[PATCH 2/5] uaccess: Add speculation barrier to copy_from_user_iter()

Christophe Leroy
2025-06-22T10:20:34Z

The results of "access_ok()" can be mis-speculated. The result is that
you can end speculatively:

if (access_ok(from, size))
// Right here

For the same reason as done in copy_from_user() by
commit 74e19ef0ff80 ("uaccess: Add speculation barrier to
copy_from_user()"), add a speculation barrier to copy_from_user_iter().

See commit 74e19ef0ff80 ("uaccess: Add speculation barrier to
copy_from_user()") for more details.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
lib/iov_iter.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index f9193f952f49..ebf524a37907 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -50,6 +50,13 @@ size_t copy_from_user_iter(void __user *iter_from, size_t progress,
if (should_fail_usercopy())
return len;
if (access_ok(iter_from, len)) {
+ /*
+ * Ensure that bad access_ok() speculation will not
+ * lead to nasty side effects *after* the copy is
+ * finished:
+ */
+ barrier_nospec();
+
to += progress;
instrument_copy_from_user_before(to, iter_from, len);
res = raw_copy_from_user(to, iter_from, len);
--
2.49.0