Message-ID: <CACT4Y+aBX-eKqEFk8C4LNDG-bpSQQWhn++jqBmQkWyqqxnDQ1A@mail.gmail.com>
Date:   Tue, 19 Nov 2019 11:06:06 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     syzbot <syzbot+9bfadf534cf2fd1b5eb8@...kaller.appspotmail.com>,
        Jens Axboe <axboe@...nel.dk>,
        Cgroups <cgroups@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: KASAN: use-after-free Read in blkcg_print_stat (2)

On Tue, Nov 19, 2019 at 10:57 AM Hillf Danton <hdanton@...a.com> 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@...kaller.appspotmail.com
> >
> > ==================================================================
> > 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.

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