linux-kernel - Re: [PATCH] efi: Replace zero-length array and use struct

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Date:   Fri, 29 May 2020 01:31:54 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Ard Biesheuvel <ardb@...nel.org>, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: Re: [PATCH] efi: Replace zero-length array and use struct_size()
 helper

On Wed, May 27, 2020 at 12:14:25PM -0500, Gustavo A. R. Silva wrote:
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
> 
> struct foo {
>         int stuff;
>         struct boo array[];
> };
> 
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on.
> 
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
> 
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
> 
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in
> which the sizeof operator is being incorrectly/erroneously applied to
> zero-length arrays and the result is zero. Such instances may be hiding
> some bugs. So, this work (flexible-array member conversions) will also
> help to get completely rid of those sorts of issues.
> 
> Lastly, make use of the sizeof_field() helper instead of an open-coded
> version.
> 
> This issue was found with the help of Coccinelle and audited _manually_.
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Reviewed-by: Kees Cook <keescook@...omium.org>

> ---
>  drivers/firmware/efi/efi.c | 3 ++-
>  include/linux/efi.h        | 7 ++-----
>  2 files changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> index 7f1657b6c30df..edc5d36caf54e 100644
> --- a/drivers/firmware/efi/efi.c
> +++ b/drivers/firmware/efi/efi.c
> @@ -622,7 +622,8 @@ int __init efi_config_parse_tables(const efi_config_table_t *config_tables,
>  			rsv = (void *)(p + prsv % PAGE_SIZE);
>  
>  			/* reserve the entry itself */
> -			memblock_reserve(prsv, EFI_MEMRESERVE_SIZE(rsv->size));
> +			memblock_reserve(prsv,
> +					 struct_size(rsv, entry, rsv->size));
>  
>  			for (i = 0; i < atomic_read(&rsv->count); i++) {
>  				memblock_reserve(rsv->entry[i].base,
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index c45ac969ea4eb..328cc52a5fd45 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -1234,14 +1234,11 @@ struct linux_efi_memreserve {
>  	struct {
>  		phys_addr_t	base;
>  		phys_addr_t	size;
> -	} entry[0];
> +	} entry[];
>  };
>  
> -#define EFI_MEMRESERVE_SIZE(count) (sizeof(struct linux_efi_memreserve) + \
> -	(count) * sizeof(((struct linux_efi_memreserve *)0)->entry[0]))
> -
>  #define EFI_MEMRESERVE_COUNT(size) (((size) - sizeof(struct linux_efi_memreserve)) \
> -	/ sizeof(((struct linux_efi_memreserve *)0)->entry[0]))
> +	/ sizeof_field(struct linux_efi_memreserve, entry[0]))

Whoa. This is kind of a "reverse struct_size()". I wonder if any other
places in the kernel do a similar calculation?

-- 
Kees Cook