[PATCH AUTOSEL 5.10 51/56] xen-blkback: fix error handling in xen_blkbk_map()

[Sasha Levin <sashal@xxxxxxxxxx>]

From: Jan Beulich <jbeulich@xxxxxxxx>

[ Upstream commit 871997bc9e423f05c7da7c9178e62dde5df2a7f8 ]

The function uses a goto-based loop, which may lead to an earlier error
getting discarded by a later iteration. Exit this ad-hoc loop when an
error was encountered.

The out-of-memory error path additionally fails to fill a structure
field looked at by xen_blkbk_unmap_prepare() before inspecting the
handle which does get properly set (to BLKBACK_INVALID_HANDLE).

Since the earlier exiting from the ad-hoc loop requires the same field
filling (invalidation) as that on the out-of-memory path, fold both
paths. While doing so, drop the pr_alert(), as extra log messages aren't
going to help the situation (the kernel will log oom conditions already
anyway).

This is XSA-365.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Julien Grall <julien@xxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 drivers/block/xen-blkback/blkback.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c 
b/drivers/block/xen-blkback/blkback.c
index 9ebf53903d7bf..9301de1386436 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -794,8 +794,13 @@ static int xen_blkbk_map(struct xen_blkif_ring *ring,
                        pages[i]->persistent_gnt = persistent_gnt;
                } else {
                        if (gnttab_page_cache_get(&ring->free_pages,
-                                                 &pages[i]->page))
-                               goto out_of_memory;
+                                                 &pages[i]->page)) {
+                               gnttab_page_cache_put(&ring->free_pages,
+                                                     pages_to_gnt,
+                                                     segs_to_map);
+                               ret = -ENOMEM;
+                               goto out;
+                       }
                        addr = vaddr(pages[i]->page);
                        pages_to_gnt[segs_to_map] = pages[i]->page;
                        pages[i]->persistent_gnt = NULL;
@@ -882,17 +887,18 @@ static int xen_blkbk_map(struct xen_blkif_ring *ring,
        }
        segs_to_map = 0;
        last_map = map_until;
-       if (map_until != num)
+       if (!ret && map_until != num)
                goto again;
 
-       return ret;
-
-out_of_memory:
-       pr_alert("%s: out of memory\n", __func__);
-       gnttab_page_cache_put(&ring->free_pages, pages_to_gnt, segs_to_map);
-       for (i = last_map; i < num; i++)
+out:
+       for (i = last_map; i < num; i++) {
+               /* Don't zap current batch's valid persistent grants. */
+               if(i >= last_map + segs_to_map)
+                       pages[i]->persistent_gnt = NULL;
                pages[i]->handle = BLKBACK_INVALID_HANDLE;
-       return -ENOMEM;
+       }
+
+       return ret;
 }
 
 static int xen_blkbk_map_seg(struct pending_req *pending_req)
-- 
2.27.0