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/[email protected]
Fixes: 47b9012 ("shmem: add sealing support to hugetlb-backed memfd")
Signed-off-by: Roberto Sassu <[email protected]>
Cc: Marc-Andr Lureau <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: robertosassu

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);