Re: [PATCH] mm/gup_test: fix race with PIN_LONGTERM_TEST ioctls

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/8/26 04:50, Yunhui Cui wrote:
> The PIN_LONGTERM_TEST helpers keep their state in global variables that
> are protected by pin_longterm_test_mutex when accessed from ioctl().
> However, gup_test_release() calls pin_longterm_test_stop() without
> holding that mutex.
> 
> This can race with PIN_LONGTERM_TEST_STOP and let two callers operate on
> the same pages array concurrently, corrupting the test state and possibly
> freeing it twice:
> 
>  CPU 0                              CPU 1
>  -----                              -----
>  ioctl(PIN_LONGTERM_TEST_STOP)
>    mutex_lock(&pin_longterm_test_mutex)
>    pin_longterm_test_stop()
>      if (pin_longterm_test_pages)
>        kvfree(pin_longterm_test_pages)
> 
>                                     close()
>                                       gup_test_release()
>                                         pin_longterm_test_stop()
>                                           if (pin_longterm_test_pages)
>                                             kvfree(pin_longterm_test_pages)
> 
>      pin_longterm_test_pages = NULL
>    mutex_unlock(&pin_longterm_test_mutex)

Okay, thinking about this some more ...

I think what's really required here is that we have two separate "struct file",
because otherwise release() cannot race with unlocked_ioctl().

Which is something we didn't expect when we added this functionality.

I think the proper way to handle this is by moving the state to the
"struct file", to actually cleanly allow concurrent usage.

So instead, I think we should do the following (untested):

From 29e3d6fe00c4bd843d11bb548efa89bca478436c Mon Sep 17 00:00:00 2001
From: "David Hildenbrand (Arm)" <david@kernel.org>
Date: Fri, 12 Jun 2026 09:22:25 +0200
Subject: [PATCH] mm/gup_test: keep longterm pin state per file

The pin longterm test currently stores its data globally, shared among
multiple concurrent users of the interface (multiple open file
descriptors -> multiple "struct file"'s). That makes
the gup_test interface problematic to use concurrently: two users, such
as concurrent selftest runs, can interfere with the same longterm
pin state.

While this has not been observed as a problem so far in practice, let's
just handle it cleanly. There could be a way to trigger selftest
failures by e.g., running the cow.c and gup_longerm.c selftests
concurrently, but we usually run them sequentially. Let's add a "Fixes"
tag to be safe.

Fixes: c77369b437f9 ("mm/gup_test: start/stop/read functionality for PIN LONGTERM test")
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/gup_test.c | 93 +++++++++++++++++++++++++++++++++------------------
 1 file changed, 61 insertions(+), 32 deletions(-)

diff --git a/mm/gup_test.c b/mm/gup_test.c
index 9dd48db897b9..16916056677e 100644
--- a/mm/gup_test.c
+++ b/mm/gup_test.c
@@ -8,6 +8,12 @@
 #include <linux/highmem.h>
 #include "gup_test.h"
 
+struct gup_test_data {
+	struct mutex longterm_mutex;
+	struct page **longterm_pages;
+	unsigned long longterm_nr_pages;
+};
+
 static void put_back_pages(unsigned int cmd, struct page **pages,
 			   unsigned long nr_pages, unsigned int gup_test_flags)
 {
@@ -204,23 +210,20 @@ static int __gup_test_ioctl(unsigned int cmd,
 	return ret;
 }
 
-static DEFINE_MUTEX(pin_longterm_test_mutex);
-static struct page **pin_longterm_test_pages;
-static unsigned long pin_longterm_test_nr_pages;
-
-static inline void pin_longterm_test_stop(void)
+static inline void pin_longterm_test_stop(struct gup_test_data *data)
 {
-	if (pin_longterm_test_pages) {
-		if (pin_longterm_test_nr_pages)
-			unpin_user_pages(pin_longterm_test_pages,
-					 pin_longterm_test_nr_pages);
-		kvfree(pin_longterm_test_pages);
-		pin_longterm_test_pages = NULL;
-		pin_longterm_test_nr_pages = 0;
+	if (data->longterm_pages) {
+		if (data->longterm_nr_pages)
+			unpin_user_pages(data->longterm_pages,
+					 data->longterm_nr_pages);
+		kvfree(data->longterm_pages);
+		data->longterm_pages = NULL;
+		data->longterm_nr_pages = 0;
 	}
 }
 
-static inline int pin_longterm_test_start(unsigned long arg)
+static inline int pin_longterm_test_start(struct gup_test_data *data,
+		unsigned long arg)
 {
 	long nr_pages, cur_pages, addr, remaining_pages;
 	int gup_flags = FOLL_LONGTERM;
@@ -229,7 +232,7 @@ static inline int pin_longterm_test_start(unsigned long arg)
 	int ret = 0;
 	bool fast;
 
-	if (pin_longterm_test_pages)
+	if (data->longterm_pages)
 		return -EINVAL;
 
 	if (copy_from_user(&args, (void __user *)arg, sizeof(args)))
@@ -259,12 +262,12 @@ static inline int pin_longterm_test_start(unsigned long arg)
 		return -EINTR;
 	}
 
-	pin_longterm_test_pages = pages;
-	pin_longterm_test_nr_pages = 0;
+	data->longterm_pages = pages;
+	data->longterm_nr_pages = 0;
 
-	while (nr_pages - pin_longterm_test_nr_pages) {
-		remaining_pages = nr_pages - pin_longterm_test_nr_pages;
-		addr = args.addr + pin_longterm_test_nr_pages * PAGE_SIZE;
+	while (nr_pages - data->longterm_nr_pages) {
+		remaining_pages = nr_pages - data->longterm_nr_pages;
+		addr = args.addr + data->longterm_nr_pages * PAGE_SIZE;
 
 		if (fast)
 			cur_pages = pin_user_pages_fast(addr, remaining_pages,
@@ -273,11 +276,11 @@ static inline int pin_longterm_test_start(unsigned long arg)
 			cur_pages = pin_user_pages(addr, remaining_pages,
 						   gup_flags, pages);
 		if (cur_pages < 0) {
-			pin_longterm_test_stop();
+			pin_longterm_test_stop(data);
 			ret = cur_pages;
 			break;
 		}
-		pin_longterm_test_nr_pages += cur_pages;
+		data->longterm_nr_pages += cur_pages;
 		pages += cur_pages;
 	}
 
@@ -286,19 +289,20 @@ static inline int pin_longterm_test_start(unsigned long arg)
 	return ret;
 }
 
-static inline int pin_longterm_test_read(unsigned long arg)
+static inline int pin_longterm_test_read(struct gup_test_data *data,
+		unsigned long arg)
 {
 	__u64 user_addr;
 	unsigned long i;
 
-	if (!pin_longterm_test_pages)
+	if (!data->longterm_pages)
 		return -EINVAL;
 
 	if (copy_from_user(&user_addr, (void __user *)arg, sizeof(user_addr)))
 		return -EFAULT;
 
-	for (i = 0; i < pin_longterm_test_nr_pages; i++) {
-		void *addr = kmap_local_page(pin_longterm_test_pages[i]);
+	for (i = 0; i < data->longterm_nr_pages; i++) {
+		void *addr = kmap_local_page(data->longterm_pages[i]);
 		unsigned long ret;
 
 		ret = copy_to_user((void __user *)(unsigned long)user_addr, addr,
@@ -314,25 +318,26 @@ static inline int pin_longterm_test_read(unsigned long arg)
 static long pin_longterm_test_ioctl(struct file *filep, unsigned int cmd,
 				    unsigned long arg)
 {
+	struct gup_test_data *data = filep->private_data;
 	int ret = -EINVAL;
 
-	if (mutex_lock_killable(&pin_longterm_test_mutex))
+	if (mutex_lock_killable(&data->longterm_mutex))
 		return -EINTR;
 
 	switch (cmd) {
 	case PIN_LONGTERM_TEST_START:
-		ret = pin_longterm_test_start(arg);
+		ret = pin_longterm_test_start(data, arg);
 		break;
 	case PIN_LONGTERM_TEST_STOP:
-		pin_longterm_test_stop();
+		pin_longterm_test_stop(data);
 		ret = 0;
 		break;
 	case PIN_LONGTERM_TEST_READ:
-		ret = pin_longterm_test_read(arg);
+		ret = pin_longterm_test_read(data, arg);
 		break;
 	}
 
-	mutex_unlock(&pin_longterm_test_mutex);
+	mutex_unlock(&data->longterm_mutex);
 	return ret;
 }
 
@@ -371,15 +376,39 @@ static long gup_test_ioctl(struct file *filep, unsigned int cmd,
 	return 0;
 }
 
+static int gup_test_open(struct inode *inode, struct file *file)
+{
+	struct gup_test_data *data;
+	int ret;
+
+	data = kzalloc(sizeof(*data), GFP_KERNEL);
+	if (!data)
+		return -ENOMEM;
+
+	ret = nonseekable_open(inode, file);
+	if (ret) {
+		kfree(data);
+		return ret;
+	}
+
+	mutex_init(&data->longterm_mutex);
+	file->private_data = data;
+	return 0;
+}
+
 static int gup_test_release(struct inode *inode, struct file *file)
 {
-	pin_longterm_test_stop();
+	struct gup_test_data *data = file->private_data;
+
+	pin_longterm_test_stop(data);
+	kfree(data);
+	file->private_data = NULL;
 
 	return 0;
 }
 
 static const struct file_operations gup_test_fops = {
-	.open = nonseekable_open,
+	.open = gup_test_open,
 	.unlocked_ioctl = gup_test_ioctl,
 	.compat_ioctl = compat_ptr_ioctl,
 	.release = gup_test_release,
-- 
2.43.0



Can you review+test that change? Thanks!

-- 
Cheers,

David