Merge tag 'xsa396-5.17-tag' of git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip

Pull xen fixes from Juergen Gross:
 "Several Linux PV device frontends are using the grant table interfaces
  for removing access rights of the backends in ways being subject to
  race conditions, resulting in potential data leaks, data corruption by
  malicious backends, and denial of service triggered by malicious
  backends:

   - blkfront, netfront, scsifront and the gntalloc driver are testing
     whether a grant reference is still in use. If this is not the case,
     they assume that a following removal of the granted access will
     always succeed, which is not true in case the backend has mapped
     the granted page between those two operations.

     As a result the backend can keep access to the memory page of the
     guest no matter how the page will be used after the frontend I/O
     has finished. The xenbus driver has a similar problem, as it
     doesn't check the success of removing the granted access of a
     shared ring buffer.

   - blkfront, netfront, scsifront, usbfront, dmabuf, xenbus, 9p,
     kbdfront, and pvcalls are using a functionality to delay freeing a
     grant reference until it is no longer in use, but the freeing of
     the related data page is not synchronized with dropping the granted
     access.

     As a result the backend can keep access to the memory page even
     after it has been freed and then re-used for a different purpose.

   - netfront will fail a BUG_ON() assertion if it fails to revoke
     access in the rx path.

     This will result in a Denial of Service (DoS) situation of the
     guest which can be triggered by the backend"

* tag 'xsa396-5.17-tag' of git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip:
  xen/netfront: react properly to failing gnttab_end_foreign_access_ref()
  xen/gnttab: fix gnttab_end_foreign_access() without page specified
  xen/pvcalls: use alloc/free_pages_exact()
  xen/9p: use alloc/free_pages_exact()
  xen/usb: don't use gnttab_end_foreign_access() in xenhcd_gnttab_done()
  xen: remove gnttab_query_foreign_access()
  xen/gntalloc: don't use gnttab_query_foreign_access()
  xen/scsifront: don't use gnttab_query_foreign_access() for mapped status
  xen/netfront: don't use gnttab_query_foreign_access() for mapped status
  xen/blkfront: don't use gnttab_query_foreign_access() for mapped status
  xen/grant-table: add gnttab_try_end_foreign_access()
  xen/xenbus: don't let xenbus_grant_ring() remove grants in error case

1 files changed, 17 insertions, 2 deletions

diff --git a/include/xen/grant_table.h b/include/xen/grant_table.h
index cb854df031ce..c9fea9389ebe 100644
--- a/include/xen/grant_table.h
+++ b/include/xen/grant_table.h
@@ -104,17 +104,32 @@ int gnttab_end_foreign_access_ref(grant_ref_t ref, int readonly);
  * access has been ended, free the given page too.  Access will be ended
  * immediately iff the grant entry is not in use, otherwise it will happen
  * some time later.  page may be 0, in which case no freeing will occur.
+ * Note that the granted page might still be accessed (read or write) by the
+ * other side after gnttab_end_foreign_access() returns, so even if page was
+ * specified as 0 it is not allowed to just reuse the page for other
+ * purposes immediately. gnttab_end_foreign_access() will take an additional
+ * reference to the granted page in this case, which is dropped only after
+ * the grant is no longer in use.
+ * This requires that multi page allocations for areas subject to
+ * gnttab_end_foreign_access() are done via alloc_pages_exact() (and freeing
+ * via free_pages_exact()) in order to avoid high order pages.
  */
 void gnttab_end_foreign_access(grant_ref_t ref, int readonly,
 			       unsigned long page);
 
+/*
+ * End access through the given grant reference, iff the grant entry is
+ * no longer in use.  In case of success ending foreign access, the
+ * grant reference is deallocated.
+ * Return 1 if the grant entry was freed, 0 if it is still in use.
+ */
+int gnttab_try_end_foreign_access(grant_ref_t ref);
+
 int gnttab_grant_foreign_transfer(domid_t domid, unsigned long pfn);
 
 unsigned long gnttab_end_foreign_transfer_ref(grant_ref_t ref);
 unsigned long gnttab_end_foreign_transfer(grant_ref_t ref);
 
-int gnttab_query_foreign_access(grant_ref_t ref);
-
 /*
  * operations on reserved batches of grant references
  */