usb: gadget: storage: Fix Spectre v1 vulnerability

[Felipe Balbi <balbi@kernel.org>]

Hi Greg,

"Gustavo A. R. Silva" <gustavo@embeddedor.com> writes:

> num can be indirectly controlled by user-space, hence leading to
> a potential exploitation of the Spectre variant 1 vulnerability.
>
> This issue was detected with the help of Smatch:
>
> drivers/usb/gadget/function/f_mass_storage.c:3177 fsg_lun_make() warn:
> potential spectre issue 'fsg_opts->common->luns' [r] (local cap)
>
> Fix this by sanitizing num before using it to index
> fsg_opts->common->luns
>
> Notice that given that speculation windows are large, the policy is
> to kill the speculation on the first load and not worry if it can be
> completed with a dependent load/store [1].
>
> [1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2
>
> Cc: stable@vger.kernel.org
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

Can you still take this as an urgent fix?

Acked-by: Felipe Balbi <felipe.balbi@intel.com>