find_vma() and variants need protection when used.
This patch adds mmap_assert_lock() calls in the functions.

To make sure the invariant is satisfied, we also need to add a
mmap_read_loc() around the get_user_pages_remote() call in
get_arg_page(). The lock is not strictly necessary because the mm
has been newly created, but the extra cost is limited because
the same mutex was also acquired shortly before in __bprm_mm_init(),
so it is hot and uncontended.

Signed-off-by: Luigi Rizzo <redacted>
 fs/exec.c | 2 ++
 mm/mmap.c | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/fs/exec.c b/fs/exec.c
index 38f63451b928..ac7603e985b4 100644
+++ b/fs/exec.c

@@ -217,8 +217,10 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
 	 * We are doing an exec().  'current' is the process
 	 * doing the exec and bprm->mm is the new process's mm.
 	 */
+	mmap_read_lock(bprm->mm);
 	ret = get_user_pages_remote(bprm->mm, pos, 1, gup_flags,
 			&page, NULL, NULL);
+	mmap_read_unlock(bprm->mm);
 	if (ret <= 0)
 		return NULL;