linux-hardening - Re: [PATCH] efi: Replace efi_memory_attributes_table

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Message-ID: <d2b9e11a-749b-47cd-9cc2-0734ec5849b0@embeddedor.com>
Date: Wed, 10 Jul 2024 17:32:13 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Kees Cook <kees@...nel.org>, Ard Biesheuvel <ardb@...nel.org>
Cc: linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH] efi: Replace efi_memory_attributes_table_t 0-sized array
 with flexible array



On 10/07/24 16:55, Kees Cook wrote:
> While efi_memory_attributes_table_t::entry isn't used directly as an
> array, it is used as a base for pointer arithmetic. The type is wrong
> as it's not technically an array of efi_memory_desc_t's; they could be
> larger. Regardless, leave the type unchanged and remove the old style
> "0" array size. Additionally replace the open-coded entry offset code
> with the existing efi_early_memdesc_ptr() helper.
> 
> Signed-off-by: Kees Cook <kees@...nel.org>
> ---
> Cc: Ard Biesheuvel <ardb@...nel.org>
> Cc: linux-efi@...r.kernel.org
> ---
>   drivers/firmware/efi/memattr.c | 2 +-
>   include/linux/efi.h            | 6 +++++-
>   2 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/firmware/efi/memattr.c b/drivers/firmware/efi/memattr.c
> index ab85bf8e165a..01142604e8df 100644
> --- a/drivers/firmware/efi/memattr.c
> +++ b/drivers/firmware/efi/memattr.c
> @@ -164,7 +164,7 @@ int __init efi_memattr_apply_permissions(struct mm_struct *mm,
>   		bool valid;
>   		char buf[64];
>   
> -		valid = entry_is_valid((void *)tbl->entry + i * tbl->desc_size,
> +		valid = entry_is_valid(efi_early_memdesc_ptr(tbl->entry, tbl->desc_size, i),
>   				       &md);
>   		size = md.num_pages << EFI_PAGE_SHIFT;
>   		if (efi_enabled(EFI_DBG) || !valid)
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index 418e555459da..b06639c4f6a5 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -607,7 +607,11 @@ typedef struct {
>   	u32 num_entries;
>   	u32 desc_size;
>   	u32 flags;
> -	efi_memory_desc_t entry[0];
> +	/*
> +	 * There are @num_entries following, each of size @desc_size bytes,
> +	 * including an efi_memory_desc_t header.
> +	 */
> +	efi_memory_desc_t entry[];

a candidate for future __counted_by(num_entries * desc_size) ? :p

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

Thanks
-- 
Gustavo

>   } efi_memory_attributes_table_t;
>   
>   typedef struct {