Re: net/can: use-after-free in bcm_rx_thr_flush

From: Oliver Hartkopp
Date: Tue Nov 22 2016 - 12:30:01 EST

Hi Andrey,

thanks for the report.

Although I can't see the issue in the code ...

On 11/22/2016 10:22 AM, Andrey Konovalov wrote:

BUG: KASAN: use-after-free in bcm_rx_thr_flush+0x284/0x2b0
Read of size 1 at addr ffff88006c1faae5 by task a.out/3874

page:ffffea0001b07e80 count:1 mapcount:0 mapping: (null) index:0x0
flags: 0x100000000000080(slab)
page dumped because: kasan: bad access detected


The buggy address belongs to the object at ffff88006c1faae0
which belongs to the cache kmalloc-32 of size 32


The buggy address ffff88006c1faae5 is located 5 bytes inside
of 32-byte region [ffff88006c1faae0, ffff88006c1fab00)


Memory state around the buggy address:
ffff88006c1fa980: fc fc fb fb fb fb fc fc fb fb fb fb fc fc fb fb
ffff88006c1faa00: fb fb fc fc fb fb fb fb fc fc fb fb fb fb fc fc
ffff88006c1faa80: fb fb fb fb fc fc fb fb fb fb fc fc fb fb fb fb
ffff88006c1fab00: fc fc fb fb fb fb fc fc 00 00 00 00 fc fc 00 00
ffff88006c1fab80: 00 00 fc fc fb fb fb fb fc fc fb fb fb fb fc fc

(should be some zero initialized memory here)

The relevant code of bcm_rx_do_flush() can be found here:

static inline int bcm_rx_do_flush(struct bcm_op *op, int update,
unsigned int index)
struct canfd_frame *lcf = op->last_frames + op->cfsiz * index;

if ((op->last_frames) && (lcf->flags & RX_THR)) { <<<----- !!!
if (update)
bcm_rx_changed(op, lcf);
return 1;
return 0;

lcf->flags points into an array of struct canfd_frame at offset 5 which is allocated here:

/* create and init array for received CAN frames */
op->last_frames = kzalloc(msg_head->nframes * op->cfsiz,

So why does KASAN complain about accessing some kind of 32 byte cache when it should point into a zero initialized allocated space?

I will write some other test cases with a similar setting of options to check if I can trigger the instability too.

Tnx & regards,