[PATCH v2 02/10] uaccess: Add speculation barrier to copy_from_user_iter()

[Christophe Leroy]

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.le...@csgroup.eu>
---
 lib/iov_iter.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 48bd0cbce8c2..8d08b3435174 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -49,11 +49,19 @@ size_t copy_from_user_iter(void __user *iter_from, size_t 
progress,
 
        if (should_fail_usercopy())
                return len;
-       if (can_do_masked_user_access())
+       if (can_do_masked_user_access()) {
                iter_from = mask_user_address(iter_from);
-       else if (!access_ok(iter_from, len))
-               return res;
+       } else {
+               if (!access_ok(iter_from, len))
+                       return res;
 
+               /*
+                * 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