xen/grant-table: add gnttab_try_end_foreign_access()

Add a new grant table function gnttab_try_end_foreign_access(), which
will remove and free a grant if it is not in use.

Its main use case is to either free a grant if it is no longer in use,
or to take some other action if it is still in use. This other action
can be an error exit, or (e.g. in the case of blkfront persistent grant
feature) some special handling.

This is CVE-2022-23036, CVE-2022-23038 / part of XSA-396.

Reported-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
V2:
- new patch
V4:
- add comments to header (Jan Beulich)

diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index 3729bea0c98956ac6124c01260dc4d7423445dca..1b82e7a3722a31bd1ad3fda7cbc7a7a87f10ce26 100644 (file)
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -435,11 +435,21 @@ static void gnttab_add_deferred(grant_ref_t ref, bool readonly,
               what, ref, page ? page_to_pfn(page) : -1);
 }
 
+int gnttab_try_end_foreign_access(grant_ref_t ref)
+{
+       int ret = _gnttab_end_foreign_access_ref(ref, 0);
+
+       if (ret)
+               put_free_entry(ref);
+
+       return ret;
+}
+EXPORT_SYMBOL_GPL(gnttab_try_end_foreign_access);
+
 void gnttab_end_foreign_access(grant_ref_t ref, int readonly,
                               unsigned long page)
 {
-       if (gnttab_end_foreign_access_ref(ref, readonly)) {
-               put_free_entry(ref);
+       if (gnttab_try_end_foreign_access(ref)) {
                if (page != 0)
                        put_page(virt_to_page(page));
        } else

diff --git a/include/xen/grant_table.h b/include/xen/grant_table.h
index cb854df031ce0715b9158fa71c7f02ad2c2dc732..358d2817741b7b089fc2ac32d92456fecff43fbf 100644 (file)
--- a/include/xen/grant_table.h
+++ b/include/xen/grant_table.h
@@ -104,10 +104,22 @@ 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.
  */
 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);