[PATCH v3] futex: Use user_write_access_begin/_end() in futex_put_value()

[PATCH v3] futex: Use user_write_access_begin/_end() in futex_put_value()

[Waiman Long]

Commit cec199c5e39b ("futex: Implement FUTEX2_NUMA") introduces a new
futex_put_value() helper function to write a value to the given user
address. However, it uses user_read_access_begin() before the write.
For arches that differentiate between read and write accesses, like
powerpc, futex_put_value() fails with a -EFAULT return value.  Fix that
by using the user_write_access_begin/user_write_access_end() pair.

Fixes: cec199c5e39b ("futex: Implement FUTEX2_NUMA")
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/futex/futex.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index c74eac572acd..2cd57096c38e 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -319,13 +319,13 @@ static __always_inline int futex_put_value(u32 val, u32 __user *to)
 {
 	if (can_do_masked_user_access())
 		to = masked_user_access_begin(to);
-	else if (!user_read_access_begin(to, sizeof(*to)))
+	else if (!user_write_access_begin(to, sizeof(*to)))
 		return -EFAULT;
 	unsafe_put_user(val, to, Efault);
-	user_read_access_end();
+	user_write_access_end();
 	return 0;
 Efault:
-	user_read_access_end();
+	user_write_access_end();
 	return -EFAULT;
 }
 
-- 
2.50.1

[tip: locking/urgent] futex: Use user_write_access_begin/_end() in futex_put_value()

[tip-bot2 for Waiman Long]

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     dfb36e4a8db0cd56f92d4cb445f54e85a9b40897
Gitweb:        https://git.kernel.org/tip/dfb36e4a8db0cd56f92d4cb445f54e85a9b40897
Author:        Waiman Long <longman@redhat.com>
AuthorDate:    Mon, 11 Aug 2025 10:11:47 -04:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Mon, 11 Aug 2025 17:53:21 +02:00

futex: Use user_write_access_begin/_end() in futex_put_value()

Commit cec199c5e39b ("futex: Implement FUTEX2_NUMA") introduced the
futex_put_value() helper to write a value to the given user
address.

However, it uses user_read_access_begin() before the write. For
architectures that differentiate between read and write accesses, like
PowerPC, futex_put_value() fails with -EFAULT.

Fix that by using the user_write_access_begin/user_write_access_end() pair
instead.

Fixes: cec199c5e39b ("futex: Implement FUTEX2_NUMA")
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/20250811141147.322261-1-longman@redhat.com
---
 kernel/futex/futex.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index c74eac5..2cd5709 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -319,13 +319,13 @@ static __always_inline int futex_put_value(u32 val, u32 __user *to)
 {
 	if (can_do_masked_user_access())
 		to = masked_user_access_begin(to);
-	else if (!user_read_access_begin(to, sizeof(*to)))
+	else if (!user_write_access_begin(to, sizeof(*to)))
 		return -EFAULT;
 	unsafe_put_user(val, to, Efault);
-	user_read_access_end();
+	user_write_access_end();
 	return 0;
 Efault:
-	user_read_access_end();
+	user_write_access_end();
 	return -EFAULT;
 }