On Tue, Nov 19, 2019 at 10:57 AM Hillf Danton <hdanton@xxxxxxxx> wrote: > > > On Mon, 18 Nov 2019 23:11:06 -0800 > > > > Hello, > > > > syzbot found the following crash on: > > > > HEAD commit: fe30021c Merge branch 'x86-urgent-for-linus' of git://git... > > git tree: upstream > > console output: https://syzkaller.appspot.com/x/log.txt?x=1196cb3ae00000 > > kernel config: https://syzkaller.appspot.com/x/.config?x=1aab6d4187ddf667 > > dashboard link: https://syzkaller.appspot.com/bug?extid=9bfadf534cf2fd1b5eb8 > > compiler: gcc (GCC) 9.0.0 20181231 (experimental) > > > > Unfortunately, I don't have any reproducer for this crash yet. > > > > IMPORTANT: if you fix the bug, please add the following tag to the commit: > > Reported-by: syzbot+9bfadf534cf2fd1b5eb8@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > ================================================================== > > BUG: KASAN: use-after-free in dev_name include/linux/device.h:1342 [inline] > > BUG: KASAN: use-after-free in blkg_dev_name block/blk-cgroup.c:498 [inline] > > BUG: KASAN: use-after-free in blkcg_print_stat+0xb26/0xc00 > > block/blk-cgroup.c:942 > > Read of size 8 at addr ffff8880a01b2050 by task syz-executor.1/16053 > > > > CPU: 0 PID: 16053 Comm: syz-executor.1 Not tainted 5.4.0-rc7+ #0 > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS > > Google 01/01/2011 > > 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/0x41 mm/kasan/report.c:506 > > kasan_report+0x12/0x20 mm/kasan/common.c:634 > > __asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:132 > > dev_name include/linux/device.h:1342 [inline] > > blkg_dev_name block/blk-cgroup.c:498 [inline] > > blkcg_print_stat+0xb26/0xc00 block/blk-cgroup.c:942 > > cgroup_seqfile_show+0x1a8/0x300 kernel/cgroup/cgroup.c:3816 > > kernfs_seq_show+0x14f/0x1b0 fs/kernfs/file.c:167 > > seq_read+0x4ca/0x1110 fs/seq_file.c:229 > > kernfs_fop_read+0xed/0x560 fs/kernfs/file.c:251 > > do_loop_readv_writev fs/read_write.c:714 [inline] > > do_loop_readv_writev fs/read_write.c:701 [inline] > > do_iter_read+0x4a4/0x660 fs/read_write.c:935 > > vfs_readv+0xf0/0x160 fs/read_write.c:997 > > do_readv+0x15b/0x330 fs/read_write.c:1034 > > __do_sys_readv fs/read_write.c:1125 [inline] > > __se_sys_readv fs/read_write.c:1122 [inline] > > __x64_sys_readv+0x75/0xb0 fs/read_write.c:1122 > > do_syscall_64+0xfa/0x760 arch/x86/entry/common.c:290 > > entry_SYSCALL_64_after_hwframe+0x49/0xbe > > RIP: 0033:0x45a639 > > Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 > > 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff > > ff 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 > > RSP: 002b:00007fd336772c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000013 > > RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 000000000045a639 > > RDX: 00000000000001a5 RSI: 00000000200002c0 RDI: 0000000000000004 > > RBP: 000000000075bfc8 R08: 0000000000000000 R09: 0000000000000000 > > R10: 0000000000000000 R11: 0000000000000246 R12: 00007fd3367736d4 > > R13: 00000000004c7ecd R14: 00000000004de160 R15: 00000000ffffffff > > > > Allocated by task 12327: > > save_stack+0x23/0x90 mm/kasan/common.c:69 > > set_track mm/kasan/common.c:77 [inline] > > __kasan_kmalloc mm/kasan/common.c:510 [inline] > > __kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:483 > > kasan_kmalloc+0x9/0x10 mm/kasan/common.c:524 > > kmem_cache_alloc_trace+0x158/0x790 mm/slab.c:3550 > > kmalloc include/linux/slab.h:556 [inline] > > kzalloc include/linux/slab.h:690 [inline] > > device_create_groups_vargs+0x8e/0x270 drivers/base/core.c:2828 > > device_create_vargs+0x45/0x60 drivers/base/core.c:2886 > > bdi_register_va.part.0+0x91/0x940 mm/backing-dev.c:940 > > bdi_register_va mm/backing-dev.c:976 [inline] > > bdi_register+0x12a/0x140 mm/backing-dev.c:973 > > bdi_register_owner+0x6a/0x110 mm/backing-dev.c:983 > > __device_add_disk+0xe09/0x1230 block/genhd.c:739 > > device_add_disk+0x2b/0x40 block/genhd.c:763 > > add_disk include/linux/genhd.h:429 [inline] > > loop_add+0x635/0x8d0 drivers/block/loop.c:2061 > > loop_control_ioctl drivers/block/loop.c:2162 [inline] > > loop_control_ioctl+0x165/0x360 drivers/block/loop.c:2144 > > vfs_ioctl fs/ioctl.c:46 [inline] > > file_ioctl fs/ioctl.c:509 [inline] > > do_vfs_ioctl+0xdb6/0x13e0 fs/ioctl.c:696 > > ksys_ioctl+0xab/0xd0 fs/ioctl.c:713 > > __do_sys_ioctl fs/ioctl.c:720 [inline] > > __se_sys_ioctl fs/ioctl.c:718 [inline] > > __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:718 > > do_syscall_64+0xfa/0x760 arch/x86/entry/common.c:290 > > entry_SYSCALL_64_after_hwframe+0x49/0xbe > > > > Freed by task 16049: > > save_stack+0x23/0x90 mm/kasan/common.c:69 > > set_track mm/kasan/common.c:77 [inline] > > kasan_set_free_info mm/kasan/common.c:332 [inline] > > __kasan_slab_free+0x102/0x150 mm/kasan/common.c:471 > > kasan_slab_free+0xe/0x10 mm/kasan/common.c:480 > > __cache_free mm/slab.c:3425 [inline] > > kfree+0x10a/0x2c0 mm/slab.c:3756 > > device_create_release+0x16/0x20 drivers/base/power/wakeup_stats.c:130 > > device_release+0x7a/0x210 drivers/base/core.c:1101 > > kobject_cleanup lib/kobject.c:693 [inline] > > kobject_release lib/kobject.c:722 [inline] > > kref_put include/linux/kref.h:65 [inline] > > kobject_put.cold+0x289/0x2e6 lib/kobject.c:739 > > put_device drivers/base/core.c:2301 [inline] > > device_unregister+0x28/0x40 drivers/base/core.c:2409 > > bdi_unregister+0x428/0x610 mm/backing-dev.c:1016 > > del_gendisk+0x896/0xb30 block/genhd.c:810 > > loop_remove+0x3c/0xd0 drivers/block/loop.c:2079 > > loop_control_ioctl drivers/block/loop.c:2178 [inline] > > loop_control_ioctl+0x320/0x360 drivers/block/loop.c:2144 > > vfs_ioctl fs/ioctl.c:46 [inline] > > file_ioctl fs/ioctl.c:509 [inline] > > do_vfs_ioctl+0xdb6/0x13e0 fs/ioctl.c:696 > > ksys_ioctl+0xab/0xd0 fs/ioctl.c:713 > > __do_sys_ioctl fs/ioctl.c:720 [inline] > > __se_sys_ioctl fs/ioctl.c:718 [inline] > > __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:718 > > do_syscall_64+0xfa/0x760 arch/x86/entry/common.c:290 > > entry_SYSCALL_64_after_hwframe+0x49/0xbe > > > > The buggy address belongs to the object at ffff8880a01b2000 > > which belongs to the cache kmalloc-2k of size 2048 > > The buggy address is located 80 bytes inside of > > 2048-byte region [ffff8880a01b2000, ffff8880a01b2800) > > The buggy address belongs to the page: > > page:ffffea0002806c80 refcount:1 mapcount:0 mapping:ffff8880aa400e00 > > index:0x0 > > raw: 01fffc0000000200 ffffea0002374088 ffffea0002730148 ffff8880aa400e00 > > raw: 0000000000000000 ffff8880a01b2000 0000000100000001 0000000000000000 > > page dumped because: kasan: bad access detected > > > > Memory state around the buggy address: > > ffff8880a01b1f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc > > ffff8880a01b1f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc > > > ffff8880a01b2000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb > > ^ > > ffff8880a01b2080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb > > ffff8880a01b2100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb > > ================================================================== > > > bdi can be unregistered without queue_lock protection, so the reference > of bdi->dev must be informed on resetting it. > > --- > > --- a/block/blk-cgroup.c > +++ b/block/blk-cgroup.c > @@ -494,8 +494,10 @@ static int blkcg_reset_stats(struct cgro > const char *blkg_dev_name(struct blkcg_gq *blkg) > { > /* some drivers (floppy) instantiate a queue w/o disk registered */ > - if (blkg->q->backing_dev_info->dev) > + if (blkg->q->backing_dev_info->dev) { > + smp_rmb(); > return dev_name(blkg->q->backing_dev_info->dev); > + } > return NULL; > } > > --- a/mm/backing-dev.c > +++ b/mm/backing-dev.c > @@ -1015,6 +1015,7 @@ void bdi_unregister(struct backing_dev_i > bdi_debug_unregister(bdi); > device_unregister(bdi->dev); > bdi->dev = NULL; > + smp_wmb(); > } > > if (bdi->owner) { This was hit on x86, so I would assume there is a more substantial bug than missed memory barriers (though, of course possible due to compiler and formal undefined behavior). But still I think there is also something like a missed refcount. The fix in blkcg_reset_stats() looks suspicious to me on it's own. Let's say these functions are racing, even if we read blkg->q->backing_dev_info->dev as non-NULL and did smp_rmb() afterwards, nothing prevents dev from becoming NULL/freed on the very next line. If there is really a race, this type of use would require stronger synchronization (locks, refcounts, etc), not a barrier. But if there is also a data race, then we will get back to this code with KCSAN later. Please don't use rmb/wmb, smp_load_acquire/release are better primitives. At the very least rmb/wmb must always be used with READ/WRITE_ONCE. rmb/wmb cannot possibly prevent a data race on dev variable itself.
