamically sized set of trailing elements in a structure. Kernel code
> should always use "flexible array members" [1] for these cases. The
> older style of one-element or zero-length arrays should no longer be
> used[2].
> 
> Also, make use of the struct_size() helper in kzalloc().
> 
> [1] https://en.wikipedia.org/wiki/Flexible_array_member
> [2] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays
> 
> Signed-off-by: Len Baker <len.baker@....com>

Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Thanks
--
Gustavo

> ---
>  arch/powerpc/include/asm/kvm_host.h | 2 +-
>  arch/powerpc/kvm/book3s_64_vio.c    | 3 +--
>  2 files changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
> index 080a7feb7731..3aed653373a5 100644
> --- a/arch/powerpc/include/asm/kvm_host.h
> +++ b/arch/powerpc/include/asm/kvm_host.h
> @@ -190,7 +190,7 @@ struct kvmppc_spapr_tce_table {
>  	u64 size;		/* window size in pages */
>  	struct list_head iommu_tables;
>  	struct mutex alloc_lock;
> -	struct page *pages[0];
> +	struct page *pages[];
>  };
> 
>  /* XICS components, defined in book3s_xics.c */
> diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c
> index 6365087f3160..d42b4b6d4a79 100644
> --- a/arch/powerpc/kvm/book3s_64_vio.c
> +++ b/arch/powerpc/kvm/book3s_64_vio.c
> @@ -295,8 +295,7 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
>  		return ret;
> 
>  	ret = -ENOMEM;
> -	stt = kzalloc(sizeof(*stt) + npages * sizeof(struct page *),
> -		      GFP_KERNEL);
> +	stt = kzalloc(struct_size(stt, pages, npages), GFP_KERNEL);
>  	if (!stt)
>  		goto fail_acct;
> 
> --
> 2.25.1
>