[greybus-dev] Re: [PATCH 1/2 RESEND] greybus: raw: fix use-after-free on cdev close

On Wed, Mar 11, 2026 at 05:25:10PM -0400, Damien Riégel wrote:

This addresses a use-after-free bug when a raw bundle is disconnected but its chardev is still opened by an application. When the application releases the cdev, it causes the following panic when init on free is enabled (CONFIG_INIT_ON_FREE_DEFAULT_ON=y):

[   78.451062] refcount_t: underflow; use-after-free.
[   78.451352] WARNING: CPU: 0 PID: 139 at lib/refcount.c:28 refcount_warn_saturate+0xd0/0x130

[   78.451698] Modules linked in: gb_raw(C)
[   78.451881] CPU: 0 UID: 0 PID: 139 Comm: raw_chardev_tes Tainted: G        WC          6.18.0-rc4 #212 PREEMPT(voluntary)
[   78.452386] Tainted: [W]=WARN, [C]=CRAP
[   78.452560] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
[   78.453049] RIP: 0010:refcount_warn_saturate+0xd0/0x130
[   78.453311] Code: 0b 90 90 c3 cc cc cc cc 80 3d 4f ec 1d 01 00 0f 85 75 ff ff ff c6 05 42 ec 1d 01 01 90 48 c7 c7 e8 5b cb b4 e8 31f
[   78.453953] RSP: 0018:ffffaa0f80203ed0 EFLAGS: 00010282
[   78.454251] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[   78.454472] RDX: 0000000000000000 RSI: ffffaa0f80203d68 RDI: 00000000ffffdfff
[   78.454690] RBP: 00000000040e001f R08: 00000000ffffdfff R09: ffffffffb510c008
[   78.454899] R10: ffffffffb505c060 R11: 0000000063666572 R12: ffff938dc210b468
[   78.455279] R13: ffff938dc1f5e1a0 R14: ffff938dc14710c0 R15: 0000000000000000
[   78.455549] FS:  00007f2f22741740(0000) GS:ffff938e11fbc000(0000) knlGS:0000000000000000
[   78.455806] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   78.456129] CR2: 00007f2f228c89c3 CR3: 00000000020d0000 CR4: 00000000000006f0

Please trim oopses like this in commit messages (and reports). The above (after the WARNING) doesn't add any value here.

[   78.456786] Call Trace:
[   78.456936]  <TASK>
[   78.457069]  cdev_put+0x18/0x30
[   78.457230]  __fput+0x255/0x2a0
[   78.457372]  __x64_sys_close+0x3d/0x80
[   78.457544]  do_syscall_64+0xa4/0x290
[   78.457697]  entry_SYSCALL_64_after_hwframe+0x77/0x7f

You can keep a (partial) call trace, and skip the rest.

[   78.457883] RIP: 0033:0x7f2f227d1cc7
[   78.458097] Code: 48 89 fa 4c 89 df e8 08 ae 00 00 8b 93 08 03 00 00 59 5e 48 83 f8 fc 74 1a 5b c3 0f 1f 84 00 00 00 00 00 48 8b 44f
[   78.458692] RSP: 002b:00007fffab36fb50 EFLAGS: 00000202 ORIG_RAX: 0000000000000003
[   78.459155] RAX: ffffffffffffffda RBX: 00007f2f22741740 RCX: 00007f2f227d1cc7
[   78.459400] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
[   78.459648] RBP: 00007fffab36fba8 R08: 0000000000000000 R09: 0000000000000000
[   78.459899] R10: 0000000000000000 R11: 0000000000000202 R12: 0000558298427128
[   78.460212] R13: 00007f2f227416d0 R14: 00005582c72cf320 R15: 00005582c72cf320
[   78.460470]  </TASK>
[   78.460571] ---[ end trace 0000000000000000 ]---

The cdev is contained in the "gb_raw" structure, which is freed in the disconnect operation. When the cdev is released at a later time, cdev_put gets an address that points to freed memory.

To fix this use-after-free, convert the struct device from a pointer to being embedded, that makes the lifetime of the cdev and of this device the same. Then, use cdev_device_add, which guarantees that the device won't be released until all references to the cdev are not released.

s/are not/have been/

Finally, delegate the freeing of the structure to the device release function, instead of freeing immediately in the disconnect callback.

Fixes: e806c7fb8e9b ("greybus: raw: add raw greybus kernel driver") Signed-off-by: Damien Riégel damien.riegel@silabs.com

---

resend: added linux-staging as Cc, this list was not part of the first submission.

drivers/staging/greybus/raw.c | 49 +++++++++++++++++++---------------- 1 file changed, 26 insertions(+), 23 deletions(-)

diff --git a/drivers/staging/greybus/raw.c b/drivers/staging/greybus/raw.c index 71de6776739..b92214f97e3 100644 --- a/drivers/staging/greybus/raw.c +++ b/drivers/staging/greybus/raw.c @@ -21,9 +21,8 @@ struct gb_raw { struct list_head list; int list_data; struct mutex list_lock;

• dev_t dev; struct cdev cdev;
• struct device *device;

• struct device dev;

}; struct raw_data { @@ -148,6 +147,13 @@ static int gb_raw_send(struct gb_raw *raw, u32 len, const char __user *data) return retval; } +static void raw_dev_release(struct device *dev) +{

• struct gb_raw *raw = dev_get_drvdata(dev);

I think it's more common to use container_of here (and skip the set_drvdata below).

• kfree(raw);

+}

static int gb_raw_probe(struct gb_bundle *bundle, const struct greybus_bundle_id *id) { @@ -168,11 +174,14 @@ static int gb_raw_probe(struct gb_bundle *bundle, if (!raw) return -ENOMEM;

• device_initialize(&raw->dev);

• dev_set_drvdata(&raw->dev, raw);

Skip this.

• connection = gb_connection_create(bundle, le16_to_cpu(cport_desc->id), gb_raw_request_handler); if (IS_ERR(connection)) { retval = PTR_ERR(connection);

• goto error_free;
  

• goto error_put_device;
  

As Dan already pointed out, you need to set the release function (initialise dev) before calling put device.

} INIT_LIST_HEAD(&raw->list); @@ -187,29 +196,26 @@ static int gb_raw_probe(struct gb_bundle *bundle, goto error_connection_destroy; }

• raw->dev = MKDEV(raw_major, minor);

• raw->dev.devt = MKDEV(raw_major, minor);
• raw->dev.class = &raw_class;
• raw->dev.parent = &connection->bundle->dev;
• raw->dev.release = raw_dev_release;
• retval = dev_set_name(&raw->dev, "gb!raw%d", minor);
• if (retval)

• goto error_remove_ida;
  

• cdev_init(&raw->cdev, &raw_fops);

retval = gb_connection_enable(connection); if (retval) goto error_remove_ida;

• retval = cdev_add(&raw->cdev, raw->dev, 1);

• retval = cdev_device_add(&raw->cdev, &raw->dev); if (retval) goto error_connection_disable;

• raw->device = device_create(&raw_class, &connection->bundle->dev,

• 		    raw->dev, raw, "gb!raw%d", minor);
  

• if (IS_ERR(raw->device)) {

• retval = PTR_ERR(raw->device);
  

• goto error_del_cdev;
  

• }
• return 0;

-error_del_cdev:

• cdev_del(&raw->cdev);

error_connection_disable: gb_connection_disable(connection); @@ -219,8 +225,8 @@ static int gb_raw_probe(struct gb_bundle *bundle, error_connection_destroy: gb_connection_destroy(connection); -error_free:

• kfree(raw);

+error_put_device:

• put_device(&raw->dev); return retval;

} @@ -231,11 +237,9 @@ static void gb_raw_disconnect(struct gb_bundle *bundle) struct raw_data *raw_data; struct raw_data *temp;

• // FIXME - handle removing a connection when the char device node is open.
• device_destroy(&raw_class, raw->dev);
• cdev_del(&raw->cdev);

• cdev_device_del(&raw->cdev, &raw->dev); gb_connection_disable(connection);

• ida_free(&minors, MINOR(raw->dev));

• ida_free(&minors, MINOR(raw->dev.devt));

I think you should move this to the release function as well so that the minor number doesn't get reused until all references are gone.

gb_connection_destroy(connection); mutex_lock(&raw->list_lock); @@ -244,8 +248,7 @@ static void gb_raw_disconnect(struct gb_bundle *bundle) kfree(raw_data); } mutex_unlock(&raw->list_lock);

• kfree(raw);

• put_device(&raw->dev);

}

Johan