[PATCH v2] drivers: xen: unmap Enlighten page before jumping to Linux

[Nastya Vicodin]

Reviewed-by: Anastasiia Lukianenko <vicooodin at gmail.com>

On Tue, Jul 19, 2022 at 5:55 PM Dmytro Firsov <Dmytro_Firsov at epam.com>
wrote:

> This commit fixes issue with usage of Xen hypervisor shared info page.
> Previously U-boot did not unmap it at the end of OS boot process. Xen
> did not prevent guest from this. So, it worked, but caused wierd
> issues - one memory page, that was returned by memalign in U-boot
> for Enlighten mapping was not unmaped by Xen (shared_info values was
> not removed from there) and returned to allocator. During the Linux
> boot, it uses shared_info page as regular RAM page, which leads to
> hypervisor shared info corruption.
>
> So, to fix this issue, as discussed on the xen-devel mailing list, the
> code should:
>    1) Unmap the page
>    2) Populate the area with memory using XENMEM_populate_physmap
>
> This patch adds page unmapping via XENMEM_remove_from_physmap, fills
> hole in address space where page was mapped via XENMEM_populate_physmap
> and return this address to memory allocator for freeing.
>
> Signed-off-by: Dmytro Firsov <dmytro_firsov at epam.com>
> ---
>
> Changes in v2:
> - Reword commit message to be more clear with purpose of the patch
> - Change BUG() helper to panic() and add error messages
> - Add struct zeroing during initialization
> - Fix typo in comment
> ---
>  drivers/xen/hypervisor.c | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
>
> diff --git a/drivers/xen/hypervisor.c b/drivers/xen/hypervisor.c
> index 2560894832..16c7c96c94 100644
> --- a/drivers/xen/hypervisor.c
> +++ b/drivers/xen/hypervisor.c
> @@ -144,6 +144,36 @@ struct shared_info *map_shared_info(void *p)
>         return HYPERVISOR_shared_info;
>  }
>
> +void unmap_shared_info(void)
> +{
> +       xen_pfn_t shared_info_pfn = virt_to_pfn(HYPERVISOR_shared_info);
> +       struct xen_remove_from_physmap xrfp = {0};
> +       struct xen_memory_reservation reservation = {0};
> +       xen_ulong_t nr_exts = 1;
> +
> +       xrfp.domid = DOMID_SELF;
> +       xrfp.gpfn = shared_info_pfn;
> +       if (HYPERVISOR_memory_op(XENMEM_remove_from_physmap, &xrfp) != 0)
> +               panic("Failed to unmap HYPERVISOR_shared_info\n");
> +
> +       /*
> +        * After removing from physmap there will be a hole in address
> space on
> +        * HYPERVISOR_shared_info address, so to free memory allocated with
> +        * memalign and prevent exceptions during access to this page we
> need to
> +        * fill this 4KB hole with XENMEM_populate_physmap before jumping
> to Linux.
> +        */
> +       reservation.domid = DOMID_SELF;
> +       reservation.extent_order = 0;
> +       reservation.address_bits = 0;
> +       set_xen_guest_handle(reservation.extent_start, &shared_info_pfn);
> +       reservation.nr_extents = nr_exts;
> +       if (HYPERVISOR_memory_op(XENMEM_populate_physmap, &reservation) !=
> nr_exts)
> +               panic("Failed to populate memory on HYPERVISOR_shared_info
> addr\n");
> +
> +       /* Now we can return this to memory allocator */
> +       free(HYPERVISOR_shared_info);
> +}
> +
>  void do_hypervisor_callback(struct pt_regs *regs)
>  {
>         unsigned long l1, l2, l1i, l2i;
> @@ -251,4 +281,5 @@ void xen_fini(void)
>         fini_gnttab();
>         fini_xenbus();
>         fini_events();
> +       unmap_shared_info();
>  }
> --
> 2.25.1
>