[PATCH 4.19 28/33] xen/gntalloc: dont use gnttab_query_foreign_access()

Greg Kroah-Hartman
2022-03-10T14:33:24Z

From: Juergen Gross <jgross@suse.com>

Commit d3b6372c5881cb54925212abb62c521df8ba4809 upstream.

Using gnttab_query_foreign_access() is unsafe, as it is racy by design.

The use case in the gntalloc driver is not needed at all. While at it
replace the call of gnttab_end_foreign_access_ref() with a call of
gnttab_end_foreign_access(), which is what is really wanted there. In
case the grant wasn't used due to an allocation failure, just free the
grant via gnttab_free_grant_reference().

This is CVE-2022-23039 / 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/xen/gntalloc.c | 25 +++++++------------------
1 file changed, 7 insertions(+), 18 deletions(-)

--- a/drivers/xen/gntalloc.c
+++ b/drivers/xen/gntalloc.c
@@ -169,20 +169,14 @@ undo:
__del_gref(gref);
}

- /* It's possible for the target domain to map the just-allocated grant
- * references by blindly guessing their IDs; if this is done, then
- * __del_gref will leave them in the queue_gref list. They need to be
- * added to the global list so that we can free them when they are no
- * longer referenced.
- */
- if (unlikely(!list_empty(&queue_gref)))
- list_splice_tail(&queue_gref, &gref_list);
mutex_unlock(&gref_mutex);
return rc;
}

static void __del_gref(struct gntalloc_gref *gref)
{
+ unsigned long addr;
+
if (gref->notify.flags & UNMAP_NOTIFY_CLEAR_BYTE) {
uint8_t *tmp = kmap(gref->page);
tmp[gref->notify.pgoff] = 0;
@@ -196,21 +190,16 @@ static void __del_gref(struct gntalloc_g
gref->notify.flags = 0;

if (gref->gref_id) {
- if (gnttab_query_foreign_access(gref->gref_id))
- return;
-
- if (!gnttab_end_foreign_access_ref(gref->gref_id, 0))
- return;
-
- gnttab_free_grant_reference(gref->gref_id);
+ if (gref->page) {
+ addr = (unsigned long)page_to_virt(gref->page);
+ gnttab_end_foreign_access(gref->gref_id, 0, addr);
+ } else
+ gnttab_free_grant_reference(gref->gref_id);
}

gref_size--;
list_del(&gref->next_gref);

- if (gref->page)
- __free_page(gref->page);
-
kfree(gref);
}