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From:   Guenter Roeck <groeck@google.com>
Date:   Fri, 6 Jan 2023 20:39:06 -0800
Message-ID: <CABXOdTfENiCZF-G4TW7=hy7eVJSQJ5t63GzW4_mtezZzTs+18g@mail.gmail.com>
Subject: Re: [PATCH v2] firmware: coreboot: Check size of table entry and
 split memcpy
To:     Kees Cook <keescook@chromium.org>
Cc:     Jack Rosenthal <jrosenth@chromium.org>,
        Paul Menzel <pmenzel@molgen.mpg.de>,
        Guenter Roeck <groeck@chromium.org>,
        Julius Werner <jwerner@chromium.org>,
        Brian Norris <briannorris@chromium.org>,
        Stephen Boyd <swboyd@chromium.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
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On Fri, Jan 6, 2023 at 7:14 PM Kees Cook <keescook@chromium.org> wrote:
>
> The memcpy() of the data following a coreboot_table_entry couldn't
> be evaluated by the compiler under CONFIG_FORTIFY_SOURCE. To make it
> easier to reason about, add an explicit flexible array member to struct
> coreboot_device so the entire entry can be copied at once. Additionally,
> validate the sizes before copying. Avoids this run-time false positive
> warning:
>
>   memcpy: detected field-spanning write (size 168) of single field "&device->entry" at drivers/firmware/google/coreboot_table.c:103 (size 8)
>
> Reported-by: Paul Menzel <pmenzel@molgen.mpg.de>
> Link: https://lore.kernel.org/all/03ae2704-8c30-f9f0-215b-7cdf4ad35a9a@molgen.mpg.de/
> Cc: Jack Rosenthal <jrosenth@chromium.org>
> Cc: Guenter Roeck <groeck@chromium.org>
> Cc: Julius Werner <jwerner@chromium.org>
> Cc: Brian Norris <briannorris@chromium.org>
> Cc: Stephen Boyd <swboyd@chromium.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Kees Cook <keescook@chromium.org>

Reviewed-by: Guenter Roeck <groeck@chromium.org>

> ---
> v2: move flex array to struct coreboot_device (julius)
> v1: https://lore.kernel.org/lkml/20230106045327.never.413-kees@kernel.org
> ---
>  drivers/firmware/google/coreboot_table.c | 9 +++++++--
>  drivers/firmware/google/coreboot_table.h | 1 +
>  2 files changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/firmware/google/coreboot_table.c b/drivers/firmware/google/coreboot_table.c
> index 2652c396c423..564a3c908838 100644
> --- a/drivers/firmware/google/coreboot_table.c
> +++ b/drivers/firmware/google/coreboot_table.c
> @@ -93,14 +93,19 @@ static int coreboot_table_populate(struct device *dev, void *ptr)
>         for (i = 0; i < header->table_entries; i++) {
>                 entry = ptr_entry;
>
> -               device = kzalloc(sizeof(struct device) + entry->size, GFP_KERNEL);
> +               if (entry->size < sizeof(*entry)) {
> +                       dev_warn(dev, "coreboot table entry too small!\n");
> +                       return -EINVAL;
> +               }
> +
> +               device = kzalloc(sizeof(device->dev) + entry->size, GFP_KERNEL);
>                 if (!device)
>                         return -ENOMEM;
>
>                 device->dev.parent = dev;
>                 device->dev.bus = &coreboot_bus_type;
>                 device->dev.release = coreboot_device_release;
> -               memcpy(&device->entry, ptr_entry, entry->size);
> +               memcpy(device->raw, entry, entry->size);
>
>                 switch (device->entry.tag) {
>                 case LB_TAG_CBMEM_ENTRY:
> diff --git a/drivers/firmware/google/coreboot_table.h b/drivers/firmware/google/coreboot_table.h
> index 37f4d335a606..d814dca33a08 100644
> --- a/drivers/firmware/google/coreboot_table.h
> +++ b/drivers/firmware/google/coreboot_table.h
> @@ -79,6 +79,7 @@ struct coreboot_device {
>                 struct lb_cbmem_ref cbmem_ref;
>                 struct lb_cbmem_entry cbmem_entry;
>                 struct lb_framebuffer framebuffer;
> +               DECLARE_FLEX_ARRAY(u8, raw);
>         };
>  };
>
> --
> 2.34.1
>