6.4 - CVE-2023-52886

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

USB: core: Fix race by not overwriting udev->descriptor in hub_port_init()

Syzbot reported an out-of-bounds read in sysfs.c:read_descriptors():

BUG: KASAN: slab-out-of-bounds in read_descriptors+0x263/0x280 drivers/usb/core/sysfs.c:883 Read of size 8 at addr ffff88801e78b8c8 by task udevd/5011

CPU: 0 PID: 5011 Comm: udevd Not tainted 6.4.0-rc6-syzkaller-00195-g40f71e7cd3c6 #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/27/2023 Call Trace:

__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xd9/0x150 lib/dump_stack.c:106
print_address_description.constprop.0+0x2c/0x3c0 mm/kasan/report.c:351
print_report mm/kasan/report.c:462 [inline]
kasan_report+0x11c/0x130 mm/kasan/report.c:572
read_descriptors+0x263/0x280 drivers/usb/core/sysfs.c:883 ... Allocated by task 758: ...
__do_kmalloc_node mm/slab_common.c:966 [inline]
__kmalloc+0x5e/0x190 mm/slab_common.c:979
kmalloc include/linux/slab.h:563 [inline]
kzalloc include/linux/slab.h:680 [inline]
usb_get_configuration+0x1f7/0x5170 drivers/usb/core/config.c:887
usb_enumerate_device drivers/usb/core/hub.c:2407 [inline]
usb_new_device+0x12b0/0x19d0 drivers/usb/core/hub.c:2545

As analyzed by Khazhy Kumykov, the cause of this bug is a race between read_descriptors() and hub_port_init(): The first routine uses a field in udev->descriptor, not expecting it to change, while the second overwrites it.

Prior to commit 45bf39f8df7f ("USB: core: Don't hold device lock while reading the "descriptors" sysfs file") this race couldn't occur, because the routines were mutually exclusive thanks to the device locking. Removing that locking from read_descriptors() exposed it to the race.

The best way to fix the bug is to keep hub_port_init() from changing udev->descriptor once udev has been initialized and registered. Drivers expect the descriptors stored in the kernel to be immutable; we should not undermine this expectation. In fact, this change should have been made long ago.

So now hub_port_init() will take an additional argument, specifying a buffer in which to store the device descriptor it reads. (If udev has not yet been initialized, the buffer pointer will be NULL and then hub_port_init() will store the device descriptor in udev as before.) This eliminates the data race responsible for the out-of-bounds read.

The changes to hub_port_init() appear more extensive than they really are, because of indentation changes resulting from an attempt to avoid writing to other parts of the usb_device structure after it has been initialized. Similar changes should be made to the code that reads the BOS descriptor, but that can be handled in a separate patch later on. This patch is sufficient to fix the bug found by syzbot.