xen/gntdev: Accommodate VMA splitting
CVE: CVE-2022-50471
Component: kernel
CWE: CWE-911
Severity: Moderate
CVSS v3: 5.6/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Release date: 2022-01-01T00:00:00+00:00

In the Linux kernel, the following vulnerability has been resolved:

xen/gntdev: Accommodate VMA splitting

Prior to this commit, the gntdev driver code did not handle the
following scenario correctly with paravirtualized (PV) Xen domains:

* User process sets up a gntdev mapping composed of two grant mappings
  (i.e., two pages shared by another Xen domain).
* User process munmap()s one of the pages.
* User process munmap()s the remaining page.
* User process exits.

In the scenario above, the user process would cause the kernel to log
the following messages in dmesg for the first munmap(), and the second
munmap() call would result in similar log messages:

  BUG: Bad page map in process doublemap.test  pte:... pmd:...
  page:0000000057c97bff refcount:1 mapcount:-1 \
    mapping:0000000000000000 index:0x0 pfn:...
  ...
  page dumped because: bad pte
  ...
  file:gntdev fault:0x0 mmap:gntdev_mmap [xen_gntdev] readpage:0x0
  ...
  Call Trace:
   <TASK>
   dump_stack_lvl+0x46/0x5e
   print_bad_pte.cold+0x66/0xb6
   unmap_page_range+0x7e5/0xdc0
   unmap_vmas+0x78/0xf0
   unmap_region+0xa8/0x110
   __do_munmap+0x1ea/0x4e0
   __vm_munmap+0x75/0x120
   __x64_sys_munmap+0x28/0x40
   do_syscall_64+0x38/0x90
   entry_SYSCALL_64_after_hwframe+0x61/0xcb
   ...

For each munmap() call, the Xen hypervisor (if built with CONFIG_DEBUG)
would print out the following and trigger a general protection fault in
the affected Xen PV domain:

  (XEN) d0v... Attempt to implicitly unmap d0's grant PTE ...
  (XEN) d0v... Attempt to implicitly unmap d0's grant PTE ...

As of this writing, gntdev_grant_map structure's vma field (referred to
as map->vma below) is mainly used for checking the start and end
addresses of mappings. However, with split VMAs, these may change, and
there could be more than one VMA associated with a gntdev mapping.
Hence, remove the use of map->vma and rely on map->pages_vm_start for
the original start address and on (map->count << PAGE_SHIFT) for the
original mapping size. Let the invalidate() and find_special_page()
hooks use these.

Also, given that there can be multiple VMAs associated with a gntdev
mapping, move the "mmu_interval_notifier_remove(&map->notifier)" call to
the end of gntdev_put_map, so that the MMU notifier is only removed
after the closing of the last remaining VMA.

Finally, use an atomic to prevent inadvertent gntdev mapping re-use,
instead of using the map->live_grants atomic counter and/or the map->vma
pointer (the latter of which is now removed). This prevents the
userspace from mmap()'ing (with MAP_FIXED) a gntdev mapping over the
same address range as a previously set up gntdev mapping. This scenario
can be summarized with the following call-trace, which was valid prior
to this commit:

  mmap
    gntdev_mmap
  mmap (repeat mmap with MAP_FIXED over the same address range)
    gntdev_invalidate
      unmap_grant_pages (sets 'being_removed' entries to true)
        gnttab_unmap_refs_async
    unmap_single_vma
    gntdev_mmap (maps the shared pages again)
  munmap
    gntdev_invalidate
      unmap_grant_pages
        (no-op because 'being_removed' entries are true)
    unmap_single_vma (For PV domains, Xen reports that a granted page
      is being unmapped and triggers a general protection fault in the
      affected domain, if Xen was built with CONFIG_DEBUG)

The fix for this last scenario could be worth its own commit, but we
opted for a single commit, because removing the gntdev_grant_map
structure's vma field requires guarding the entry to gntdev_mmap(), and
the live_grants atomic counter is not sufficient on its own to prevent
the mmap() over a pre-existing mapping.

Statement: The gntdev driver manages memory mappings of pages granted by other Xen domains. The code maintained a single VMA pointer per grant mapping, but Linux can split VMAs when users partially unmap regions. When a mapping spanning two pages is split by unmapping one page, the driver loses track of the relationship between VMAs and grants. Additionally, the driver allowed remapping the same address range with MAP_FIXED, creating a scenario where grants are unmapped while marked as "being_removed," then remapped, then unmapped again—triggering Xen's protection against implicit unmapping of granted pages. The fix removes dependence on VMA tracking, uses atomic guards to prevent mapping reuse, and defers MMU notifier removal until all associated VMAs are closed. Triggering requires running as a Xen PV domain guest with the ability to manipulate gntdev mappings.

Remediation: