memfd: check for non-NULL file_seals in memfd_create() syscall

Ensure that file_seals is non-NULL before using it in the memfd_create()
syscall.  One situation in which memfd_file_seals_ptr() could return a
NULL pointer when CONFIG_SHMEM=n, oopsing the kernel.

Link: https://lkml.kernel.org/r/20230607132427.2867435-1-roberto.sassu@huaweicloud.com
Fixes: 47b9012ecdc7 ("shmem: add sealing support to hugetlb-backed memfd")
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Cc: Marc-Andr Lureau <marcandre.lureau@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memfd.c b/mm/memfd.c
index 69b90c3..e763e76 100644 (file)
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -371,12 +371,15 @@ SYSCALL_DEFINE2(memfd_create,
 
                inode->i_mode &= ~0111;
                file_seals = memfd_file_seals_ptr(file);
-               *file_seals &= ~F_SEAL_SEAL;
-               *file_seals |= F_SEAL_EXEC;
+               if (file_seals) {
+                       *file_seals &= ~F_SEAL_SEAL;
+                       *file_seals |= F_SEAL_EXEC;
+               }
        } else if (flags & MFD_ALLOW_SEALING) {
                /* MFD_EXEC and MFD_ALLOW_SEALING are set */
                file_seals = memfd_file_seals_ptr(file);
-               *file_seals &= ~F_SEAL_SEAL;
+               if (file_seals)
+                       *file_seals &= ~F_SEAL_SEAL;
        }
 
        fd_install(fd, file);