Re: [PATCH] vmcore: replace strncpy with strtomem

[Kees Cook <keescook@chromium.org>]

On Wed, Mar 27, 2024 at 09:10:52PM +0000, Justin Stitt wrote:
> strncpy() is in the process of being replaced as it is deprecated in
> some situations [1]. While the specific use of strncpy that this patch
> targets is not exactly deprecated, the real mission is to rid the kernel
> of all its uses.
> 
> Looking at vmcoredd_header's definition:
> |	struct vmcoredd_header {
> |		__u32 n_namesz; /* Name size */
> |		__u32 n_descsz; /* Content size */
> |		__u32 n_type;   /* NT_VMCOREDD */
> |		__u8 name[8];   /* LINUX\0\0\0 */
> |		__u8 dump_name[VMCOREDD_MAX_NAME_BYTES]; /* Device dump's name */
> |	};
> ... we can see that both `name` and `dump_name` are u8s. It seems `name`
> wants to be NUL-padded (based on the comment above), but for the sake of
> symmetry lets NUL-pad both of these.

Do we have a way to know that dump_name is not parsed by userspace as a
NUL-terminated string?

> 
> Mark these buffers as __nonstring and use strtomem_pad.
> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@vger.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@google.com>
> ---
> Note: build-tested only.
> 
> Found with: $ rg "strncpy\("
> ---
>  fs/proc/vmcore.c            | 5 ++---
>  include/uapi/linux/vmcore.h | 4 ++--
>  2 files changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
> index 1fb213f379a5..5d7ecf3b75e8 100644
> --- a/fs/proc/vmcore.c
> +++ b/fs/proc/vmcore.c
> @@ -1370,9 +1370,8 @@ static void vmcoredd_write_header(void *buf, struct vmcoredd_data *data,
>  	vdd_hdr->n_descsz = size + sizeof(vdd_hdr->dump_name);
>  	vdd_hdr->n_type = NT_VMCOREDD;
>  
> -	strncpy((char *)vdd_hdr->name, VMCOREDD_NOTE_NAME,
> -		sizeof(vdd_hdr->name));
> -	memcpy(vdd_hdr->dump_name, data->dump_name, sizeof(vdd_hdr->dump_name));
> +	strtomem_pad(vdd_hdr->name, VMCOREDD_NOTE_NAME, 0);
> +	strtomem_pad(vdd_hdr->dump_name, data->dump_name, 0);
>  }
>  
>  /**
> diff --git a/include/uapi/linux/vmcore.h b/include/uapi/linux/vmcore.h
> index 3e9da91866ff..7053e2b62fa0 100644
> --- a/include/uapi/linux/vmcore.h
> +++ b/include/uapi/linux/vmcore.h
> @@ -11,8 +11,8 @@ struct vmcoredd_header {
>  	__u32 n_namesz; /* Name size */
>  	__u32 n_descsz; /* Content size */
>  	__u32 n_type;   /* NT_VMCOREDD */
> -	__u8 name[8];   /* LINUX\0\0\0 */
> -	__u8 dump_name[VMCOREDD_MAX_NAME_BYTES]; /* Device dump's name */
> +	__u8 name[8] __nonstring;   /* LINUX\0\0\0 */
> +	__u8 dump_name[VMCOREDD_MAX_NAME_BYTES] __nonstring; /* Device dump's name */
>  };

Unfortunately since this is UAPI, we can't sanely use __nonstring here.
:(


-- 
Kees Cook