xen/blkfront: don't use gnttab_query_foreign_access() for mapped status

Commit abf1fd5919d6238ee3bc5eb4a9b6c3947caa6638 upstream.

It isn't enough to check whether a grant is still being in use by
calling gnttab_query_foreign_access(), as a mapping could be realized
by the other side just after having called that function.

In case the call was done in preparation of revoking a grant it is
better to do so via gnttab_end_foreign_access_ref() and check the
success of that operation instead.

For the ring allocation use alloc_pages_exact() in order to avoid
high order pages in case of a multi-page ring.

If a grant wasn't unmapped by the backend without persistent grants
being used, set the device state to "error".

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

Reported-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 39 insertions, 28 deletions

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 448753021724..a97a306c3850 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -1278,17 +1278,16 @@ static void blkif_free_ring(struct blkfront_ring_info *rinfo)
 		list_for_each_entry_safe(persistent_gnt, n,
 					 &rinfo->grants, node) {
 			list_del(&persistent_gnt->node);
-			if (persistent_gnt->gref != GRANT_INVALID_REF) {
-				gnttab_end_foreign_access(persistent_gnt->gref,
-							  0, 0UL);
-				rinfo->persistent_gnts_c--;
-			}
+			if (persistent_gnt->gref == GRANT_INVALID_REF ||
+			    !gnttab_try_end_foreign_access(persistent_gnt->gref))
+				continue;
+
+			rinfo->persistent_gnts_c--;
 			if (info->feature_persistent)
 				__free_page(persistent_gnt->page);
 			kfree(persistent_gnt);
 		}
 	}
-	BUG_ON(rinfo->persistent_gnts_c != 0);
 
 	for (i = 0; i < BLK_RING_SIZE(info); i++) {
 		/*
@@ -1345,7 +1344,8 @@ free_shadow:
 			rinfo->ring_ref[i] = GRANT_INVALID_REF;
 		}
 	}
-	free_pages((unsigned long)rinfo->ring.sring, get_order(info->nr_ring_pages * XEN_PAGE_SIZE));
+	free_pages_exact(rinfo->ring.sring,
+			 info->nr_ring_pages * XEN_PAGE_SIZE);
 	rinfo->ring.sring = NULL;
 
 	if (rinfo->irq)
@@ -1429,9 +1429,15 @@ static int blkif_get_final_status(enum blk_req_status s1,
 	return BLKIF_RSP_OKAY;
 }
 
-static bool blkif_completion(unsigned long *id,
-			     struct blkfront_ring_info *rinfo,
-			     struct blkif_response *bret)
+/*
+ * Return values:
+ *  1 response processed.
+ *  0 missing further responses.
+ * -1 error while processing.
+ */
+static int blkif_completion(unsigned long *id,
+			    struct blkfront_ring_info *rinfo,
+			    struct blkif_response *bret)
 {
 	int i = 0;
 	struct scatterlist *sg;
@@ -1505,42 +1511,43 @@ static bool blkif_completion(unsigned long *id,
 	}
 	/* Add the persistent grant into the list of free grants */
 	for (i = 0; i < num_grant; i++) {
-		if (gnttab_query_foreign_access(s->grants_used[i]->gref)) {
+		if (!gnttab_try_end_foreign_access(s->grants_used[i]->gref)) {
 			/*
 			 * If the grant is still mapped by the backend (the
 			 * backend has chosen to make this grant persistent)
 			 * we add it at the head of the list, so it will be
 			 * reused first.
 			 */
-			if (!info->feature_persistent)
-				pr_alert_ratelimited("backed has not unmapped grant: %u\n",
-						     s->grants_used[i]->gref);
+			if (!info->feature_persistent) {
+				pr_alert("backed has not unmapped grant: %u\n",
+					 s->grants_used[i]->gref);
+				return -1;
+			}
 			list_add(&s->grants_used[i]->node, &rinfo->grants);
 			rinfo->persistent_gnts_c++;
 		} else {
 			/*
-			 * If the grant is not mapped by the backend we end the
-			 * foreign access and add it to the tail of the list,
-			 * so it will not be picked again unless we run out of
-			 * persistent grants.
+			 * If the grant is not mapped by the backend we add it
+			 * to the tail of the list, so it will not be picked
+			 * again unless we run out of persistent grants.
 			 */
-			gnttab_end_foreign_access(s->grants_used[i]->gref, 0, 0UL);
 			s->grants_used[i]->gref = GRANT_INVALID_REF;
 			list_add_tail(&s->grants_used[i]->node, &rinfo->grants);
 		}
 	}
 	if (s->req.operation == BLKIF_OP_INDIRECT) {
 		for (i = 0; i < INDIRECT_GREFS(num_grant); i++) {
-			if (gnttab_query_foreign_access(s->indirect_grants[i]->gref)) {
-				if (!info->feature_persistent)
-					pr_alert_ratelimited("backed has not unmapped grant: %u\n",
-							     s->indirect_grants[i]->gref);
+			if (!gnttab_try_end_foreign_access(s->indirect_grants[i]->gref)) {
+				if (!info->feature_persistent) {
+					pr_alert("backed has not unmapped grant: %u\n",
+						 s->indirect_grants[i]->gref);
+					return -1;
+				}
 				list_add(&s->indirect_grants[i]->node, &rinfo->grants);
 				rinfo->persistent_gnts_c++;
 			} else {
 				struct page *indirect_page;
 
-				gnttab_end_foreign_access(s->indirect_grants[i]->gref, 0, 0UL);
 				/*
 				 * Add the used indirect page back to the list of
 				 * available pages for indirect grefs.
@@ -1621,12 +1628,17 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
 		}
 
 		if (bret.operation != BLKIF_OP_DISCARD) {
+			int ret;
+
 			/*
 			 * We may need to wait for an extra response if the
 			 * I/O request is split in 2
 			 */
-			if (!blkif_completion(&id, rinfo, &bret))
+			ret = blkif_completion(&id, rinfo, &bret);
+			if (!ret)
 				continue;
+			if (unlikely(ret < 0))
+				goto err;
 		}
 
 		if (add_id_to_freelist(rinfo, id)) {
@@ -1732,8 +1744,7 @@ static int setup_blkring(struct xenbus_device *dev,
 	for (i = 0; i < info->nr_ring_pages; i++)
 		rinfo->ring_ref[i] = GRANT_INVALID_REF;
 
-	sring = (struct blkif_sring *)__get_free_pages(GFP_NOIO | __GFP_HIGH,
-						       get_order(ring_size));
+	sring = alloc_pages_exact(ring_size, GFP_NOIO);
 	if (!sring) {
 		xenbus_dev_fatal(dev, -ENOMEM, "allocating shared ring");
 		return -ENOMEM;
@@ -1743,7 +1754,7 @@ static int setup_blkring(struct xenbus_device *dev,
 
 	err = xenbus_grant_ring(dev, rinfo->ring.sring, info->nr_ring_pages, gref);
 	if (err < 0) {
-		free_pages((unsigned long)sring, get_order(ring_size));
+		free_pages_exact(sring, ring_size);
 		rinfo->ring.sring = NULL;
 		goto fail;
 	}