linux-kernel - [PATCH 2/5] uaccess: Add speculation barrier to copy_from_user

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Message-ID: <f4b2a32853b5daba7aeac9e9b96ec1ab88981589.1750585239.git.christophe.leroy@csgroup.eu>
Date: Sun, 22 Jun 2025 11:52:40 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Naveen N Rao <naveen@...nel.org>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	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>,
	David Laight <david.laight.linux@...il.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
	linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org,
	linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org
Subject: [PATCH 2/5] 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 | 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