Re: [PATCH] binfmt_flat: Fix corruption when not offsetting data start

From: Greg Ungerer <gerg@linux-m68k.org>
To: Stefan O'Rear <sorear@fastmail.com>,
	linux-mm@kvack.org, linux-riscv@lists.infradead.org
Cc: Eric Biederman <ebiederm@xmission.com>,
	Kees Cook <keescook@chromium.org>,
	Damien Le Moal <damien.lemoal@wdc.com>,
	Waldemar Brodkorb <wbx@openadk.org>
Subject: Re: [PATCH] binfmt_flat: Fix corruption when not offsetting data start
Date: Thu, 4 Apr 2024 17:40:28 +1000	[thread overview]
Message-ID: <edb65612-3772-485a-b04f-28d2a453f3b3@linux-m68k.org> (raw)
In-Reply-To: <20240326032037.2478816-1-sorear@fastmail.com>

Hi Stefan,

On 26/3/24 13:20, Stefan O'Rear wrote:
> Commit 04d82a6d0881 ("binfmt_flat: allow not offsetting data start")
> introduced a RISC-V specific variant of the FLAT format which does not
> allocate any space for the (obsolescent) array of shared library
> pointers. However, it did not disable the code which initializes the
> array, resulting in the corruption of sizeof(long) bytes before the DATA
> segment, generally the end of the TEXT segment.
> 
> Use CONFIG_BINFMT_FLAT_NO_DATA_START_OFFSET to guard initialization the
> shared library pointer region so that it will only be initialized if
> space is reserved for it.
> 
> Fixes: 04d82a6d0881 ("binfmt_flat: allow not offsetting data start")
> Signed-off-by: Stefan O'Rear <sorear@fastmail.com>
> Tested-by: Waldemar Brodkorb <wbx@openadk.org>
> ---
>   fs/binfmt_flat.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/fs/binfmt_flat.c b/fs/binfmt_flat.c
> index c26545d71d39..70c2b68988f4 100644
> --- a/fs/binfmt_flat.c
> +++ b/fs/binfmt_flat.c
> @@ -879,6 +879,7 @@ static int load_flat_binary(struct linux_binprm *bprm)
>   	if (res < 0)
>   		return res;
>   
> +#ifndef CONFIG_BINFMT_FLAT_NO_DATA_START_OFFSET
>   	/* Update data segment pointers for all libraries */
>   	for (i = 0; i < MAX_SHARED_LIBS; i++) {
                         ^^^^^^^^^^^^^^^

It may be cleaner (ie no # conditional required) if this was changed to
DATA_START_OFFSET_WORDS. So becoming:

         for (i = 0; i < DATA_START_OFFSET_WORDS; i++) {

My only concern is does this make it less clear what the code is doing?

Regards
Greg

>   		if (!libinfo.lib_list[i].loaded)
> @@ -893,6 +894,7 @@ static int load_flat_binary(struct linux_binprm *bprm)
>   				return -EFAULT;
>   		}
>   	}
> +#endif
>   
>   	set_binfmt(&flat_format);
>   

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