exec: Move bprm_mm_init into alloc_bprm

Currently it is necessary for the usermode helper code and the code that
launches init to use set_fs so that pages coming from the kernel look like
they are coming from userspace.

To allow that usage of set_fs to be removed cleanly the argument copying
from userspace needs to happen earlier.  Move the allocation and
initialization of bprm->mm into alloc_bprm so that the bprm->mm is
available early to store the new user stack into.  This is a prerequisite
for copying argv and envp into the new user stack early before ther rest of
exec.

To keep the things consistent the cleanup of bprm->mm is moved into
free_bprm.  So that bprm->mm will be cleaned up whenever bprm->mm is
allocated and free_bprm are called.

Moving bprm_mm_init earlier is safe as it does not depend on any files,
current->in_execve, current->fs->in_exec, bprm->unsafe, or the if the file
table is shared. (AKA bprm_mm_init does not depend on any of the code that
happens between alloc_bprm and where it was previously called.)

This moves bprm->mm cleanup after current->fs->in_exec is set to 0.  This
is safe because current->fs->in_exec is only used to preventy taking an
additional reference on the fs_struct.

This moves bprm->mm cleanup after current->in_execve is set to 0.  This is
safe because current->in_execve is only used by the lsms (apparmor and
tomoyou) and always for LSM specific functions, never for anything to do
with the mm.

This adds bprm->mm cleanup into the successful return path.  This is safe
because being on the successful return path implies that begin_new_exec
succeeded and set brpm->mm to NULL.  As bprm->mm is NULL bprm cleanup I am
moving into free_bprm will do nothing.
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lkml.kernel.org/r/87eepe6x7p.fsf@x220.int.ebiederm.orgSigned-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

fs/exec.c

@@ -1543,6 +1543,10 @@ static int prepare_bprm_creds(struct linux_binprm *bprm)
 
 static void free_bprm(struct linux_binprm *bprm)
 {
+	if (bprm->mm) {
+		acct_arg_size(bprm, 0);
+		mmput(bprm->mm);
+	}
 	free_arg_pages(bprm);
 	if (bprm->cred) {
 		mutex_unlock(&current->signal->cred_guard_mutex);
@@ -1582,6 +1586,10 @@ static struct linux_binprm *alloc_bprm(int fd, struct filename *filename)
 		bprm->filename = bprm->fdpath;
 	}
 	bprm->interp = bprm->filename;
+
+	retval = bprm_mm_init(bprm);
+	if (retval)
+		goto out_free;
 	return bprm;
 
 out_free:
@@ -1911,10 +1919,6 @@ static int do_execveat_common(int fd, struct filename *filename,
 	    close_on_exec(fd, rcu_dereference_raw(current->files->fdt)))
 		bprm->interp_flags |= BINPRM_FLAGS_PATH_INACCESSIBLE;
 
-	retval = bprm_mm_init(bprm);
-	if (retval)
-		goto out_unmark;
-
 	retval = prepare_arg_pages(bprm, argv, envp);
 	if (retval < 0)
 		goto out;
@@ -1962,10 +1966,6 @@ static int do_execveat_common(int fd, struct filename *filename,
 	 */
 	if (bprm->point_of_no_return && !fatal_signal_pending(current))
 		force_sigsegv(SIGSEGV);
-	if (bprm->mm) {
-		acct_arg_size(bprm, 0);
-		mmput(bprm->mm);
-	}
 
 out_unmark:
 	current->fs->in_exec = 0;