linux-hardening - [RFC v1 06/17] futex: add get_user_no

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Message-Id: <20250712192202.707192-7-gatlin.newhouse@gmail.com>
Date: Sat, 12 Jul 2025 19:21:51 +0000
From: Gatlin Newhouse <gatlin.newhouse@...il.com>
To: linux-hardening@...r.kernel.org
Cc: Gatlin Newhouse <gatlin.newhouse@...il.com>
Subject: [RFC v1 06/17] futex: add get_user_no_dfcache() functions

Add disabled cache get_user function calls to futex functions to disable
using the SafeFetch cache on any fast userspace mutexes.
---
 kernel/futex/core.c     | 5 +++++
 kernel/futex/futex.h    | 4 ++++
 kernel/futex/pi.c       | 5 +++++
 kernel/futex/requeue.c  | 5 ++++-
 kernel/futex/waitwake.c | 4 ++++
 5 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 90d53fb0ee9e..0ad5e0dba881 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1023,8 +1023,13 @@ static int handle_futex_death(u32 __user *uaddr, struct task_struct *curr,
 		return -1;
 
 retry:
+#ifdef CONFIG_SAFEFETCH
+	if (get_user_no_dfcache(uval, uaddr))
+		return -1;
+#else
 	if (get_user(uval, uaddr))
 		return -1;
+#endif
 
 	/*
 	 * Special case for regular (non PI) futexes. The unlock path in
diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index fcd1617212ee..515338cf4289 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -308,7 +308,11 @@ static __always_inline int futex_get_value(u32 *dest, u32 __user *from)
 		from = masked_user_access_begin(from);
 	else if (!user_read_access_begin(from, sizeof(*from)))
 		return -EFAULT;
+#ifdef CONFIG_SAFEFETCH
+	unsafe_get_user_no_dfcache(val, from, Efault);
+#else
 	unsafe_get_user(val, from, Efault);
+#endif
 	user_read_access_end();
 	*dest = val;
 	return 0;
diff --git a/kernel/futex/pi.c b/kernel/futex/pi.c
index dacb2330f1fb..f9f4ac192338 100644
--- a/kernel/futex/pi.c
+++ b/kernel/futex/pi.c
@@ -1140,8 +1140,13 @@ int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
 		return -ENOSYS;
 
 retry:
+#ifdef CONFIG_SAFEFETCH
+	if (get_user_no_dfcache(uval, uaddr))
+		return -EFAULT;
+#else
 	if (get_user(uval, uaddr))
 		return -EFAULT;
+#endif
 	/*
 	 * We release only a lock we actually own:
 	 */
diff --git a/kernel/futex/requeue.c b/kernel/futex/requeue.c
index c716a66f8692..3ebc08a9a8e8 100644
--- a/kernel/futex/requeue.c
+++ b/kernel/futex/requeue.c
@@ -468,8 +468,11 @@ int futex_requeue(u32 __user *uaddr1, unsigned int flags1,
 			if (unlikely(ret)) {
 				futex_hb_waiters_dec(hb2);
 				double_unlock_hb(hb1, hb2);
-
+#ifdef CONFIG_SAFEFETCH
+				ret = get_user_no_dfcache(curval, uaddr1);
+#else
 				ret = get_user(curval, uaddr1);
+#endif
 				if (ret)
 					return ret;
 
diff --git a/kernel/futex/waitwake.c b/kernel/futex/waitwake.c
index e2bbe5509ec2..bf4ed107aff8 100644
--- a/kernel/futex/waitwake.c
+++ b/kernel/futex/waitwake.c
@@ -629,7 +629,11 @@ int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
 		if (ret) {
 			futex_q_unlock(hb);
 
+#ifdef CONFIG_SAFEFETCH
+			ret = get_user_no_dfcache(uval, uaddr);
+#else
 			ret = get_user(uval, uaddr);
+#endif
 			if (ret)
 				return ret;
 
-- 
2.25.1