linux-hardening - Re: [PATCH] cxl/acpi: Annotate struct cxl_cxims_data with __counted

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Date: Thu, 28 Sep 2023 13:48:34 +0900
From: Justin Stitt <justinstitt@...gle.com>
To: Kees Cook <keescook@...omium.org>
Cc: Davidlohr Bueso <dave@...olabs.net>, Jonathan Cameron <jonathan.cameron@...wei.com>, 
	Dave Jiang <dave.jiang@...el.com>, Alison Schofield <alison.schofield@...el.com>, 
	Vishal Verma <vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>, 
	Dan Williams <dan.j.williams@...el.com>, linux-cxl@...r.kernel.org, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, Tom Rix <trix@...hat.com>, 
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] cxl/acpi: Annotate struct cxl_cxims_data with __counted_by

On Sat, Sep 23, 2023 at 2:53 AM Kees Cook <keescook@...omium.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 checking via CONFIG_UBSAN_BOUNDS
> (for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
> functions).
>
> As found with Coccinelle[1], add __counted_by for struct cxl_cxims_data.
> Additionally, since the element count member must be set before accessing
> the annotated flexible array member, move its initialization earlier.
>
> [1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci
>
> Cc: Davidlohr Bueso <dave@...olabs.net>
> Cc: Jonathan Cameron <jonathan.cameron@...wei.com>
> Cc: Dave Jiang <dave.jiang@...el.com>
> Cc: Alison Schofield <alison.schofield@...el.com>
> Cc: Vishal Verma <vishal.l.verma@...el.com>
> Cc: Ira Weiny <ira.weiny@...el.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: linux-cxl@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>

Looks good.

The reordering of the count assignment is crucial here otherwise
the runtime checks will trip when `cximsd->xormaps` is used as
memcpy destination.

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

> ---
>  drivers/cxl/acpi.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> index d1c559879dcc..40d055560e52 100644
> --- a/drivers/cxl/acpi.c
> +++ b/drivers/cxl/acpi.c
> @@ -14,7 +14,7 @@
>
>  struct cxl_cxims_data {
>         int nr_maps;
> -       u64 xormaps[];
> +       u64 xormaps[] __counted_by(nr_maps);
>  };
>
>  /*
> @@ -112,9 +112,9 @@ static int cxl_parse_cxims(union acpi_subtable_headers *header, void *arg,
>                               GFP_KERNEL);
>         if (!cximsd)
>                 return -ENOMEM;
> +       cximsd->nr_maps = nr_maps;
>         memcpy(cximsd->xormaps, cxims->xormap_list,
>                nr_maps * sizeof(*cximsd->xormaps));
> -       cximsd->nr_maps = nr_maps;
>         cxlrd->platform_data = cximsd;
>
>         return 0;
> --
> 2.34.1
>
>