On Mon, Sep 25, 2023 at 06:54:51AM +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings
> [1] and as such we should prefer more robust and less ambiguous string
> interfaces.
>
> We expect `spec->target_type` to be NUL-terminated based on its use with
> a format string after `dm_table_add_target()` is called
> | r = dm_table_add_target(table, spec->target_type,
> | (sector_t) spec->sector_start,
> | (sector_t) spec->length,
> | target_params);
> ... wherein `spec->target_type` is passed as parameter `type` and later
> printed with DMERR:
> | DMERR("%s: %s: unknown target type", dm_device_name(t->md), type);
>
> It appears that `spec` is not zero-allocated and thus NUL-padding may be
> required in this ioctl context.
Yeah... this one was a bit odd. I see the memory being filled from
userspace in copy_params(), so it's unlikely there was any kernel memory
content leaking, but I think the old code may have been buggy:
strncpy(spec->target_type, ti->type->name,
sizeof(spec->target_type) - 1);
I can't find any reason to think that
spec->target_type[sizeof(spec->target_type) - 1] would be %NUL after
this code runs.
get_result_buffer() is basically:
return ((void *) param) + param->data_start;
then:
outptr = outbuf = get_result_buffer(param, param_size, &len);
...
spec = (struct dm_target_spec *) outptr;
and before that:
r = copy_params(user, ¶m_kernel, ioctl_flags, ¶m, ¶m_flags);
if (r)
return r;
input_param_size = param->data_size;
r = validate_params(cmd, param);
if (r)
goto out;
param->data_size = offsetof(struct dm_ioctl, data);
r = fn(file, param, input_param_size);
Notably validate_params() %NUL-terminates some _other_ strings, but
doesn't clear the param->data area.
So I think this is actually fixing a bug too, but I can't quite tell.
Regardless, the change looks correct:
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>
> Considering the above, a suitable replacement is `strscpy_pad` due to
> the fact that it guarantees NUL-termination whilst maintaining the
> NUL-padding behavior that strncpy provides.
>
> 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@xxxxxxxxxxxxxxx
> Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
> ---
> Note: build-tested only.
> ---
> drivers/md/dm-ioctl.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
> index 21ebb6c39394..e65058e0ed06 100644
> --- a/drivers/md/dm-ioctl.c
> +++ b/drivers/md/dm-ioctl.c
> @@ -1295,8 +1295,8 @@ static void retrieve_status(struct dm_table *table,
> spec->status = 0;
> spec->sector_start = ti->begin;
> spec->length = ti->len;
> - strncpy(spec->target_type, ti->type->name,
> - sizeof(spec->target_type) - 1);
> + strscpy_pad(spec->target_type, ti->type->name,
> + sizeof(spec->target_type));
>
> outptr += sizeof(struct dm_target_spec);
> remaining = len - (outptr - outbuf);
>
> ---
> base-commit: 6465e260f48790807eef06b583b38ca9789b6072
> change-id: 20230925-strncpy-drivers-md-dm-ioctl-c-ea5c10e77981
>
> Best regards,
> --
> Justin Stitt <justinstitt@xxxxxxxxxx>
>
>
--
Kees Cook
--
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