[PATCH 6.11 484/558] mm/gup: fix memfd_pin_folios alloc race panic

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

6.11-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Steve Sistare <steven.sistare@xxxxxxxxxx>

commit ce645b9fdc78ec5d28067286e92871ddae6817d5 upstream.

If memfd_pin_folios tries to create a hugetlb page, but someone else
already did, then folio gets the value -EEXIST here:

        folio = memfd_alloc_folio(memfd, start_idx);
        if (IS_ERR(folio)) {
                ret = PTR_ERR(folio);
                if (ret != -EEXIST)
                        goto err;

then on the next trip through the "while start_idx" loop we panic here:

        if (folio) {
                folio_put(folio);

To fix, set the folio to NULL on error.

Link: https://lkml.kernel.org/r/1725373521-451395-6-git-send-email-steven.sistare@xxxxxxxxxx
Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")
Signed-off-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
Acked-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 mm/gup.c |    1 +
 1 file changed, 1 insertion(+)

--- a/mm/gup.c
+++ b/mm/gup.c
@@ -3705,6 +3705,7 @@ long memfd_pin_folios(struct file *memfd
 					ret = PTR_ERR(folio);
 					if (ret != -EEXIST)
 						goto err;
+					folio = NULL;
 				}
 			}
 		}