CVE-2022-48838

In the Linux kernel, the following vulnerability has been resolved:

usb: gadget: Fix use-after-free bug by not setting udc->dev.driver

The syzbot fuzzer found a use-after-free bug:

BUG: KASAN: use-after-free in dev_uevent+0x712/0x780 drivers/base/core.c:2320 Read of size 8 at addr ffff88802b934098 by task udevd/3689

CPU: 2 PID: 3689 Comm: udevd Not tainted 5.17.0-rc4-syzkaller-00229-g4f12b742eb2b #0 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-2 04/01/2014 Call Trace: __dump_stack lib/dump_stack.c:88 [inline] dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106 print_address_description.constprop.0.cold+0x8d/0x303 mm/kasan/report.c:255 __kasan_report mm/kasan/report.c:442 [inline] kasan_report.cold+0x83/0xdf mm/kasan/report.c:459 dev_uevent+0x712/0x780 drivers/base/core.c:2320 uevent_show+0x1b8/0x380 drivers/base/core.c:2391 dev_attr_show+0x4b/0x90 drivers/base/core.c:2094

Although the bug manifested in the driver core, the real cause was a race with the gadget core. dev_uevent() does:

if (dev->driver)
	add_uevent_var(env, DRIVER=%s, dev->driver->name);

and between the test and the dereference of dev->driver, the gadget core sets dev->driver to NULL.

The race wouldnt occur if the gadget core registered its devices on a real bus, using the standard synchronization techniques of the driver core. However, its not necessary to make such a large change in order to fix this bug; all we need to do is make sure that udc->dev.driver is always NULL.

In fact, there is no reason for udc->dev.driver ever to be set to anything, let alone to the value it currently gets: the address of the gadgets driver. After all, a gadget driver only knows how to manage a gadget, not how to manage a UDC.

This patch simply removes the statements in the gadget core that touch udc->dev.driver.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/00bdd9bf1ac6d401ad926d3d8df41b9f1399f646
• https://git.kernel.org/stable/c/16b1941eac2bd499f065a6739a40ce0011a3d740
• https://git.kernel.org/stable/c/2015c23610cd0efadaeca4d3a8d1dae9a45aa35a
• https://git.kernel.org/stable/c/2282a6eb6d4e118e294e43dcc421e0e0fe4040b5
• https://git.kernel.org/stable/c/27d64436984fb8835a8b7e95993193cc478b162e
• https://git.kernel.org/stable/c/4325124dde6726267813c736fee61226f1d38f0b
• https://git.kernel.org/stable/c/609a7119bffe3ddd7c93f2fa65be8917e02a0b7e
• https://git.kernel.org/stable/c/e2d3a7009e505e120805f449c832942660f3f7f3
• https://git.kernel.org/stable/c/00bdd9bf1ac6d401ad926d3d8df41b9f1399f646
• https://git.kernel.org/stable/c/16b1941eac2bd499f065a6739a40ce0011a3d740
• https://git.kernel.org/stable/c/2015c23610cd0efadaeca4d3a8d1dae9a45aa35a
• https://git.kernel.org/stable/c/2282a6eb6d4e118e294e43dcc421e0e0fe4040b5
• https://git.kernel.org/stable/c/27d64436984fb8835a8b7e95993193cc478b162e
• https://git.kernel.org/stable/c/4325124dde6726267813c736fee61226f1d38f0b
• https://git.kernel.org/stable/c/609a7119bffe3ddd7c93f2fa65be8917e02a0b7e
• https://git.kernel.org/stable/c/e2d3a7009e505e120805f449c832942660f3f7f3