CVE-2023-52894

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

usb: gadget: fncm: fix potential NULL ptr deref in ncmbitrate()

In Google internal bug 265639009 we've received an (as yet) unreproducible crash report from an aarch64 GKI 5.10.149-android13 running device.

AFAICT the source code is at: https://android.googlesource.com/kernel/common/+/refs/tags/ASB-2022-12-05_13-5.10

The call stack is: ncmclose() -> ncmnotify() -> ncmdonotify() with the crash at: ncmdonotify+0x98/0x270 Code: 79000d0b b9000a6c f940012a f9400269 (b9405d4b)

Which I believe disassembles to (I don't know ARM assembly, but it looks sane enough to me...):

// halfword (16-bit) store presumably to event->wLength (at offset 6 of struct usbcdcnotification) 0B 0D 00 79 strh w11, [x8, #6]

// word (32-bit) store presumably to req->Length (at offset 8 of struct usb_request) 6C 0A 00 B9 str w12, [x19, #8]

// x10 (NULL) was read here from offset 0 of valid pointer x9 // IMHO we're reading 'cdev->gadget' and getting NULL // gadget is indeed at offset 0 of struct usbcompositedev 2A 01 40 F9 ldr x10, [x9]

// loading req->buf pointer, which is at offset 0 of struct usb_request 69 02 40 F9 ldr x9, [x19]

// x10 is null, crash, appears to be attempt to read cdev->gadget->max_speed 4B 5D 40 B9 ldr w11, [x10, #0x5c]

which seems to line up with ncmdonotify() case NCMNOTIFYSPEED code fragment:

event->wLength = cputole16(8); req->length = NCMSTATUSBYTECOUNT;

/* SPEEDCHANGE data is up/down speeds in bits/sec */ data = req->buf + sizeof *event; data[0] = cputole32(ncmbitrate(cdev->gadget));

My analysis of registers and NULL ptr deref crash offset (Unable to handle kernel NULL pointer dereference at virtual address 000000000000005c) heavily suggests that the crash is due to 'cdev->gadget' being NULL when executing: data[0] = cputole32(ncmbitrate(cdev->gadget)); which calls: ncmbitrate(NULL) which then calls: gadgetissuperspeed(NULL) which reads ((struct usbgadget *)NULL)->maxspeed and hits a panic.

AFAICT, if I'm counting right, the offset of maxspeed is indeed 0x5C. (remember there's a GKI KABI reservation of 16 bytes in struct workstruct)

It's not at all clear to me how this is all supposed to work... but returning 0 seems much better than panic-ing...