[tip: core/uaccess] iov_iter: Add missing speculation barrier to copy_from_user_iter()

[tip-bot2 for Christophe Leroy]

The following commit has been merged into the core/uaccess branch of tip:

Commit-ID:     803abedbd540617f136a2c4d7066ff2e304f016d
Gitweb:        https://git.kernel.org/tip/803abedbd540617f136a2c4d7066ff2e304f016d
Author:        Christophe Leroy <christophe.leroy@xxxxxxxxxx>
AuthorDate:    Mon, 17 Nov 2025 17:43:42 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 18 Nov 2025 15:27:34 +01:00

iov_iter: Add missing speculation barrier to copy_from_user_iter()

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

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

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

Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://patch.msgid.link/6b73e69cc7168c89df4eab0a216e3ed4cca36b0a.1763396724.git.christophe.leroy@xxxxxxxxxx
---
 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 a589935..896760b 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);