linux-hardening - Re: [PATCH][next] iommu/virtio: Add __counted_by for struct viommu_request and use struct

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Date: Mon, 9 Oct 2023 16:48:55 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Jean-Philippe Brucker <jean-philippe@...aro.org>, Joerg Roedel <joro@...tes.org>, 
	Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>, 
	virtualization@...ts.linux-foundation.org, iommu@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] iommu/virtio: Add __counted_by for struct
 viommu_request and use struct_size()

On Mon, Oct 9, 2023 at 11:24 AM Gustavo A. R. Silva
<gustavoars@...nel.org> wrote:
>
> Prepare for the coming implementation by GCC and Clang of the __counted_by
> attribute. Flexible array members annotated with __counted_by can have
> their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for
> array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
> functions).
>
> While there, use struct_size() helper, instead of the open-coded
> version, to calculate the size for the allocation of the whole
> flexible structure, including of course, the flexible-array member.
>
> This code was found with the help of Coccinelle, and audited and
> fixed manually.
>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
>  drivers/iommu/virtio-iommu.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/iommu/virtio-iommu.c b/drivers/iommu/virtio-iommu.c
> index 17dcd826f5c2..379ebe03efb6 100644
> --- a/drivers/iommu/virtio-iommu.c
> +++ b/drivers/iommu/virtio-iommu.c
> @@ -85,7 +85,7 @@ struct viommu_request {
>         void                            *writeback;
>         unsigned int                    write_offset;
>         unsigned int                    len;
> -       char                            buf[];
> +       char                            buf[] __counted_by(len);
>  };
>
>  #define VIOMMU_FAULT_RESV_MASK         0xffffff00
> @@ -230,7 +230,7 @@ static int __viommu_add_req(struct viommu_dev *viommu, void *buf, size_t len,
>         if (write_offset <= 0)
>                 return -EINVAL;
>
> -       req = kzalloc(sizeof(*req) + len, GFP_ATOMIC);
> +       req = kzalloc(struct_size(req, buf, len), GFP_ATOMIC);
>         if (!req)
>                 return -ENOMEM;
>
> --
> 2.34.1
>
>

The __counted_by annotation and the usage of struct_size look right!

Reviewed-by: Justin Stitt <justinstitt@...gle.com>