mm/memfd: add write seals when apply SEAL_EXEC to executable memfd

In order to avoid WX mappings, add F_SEAL_WRITE when apply F_SEAL_EXEC to
an executable memfd, so W^X from start.

This implys application need to fill the content of the memfd first, after
F_SEAL_EXEC is applied, application can no longer modify the content of
the memfd.

Typically, application seals the memfd right after writing to it.
For example:
1. memfd_create(MFD_EXEC).
2. write() code to the memfd.
3. fcntl(F_ADD_SEALS, F_SEAL_EXEC) to convert the memfd to W^X.
4. call exec() on the memfd.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Jeff Xu <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Cc: Daniel Verkamp <[email protected]>
Cc: David Herrmann <[email protected]>
Cc: Dmitry Torokhov <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Jorge Lucangeli Obes <[email protected]>
Cc: kernel test robot <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/memfd.c

@@ -222,6 +222,12 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
 		}
 	}
 
+	/*
+	 * SEAL_EXEC implys SEAL_WRITE, making W^X from the start.
+	 */
+	if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
+		seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
+
 	*file_seals |= seals;
 	error = 0;