Re: Possible race at user mode helper in request_firmware

From: Chandra Sekhar Lingutla
Date: Wed Mar 16 2016 - 03:02:17 EST

Hi Ming,

On 03/16/2016 07:36 AM, Ming Lei wrote:
On Tue, Mar 15, 2016 at 11:09 PM, Ming Lei <ming.lei@xxxxxxxxxxxxx> wrote:

On Mon, Mar 14, 2016 at 8:00 PM, <clingutla@xxxxxxxxxxxxxx> wrote:

I see possible race in _request_firmware_load function, on which I wanted to
take your opinion.

When system is going to low power mode, device_cache_fw_images() is called
from pm notifier which schedules asynchronous workers to cache devices
If more than 2 async requests falls under user helper mode, then there is a
use after free of "firmware" directory kobject.

If two requests are for getting same firmware image, only one can be sent
out and be completed with user helper, please see fw_lookup_and_allocate_buf(),
so the race shouldn't have been triggered in this case.

So I guess your test must be involved with at least two different requests.

The race will hit, when one asynchronous worker calls device_del() while
other asynchronous worker calls get_device_parent() in
After loading firmware, first worker calls device_del(), and "firmware"
directory ref count is 1, so cleanup_glue_dir() calls kobject_put() with
gdp_mutex lock held.
Meanwhile, second async worker calls device_add(), and get_device_parent()
is still able to find the firmware directory kobject in ksets after getting
This kobject gets added as parent to the second device kobject.
By the time of accessing the parent kobject, its ref count was 0 and prints
below warning stack followed by crash.

The trick here is just that we don't use 'struct device' in
then the firmware device's parent is null during calling device_add() from
_request_firmware_load(), but this should be allowed by driver core.

From firmware class view, both device_add()/device_del() is run with pair,
and driver core still complains during get_device_parent(), so something
inside driver core should be be wrong, IMO.

Thinking about the issue further, I believe it is a false positive which is
caused by CONFIG_DEBUG_KOBJECT_RELEASE, follows the story:

1) device_add() path


/* find our class-directory at the parent and reference it */
list_for_each_entry(k, &dev->class->p->glue_dirs.list, entry)
if (k->parent == parent_kobj) {
kobj = kobject_get(k);
if (kobj) {
return kobj;
k = class_dir_create_and_add(dev->class, parent_kobj);

2) device_del() path


So gdp_mutex is held wrt. both looking up/adding the parent kobject
and removing the parent kobject. Actually with this lock, both glur_dir
kobject's referece counting and joining/leaving kset are run atomically.

The race you found is triggered by CONFIG_DEBUG_KOBJECT_RELEASE,
which causes kobject_cleanup() to be run in workqueue context and the
lock of gdp_mutex can't be hold in that situation anymore. That is the root
cause for your race.

That also said DEBUG_KOBJECT_RELEASE might cause trouble in
some situations.


I agree that, CONFIG_DEBUG_KOBJECT_RELEASE can cause the race, as it delays
call to kobject_cleanup() which deletes kobj entry in glue_dir list.

I see the race even after disabling CONFIG_DEBUG_KOBJECT_RELEASE.

The race is between kobject_put() and kobject_get() for the "firmware" directory.

In device_del() path, I see the race after the call of cleanup_device_parent().
cleanup_device_parent() calls kobject_put(glue_dir) with gdp_mutex lock holding,
So ideally, to call kobject_put/kobject_get(glue_dir) we need to acquire gdp_mutex.

but kobject_del(kobj) will not hold the gdp_mutex lock while calling kobject_put(kobj->parent).

Here is the race:

Consider 2 async works calls request_firmware() for 2 different binaries (fw1.bin, fw2.bin)
Async work1 got scheduled first, and it falls in user helper mode so it creates "firmware" dir
and then creates "fw1.bin" directory to load data from user.
After load completion or timeout, it starts cleanup dir's, meantime async work2 got scheduled and
it also falls in user helper mode, and story starts here:

PATH1 req fw for "fw1.bin" PATH2
req fw for "fw2.bin"


kobject_put(kobj->parent); --> firmware ref ->1
mutex_unlock(&gdp_mutex); ->get_device_parent()
->kobject_del() mutex_lock(&gdp_mutex)
kobj_kset_leave(kobj); -->del "fw1.bin" kobj in kset list_for_each_entry(k, &dev->class->p->glue_dirs.list, entry)
kobject_put(kobj->parent); /* No lock :)*/ if (k->parent == parent_kobj) {

->kref_put() --> firmware ref ->0 kobj = kobject_get(k); -> k is "firmware"
->kboject_release() break;

->kobject_del() }

/*here deletes firmware entry in kset*/ mutex_unlock(&gdp_mutex)
->dev2->kobj.parent = kobj -> kobj is released already

PATH 1 is started removing the kobject of "fw1.bin" and its parent "firmware"
cleanup_device_parent(dev1) makes firmware ref count to 1, by calling kobject_put(firmware) with gdp_mutex held.

and kobject_del(dev1->kobj) will again calls kobject_put(kobj->parent) without holding gdp_mutex.

In PATH2, glue_dir is able to find "firmware" kobj, as firmware dir kobj is deleted from the glue_dir list at kboject_release().
and it calls kobject_get(firmware), by the time PATH1 calls kobject_release(firmware).

So PATH2 got reference of freed firmware kobj and added as parent to "fw2.bin" kobj.

I think, here is possible fix for this race:

Signed-off-by: Lingutla Chandrasekhar <clingutla@xxxxxxxxxxxxxx>

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 5d8b713..47bc082 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -1224,7 +1224,13 @@ void device_del(struct device *dev)
kobject_uevent(&dev->kobj, KOBJ_REMOVE);
- kobject_del(&dev->kobj);
+ if (dev->kobj.parent->kset == &dev->class->p->glue_dirs) {
+ mutex_lock(&gdp_mutex);
+ kobject_del(&dev->kobj);
+ mutex_unlock(&gdp_mutex);
+ } else
+ kobject_del(&dev->kobj);


[21147.455038] ------------[ cut here ]------------
[21147.458910] WARNING: at <...>/kernel/include/linux/kref.h:47
[21147.481199] Modules linked in: core_ctl(PO) qdrbg_module(O)
[21147.481256] CPU: 2 PID: 23935 Comm: kworker/u16:8 Tainted: P W O
3.10.84-g0957845-00427-g56a05c2 #1
[21147.481284] Workqueue: events_unbound async_run_entry_fn
[21147.481296] Call trace:
[21147.481315] [<ffffffc000206b60>] dump_backtrace+0x0/0x270
[21147.481331] [<ffffffc000206de0>] show_stack+0x10/0x1c
[21147.481353] [<ffffffc000d569ec>] dump_stack+0x1c/0x28
[21147.481370] [<ffffffc00021cc18>] warn_slowpath_common+0x74/0x9c
[21147.481383] [<ffffffc00021ce4c>] warn_slowpath_null+0x14/0x20
[21147.481397] [<ffffffc000456b10>] kobject_get+0x4c/0x68
[21147.481416] [<ffffffc000609f64>] get_device_parent+0xa0/0x194
[21147.481430] [<ffffffc00060a3f4>] device_add+0x100/0x600
[21147.481446] [<ffffffc00061c390>] _request_firmware+0x8b4/0xa80
[21147.481459] [<ffffffc00061c58c>] request_firmware+0x30/0x3c
[21147.481473] [<ffffffc00061c5ec>] cache_firmware+0x54/0xb0
[21147.481490] [<ffffffc00061c65c>] __async_dev_cache_fw_image+0x14/0x54
[21147.481505] [<ffffffc000243794>] async_run_entry_fn+0x6c/0x12c
[21147.481521] [<ffffffc000237e88>] process_one_work+0x264/0x3dc
[21147.481535] [<ffffffc0002392c0>] worker_thread+0x1f0/0x340
[21147.481553] [<ffffffc00023e500>] kthread+0xac/0xb8
[21147.481563] ---[ end trace dabc98ea48b8ba59 ]---

[21147.486436] Unable to handle kernel paging request at virtual address
[21147.493816] pgd = ffffffc0a982d000
[21147.498337] [6b6b6b6b6b6b6bcb] *pgd=0000000000000000
[21147.502287] Internal error: Oops: 96000004 [#1] PREEMPT SMP
[21147.507826] Modules linked in: core_ctl(PO) qdrbg_module(O)
[21147.514951] CPU: 2 PID: 23935 Comm: kworker/u16:8 Tainted: P W O
3.10.84-g0957845-00427-g56a05c2 #1
[21147.524686] Workqueue: events_unbound async_run_entry_fn
[21147.529965] task: ffffffc008138000 ti: ffffffc007880000 task.ti:
[21147.537438] PC is at sysfs_create_dir+0x34/0xd4
[21147.541949] LR is at kobject_add_internal+0x120/0x24c
[21147.546979] pc : [<ffffffc000361fa8>] lr : [<ffffffc000456f5c>] pstate:
[21147.996295] Process kworker/u16:8 (pid: 23935, stack limit =
[21148.003673] Call trace:
[21148.006117] [<ffffffc000361fa8>] sysfs_create_dir+0x34/0xd4
[21148.011663] [<ffffffc000456f58>] kobject_add_internal+0x11c/0x24c
[21148.017737] [<ffffffc0004573ec>] kobject_add+0xc8/0xe4
[21148.022863] [<ffffffc00060a410>] device_add+0x11c/0x600
[21148.028069] [<ffffffc00061c390>] _request_firmware+0x8b4/0xa80
[21148.033886] [<ffffffc00061c58c>] request_firmware+0x30/0x3c
[21148.039439] [<ffffffc00061c5ec>] cache_firmware+0x54/0xb0
[21148.044822] [<ffffffc00061c65c>] __async_dev_cache_fw_image+0x14/0x54
[21148.051249] [<ffffffc000243794>] async_run_entry_fn+0x6c/0x12c
[21148.057064] [<ffffffc000237e88>] process_one_work+0x264/0x3dc
[21148.062792] [<ffffffc0002392c0>] worker_thread+0x1f0/0x340
[21148.068263] [<ffffffc00023e500>] kthread+0xac/0xb8
[21148.073039] Code: b5000094 14000026 d000ac54 9125a294 (7940c281)
[21148.080312] ---[ end trace dabc98ea48b8ba5a ]---

Below is the callstack where the parent is freed from the slub_debug info.
0xFFFFFFC000608D74 \\vmlinux\base/core\class_dir_release+0x0C
0xFFFFFFC0002FE6BC \\vmlinux\slub\free_debug_processing\fail+0x114
0xFFFFFFC0002FE7A0 \\vmlinux\slub\__slab_free+0x44
0xFFFFFFC0002FEE60 \\vmlinux\slub\kfree+0x1F8
0xFFFFFFC000608D70 \\vmlinux\base/core\class_dir_release+0x8
0xFFFFFFC000456D70 \\vmlinux\kobject\kobject_release+0x134
0xFFFFFFC000456E10 \\vmlinux\kobject\kobject_put+0x58
0xFFFFFFC000456C28 \\vmlinux\kobject\kobject_del+0x64
0xFFFFFFC000609D78 \\vmlinux\base/core\device_del+0x150
0xFFFFFFC00061C524 \\vmlinux\firmware_class\_request_firmware\out+0x71C
0xFFFFFFC00061C58C \\vmlinux\firmware_class\request_firmware+0x30
0xFFFFFFC00061C5EC \\vmlinux\firmware_class\cache_firmware+0x54
0xFFFFFFC00061C65C \\vmlinux\firmware_class\__async_dev_cache_fw_image+0x14
0xFFFFFFC000243794 \\vmlinux\async\async_run_entry_fn+0x6C
0xFFFFFFC000237E88 \\vmlinux\workqueue\process_one_work+0x264
0xFFFFFFC0002392C0 \\vmlinux\workqueue\worker_thread\recheck+0x1A0
0xFFFFFFC00023E500 \\vmlinux\kthread\kthread+0xAC

For similar type of issue, I see there is an existing fix: "sysfs: driver
core: Fix glue dir race condition by gdp_mutex".

Still I am able to reproduce the issue. I have verified this race on
kernels: 3.10, 3.18 and 4.4.

I followed below procedure to reproduce the issue:
2. Use test_firmware, modified the test for async calls
3. Replace WARN_ON with BUG_ON in kref_get().
I ran below script from shell:
while [ 1 ]
echo 3 > /sys/devices/virtual/misc/test_firmware/trigger_ufw
if [ $? -ne 0 ]; then
print "Exiting.. "
exit 1
count=$(($count +1))
echo "count $count"

With below patch, I could reproduce the issue in the 2nd iteration.

I will run your test script to see if there is new findings.


To reproduce the race condition, use firmware test and
CONFIG_DEBUG_KOBJECT_RELEASE=y, which delays kobject
releases. So that we can catch the bug easily.

Enabled below flags in kernel config file:


Looks I can't reproduce the issue with your approach.

diff --git a/include/linux/kref.h b/include/linux/kref.h
index 484604d..727fb24 100644
--- a/include/linux/kref.h
+++ b/include/linux/kref.h
@@ -44,7 +44,7 @@ static inline void kref_get(struct kref *kref)
* condition when this kref is freeing by some other thread right
* In this case one should use kref_get_unless_zero()
- WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
+ BUG_ON(atomic_inc_return(&kref->refcount) < 2);

diff --git a/lib/test_firmware.c b/lib/test_firmware.c
old mode 100644
new mode 100755
index 86374c1..14c9598
--- a/lib/test_firmware.c
+++ b/lib/test_firmware.c
@@ -18,6 +18,7 @@
#include <linux/miscdevice.h>
#include <linux/slab.h>
#include <linux/uaccess.h>
+#include <linux/async.h>

+static ASYNC_DOMAIN_EXCLUSIVE(ufw_domain);
+static void __async_ufw_work(void *name,
+ async_cookie_t cookie)
+ const struct firmware *fw;
+ const char *fw_name = name;
+ int ret;
+ pr_err("requesting %s\n", fw_name);
+ ret = request_firmware(&fw, fw_name, NULL);
+ pr_err("loaded: %zu\n", fw ? fw->size : 0);
+ if (!ret)
+ kfree(fw);
+static char *name[] = { "fw1.bin", "fw2.bin"};
+static ssize_t trigger_ufw_store(struct device *dev,
+ struct device_attribute *attr,
+ const char *buf, size_t count)
+ int rc, i, c=0;
+ u32 start;
+ char *fw_name;
+ rc = kstrtou32(buf, 0, &start);
+ if (rc){
+ pr_err("Invalid option\n");
+ return rc;
+ }
+ pr_err(" no of iterations %d\n", start);
+ for (i=0; i< start; i++)
+ {
+ if (c >= 2)
+ c = 0;
+ fw_name = name[c++];
+ async_schedule_domain(__async_ufw_work, (void *)fw_name,
+ }
+ async_synchronize_full_domain(&ufw_domain);
+ return count;

static DEVICE_ATTR_WO(trigger_request);
+static DEVICE_ATTR_WO(trigger_ufw);

static int __init test_firmware_init(void)
@@ -92,6 +140,12 @@ static int __init test_firmware_init(void)
goto dereg;

+ rc = device_create_file(test_fw_misc_device.this_device,
+ &dev_attr_trigger_ufw);
+ if (rc) {
+ pr_err("could not create sysfs interface: %d\n", rc);
+ goto dereg;
+ }
pr_warn("interface ready\n");

return 0;
@@ -106,7 +160,9 @@ static void __exit test_firmware_exit(void)
- &dev_attr_trigger_request);
+ &dev_attr_trigger_request);
+ device_remove_file(test_fw_misc_device.this_device,
+ &dev_attr_trigger_ufw);
pr_warn("removed interface\n");


Thanks and regards,
Chandrasekhar L.
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
Code Aurora Forum,
a Linux Foundation Collaborative Project.