On Thu, Feb 27, 2020 at 11:50:11PM -0800, syzbot wrote: > Hello, > > syzbot found the following crash on: > > HEAD commit: f8788d86 Linux 5.6-rc3 > git tree: upstream > console output: https://syzkaller.appspot.com/x/log.txt?x=13005fd9e00000 > kernel config: https://syzkaller.appspot.com/x/.config?x=9833e26bab355358 > dashboard link: https://syzkaller.appspot.com/bug?extid=7dd7f2f77a7a01d1dc14 > compiler: gcc (GCC) 9.0.0 20181231 (experimental) > userspace arch: i386 > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=17e3ebede00000 > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16a9f8f9e00000 > > IMPORTANT: if you fix the bug, please add the following tag to the commit: > Reported-by: syzbot+7dd7f2f77a7a01d1dc14@xxxxxxxxxxxxxxxxxxxxxxxxx > > ================================================================== > BUG: KASAN: use-after-free in atomic_set include/asm-generic/atomic-instrumented.h:44 [inline] > BUG: KASAN: use-after-free in refcount_set include/linux/refcount.h:123 [inline] > BUG: KASAN: use-after-free in refcount_warn_saturate+0x1f/0x1f0 lib/refcount.c:15 > Write of size 4 at addr ffff888090eb4018 by task kworker/1:24/2888 > > CPU: 1 PID: 2888 Comm: kworker/1:24 Not tainted 5.6.0-rc3-syzkaller #0 > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 > Workqueue: events do_enable_set > Call Trace: > __dump_stack lib/dump_stack.c:77 [inline] > dump_stack+0x197/0x210 lib/dump_stack.c:118 > print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374 > __kasan_report.cold+0x1b/0x32 mm/kasan/report.c:506 > kasan_report+0x12/0x20 mm/kasan/common.c:641 > check_memory_region_inline mm/kasan/generic.c:185 [inline] > check_memory_region+0x134/0x1a0 mm/kasan/generic.c:192 > __kasan_check_write+0x14/0x20 mm/kasan/common.c:101 > atomic_set include/asm-generic/atomic-instrumented.h:44 [inline] > refcount_set include/linux/refcount.h:123 [inline] > refcount_warn_saturate+0x1f/0x1f0 lib/refcount.c:15 > refcount_sub_and_test include/linux/refcount.h:261 [inline] > refcount_dec_and_test include/linux/refcount.h:281 [inline] > kref_put include/linux/kref.h:64 [inline] > l2cap_chan_put+0x1d9/0x240 net/bluetooth/l2cap_core.c:498 > do_enable_set+0x54b/0x960 net/bluetooth/6lowpan.c:1075 > process_one_work+0xa05/0x17a0 kernel/workqueue.c:2264 > worker_thread+0x98/0xe40 kernel/workqueue.c:2410 > kthread+0x361/0x430 kernel/kthread.c:255 > ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352 > > Allocated by task 2888: > save_stack+0x23/0x90 mm/kasan/common.c:72 > set_track mm/kasan/common.c:80 [inline] > __kasan_kmalloc mm/kasan/common.c:515 [inline] > __kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:488 > kasan_kmalloc+0x9/0x10 mm/kasan/common.c:529 > kmem_cache_alloc_trace+0x158/0x790 mm/slab.c:3551 > kmalloc include/linux/slab.h:555 [inline] > kzalloc include/linux/slab.h:669 [inline] > l2cap_chan_create+0x45/0x3a0 net/bluetooth/l2cap_core.c:446 > chan_create+0x10/0xe0 net/bluetooth/6lowpan.c:640 > bt_6lowpan_listen net/bluetooth/6lowpan.c:959 [inline] > do_enable_set+0x583/0x960 net/bluetooth/6lowpan.c:1078 > process_one_work+0xa05/0x17a0 kernel/workqueue.c:2264 > worker_thread+0x98/0xe40 kernel/workqueue.c:2410 > kthread+0x361/0x430 kernel/kthread.c:255 > ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352 > > Freed by task 2975: > save_stack+0x23/0x90 mm/kasan/common.c:72 > set_track mm/kasan/common.c:80 [inline] > kasan_set_free_info mm/kasan/common.c:337 [inline] > __kasan_slab_free+0x102/0x150 mm/kasan/common.c:476 > kasan_slab_free+0xe/0x10 mm/kasan/common.c:485 > __cache_free mm/slab.c:3426 [inline] > kfree+0x10a/0x2c0 mm/slab.c:3757 > l2cap_chan_destroy net/bluetooth/l2cap_core.c:484 [inline] > kref_put include/linux/kref.h:65 [inline] > l2cap_chan_put+0x1b7/0x240 net/bluetooth/l2cap_core.c:498 > do_enable_set+0x54b/0x960 net/bluetooth/6lowpan.c:1075 > process_one_work+0xa05/0x17a0 kernel/workqueue.c:2264 > worker_thread+0x98/0xe40 kernel/workqueue.c:2410 > kthread+0x361/0x430 kernel/kthread.c:255 > ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352 The free and use-after-free are both via the same do_enable_set() path which implies there are racing writes to the debugfs write handler. It seems locking is missing for both listen_chan and enable_6lowpan. The latter seems misused in is_bt_6lowpan(), which should likely just be checking for chan->ops == &bt_6lowpan_chan_ops, I think? I have no way to actually test changes to this code... -- Kees Cook
