netdev - [PATCH v3 02/10] uaccess: Add speculation barrier to copy_from_user

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Message-ID: <69e4b8d8a9bad08848945ae29573045a847755d6.1760529207.git.christophe.leroy@csgroup.eu>
Date: Fri, 17 Oct 2025 12:20:58 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
	Jan Kara <jack@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	"Andre Almeida" <andrealmeid@...lia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Kuniyuki Iwashima <kuniyu@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>,
	Willem de Bruijn <willemb@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>,
	Simon Horman <horms@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
	linux-block@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH v3 02/10] uaccess: Add speculation barrier to copy_from_user_iter()

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@...roup.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 a589935bf3025..896760bad455f 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