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

Reviewed-by: Anastasiia Lukianenko vicooodin@gmail.com

On Tue, Jul 19, 2022 at 5:55 PM Dmytro Firsov Dmytro_Firsov@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@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