linux-hardening - Re: [PATCH] MIPS: boot/compressed: Use array reference for image bounds

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Date:   Wed, 9 Mar 2022 13:11:43 -0600
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-mips@...r.kernel.org, kernel test robot <lkp@...el.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] MIPS: boot/compressed: Use array reference for image
 bounds

On Wed, Mar 09, 2022 at 10:50:32AM -0800, Kees Cook wrote:
> As done with other image addresses in other architectures, use an
> explicit flexible array instead of "address of char", which can trip
> bounds checking done by the compiler. Found when building with
> -Warray-bounds:
> 
> In file included from ./include/linux/byteorder/little_endian.h:5,
>                  from ./arch/mips/include/uapi/asm/byteorder.h:15,
>                  from ./arch/mips/include/asm/bitops.h:21,
>                  from ./include/linux/bitops.h:33,
>                  from ./include/linux/kernel.h:22,
>                  from arch/mips/boot/compressed/decompress.c:13:
> arch/mips/boot/compressed/decompress.c: In function 'decompress_kernel':
> ./include/asm-generic/unaligned.h:14:8: warning: array subscript -1 is outside array bounds of 'unsigned char[1]' [-Warray-bounds]
>    14 |  __pptr->x;        \
>       |  ~~~~~~^~~
> ./include/uapi/linux/byteorder/little_endian.h:35:51: note: in definition of macro '__le32_to_cpu'
>    35 | #define __le32_to_cpu(x) ((__force __u32)(__le32)(x))
>       |                                                   ^
> ./include/asm-generic/unaligned.h:32:21: note: in expansion of macro '__get_unaligned_t'
>    32 |  return le32_to_cpu(__get_unaligned_t(__le32, p));
>       |                     ^~~~~~~~~~~~~~~~~
> arch/mips/boot/compressed/decompress.c:29:37: note: while referencing '__image_end'
>    29 | extern unsigned char __image_begin, __image_end;
>       |                                     ^~~~~~~~~~~
> 
> Cc: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
> Cc: Randy Dunlap <rdunlap@...radead.org>
> Cc: linux-mips@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>

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

Thanks
--
Gustavo

> ---
>  arch/mips/boot/compressed/decompress.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/mips/boot/compressed/decompress.c b/arch/mips/boot/compressed/decompress.c
> index aae1346a509a..5b38a802e101 100644
> --- a/arch/mips/boot/compressed/decompress.c
> +++ b/arch/mips/boot/compressed/decompress.c
> @@ -26,7 +26,7 @@ unsigned long free_mem_ptr;
>  unsigned long free_mem_end_ptr;
>  
>  /* The linker tells us where the image is. */
> -extern unsigned char __image_begin, __image_end;
> +extern unsigned char __image_begin[], __image_end[];
>  
>  /* debug interfaces  */
>  #ifdef CONFIG_DEBUG_ZBOOT
> @@ -91,9 +91,9 @@ void decompress_kernel(unsigned long boot_heap_start)
>  {
>  	unsigned long zimage_start, zimage_size;
>  
> -	zimage_start = (unsigned long)(&__image_begin);
> -	zimage_size = (unsigned long)(&__image_end) -
> -	    (unsigned long)(&__image_begin);
> +	zimage_start = (unsigned long)(__image_begin);
> +	zimage_size = (unsigned long)(__image_end) -
> +	    (unsigned long)(__image_begin);
>  
>  	puts("zimage at:     ");
>  	puthex(zimage_start);
> @@ -121,7 +121,7 @@ void decompress_kernel(unsigned long boot_heap_start)
>  		dtb_size = fdt_totalsize((void *)&__appended_dtb);
>  
>  		/* last four bytes is always image size in little endian */
> -		image_size = get_unaligned_le32((void *)&__image_end - 4);
> +		image_size = get_unaligned_le32((void *)__image_end - 4);
>  
>  		/* The device tree's address must be properly aligned  */
>  		image_size = ALIGN(image_size, STRUCT_ALIGNMENT);
> -- 
> 2.32.0
>