Syzbot reported a possible use-after-free when printing a duplicate device
warning device_list_add().

At this point it can happen that a btrfs_device::fs_info is not correctly
setup yet, so we're accessing stale data, when printing the warning
message using the btrfs_printk() wrappers.

  ==================================================================
  BUG: KASAN: use-after-free in btrfs_printk+0x3eb/0x435 fs/btrfs/super.c:245
  Read of size 8 at addr ffff8880878e06a8 by task syz-executor225/7068

  CPU: 1 PID: 7068 Comm: syz-executor225 Not tainted 5.9.0-rc5-syzkaller #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+0x1d6/0x29e lib/dump_stack.c:118
   print_address_description+0x66/0x620 mm/kasan/report.c:383
   __kasan_report mm/kasan/report.c:513 [inline]
   kasan_report+0x132/0x1d0 mm/kasan/report.c:530
   btrfs_printk+0x3eb/0x435 fs/btrfs/super.c:245
   device_list_add+0x1a88/0x1d60 fs/btrfs/volumes.c:943
   btrfs_scan_one_device+0x196/0x490 fs/btrfs/volumes.c:1359
   btrfs_mount_root+0x48f/0xb60 fs/btrfs/super.c:1634
   legacy_get_tree+0xea/0x180 fs/fs_context.c:592
   vfs_get_tree+0x88/0x270 fs/super.c:1547
   fc_mount fs/namespace.c:978 [inline]
   vfs_kern_mount+0xc9/0x160 fs/namespace.c:1008
   btrfs_mount+0x33c/0xae0 fs/btrfs/super.c:1732
   legacy_get_tree+0xea/0x180 fs/fs_context.c:592
   vfs_get_tree+0x88/0x270 fs/super.c:1547
   do_new_mount fs/namespace.c:2875 [inline]
   path_mount+0x179d/0x29e0 fs/namespace.c:3192
   do_mount fs/namespace.c:3205 [inline]
   __do_sys_mount fs/namespace.c:3413 [inline]
   __se_sys_mount+0x126/0x180 fs/namespace.c:3390
   do_syscall_64+0x31/0x70 arch/x86/entry/common.c:46
   entry_SYSCALL_64_after_hwframe+0x44/0xa9
  RIP: 0033:0x44840a
  RSP: 002b:00007ffedfffd608 EFLAGS: 00000293 ORIG_RAX: 00000000000000a5
  RAX: ffffffffffffffda RBX: 00007ffedfffd670 RCX: 000000000044840a
  RDX: 0000000020000000 RSI: 0000000020000100 RDI: 00007ffedfffd630
  RBP: 00007ffedfffd630 R08: 00007ffedfffd670 R09: 0000000000000000
  R10: 0000000000000000 R11: 0000000000000293 R12: 000000000000001a
  R13: 0000000000000004 R14: 0000000000000003 R15: 0000000000000003

  Allocated by task 6945:
   kasan_save_stack mm/kasan/common.c:48 [inline]
   kasan_set_track mm/kasan/common.c:56 [inline]
   __kasan_kmalloc+0x100/0x130 mm/kasan/common.c:461
   kmalloc_node include/linux/slab.h:577 [inline]
   kvmalloc_node+0x81/0x110 mm/util.c:574
   kvmalloc include/linux/mm.h:757 [inline]
   kvzalloc include/linux/mm.h:765 [inline]
   btrfs_mount_root+0xd0/0xb60 fs/btrfs/super.c:1613
   legacy_get_tree+0xea/0x180 fs/fs_context.c:592
   vfs_get_tree+0x88/0x270 fs/super.c:1547
   fc_mount fs/namespace.c:978 [inline]
   vfs_kern_mount+0xc9/0x160 fs/namespace.c:1008
   btrfs_mount+0x33c/0xae0 fs/btrfs/super.c:1732
   legacy_get_tree+0xea/0x180 fs/fs_context.c:592
   vfs_get_tree+0x88/0x270 fs/super.c:1547
   do_new_mount fs/namespace.c:2875 [inline]
   path_mount+0x179d/0x29e0 fs/namespace.c:3192
   do_mount fs/namespace.c:3205 [inline]
   __do_sys_mount fs/namespace.c:3413 [inline]
   __se_sys_mount+0x126/0x180 fs/namespace.c:3390
   do_syscall_64+0x31/0x70 arch/x86/entry/common.c:46
   entry_SYSCALL_64_after_hwframe+0x44/0xa9

  Freed by task 6945:
   kasan_save_stack mm/kasan/common.c:48 [inline]
   kasan_set_track+0x3d/0x70 mm/kasan/common.c:56
   kasan_set_free_info+0x17/0x30 mm/kasan/generic.c:355
   __kasan_slab_free+0xdd/0x110 mm/kasan/common.c:422
   __cache_free mm/slab.c:3418 [inline]
   kfree+0x113/0x200 mm/slab.c:3756
   deactivate_locked_super+0xa7/0xf0 fs/super.c:335
   btrfs_mount_root+0x72b/0xb60 fs/btrfs/super.c:1678
   legacy_get_tree+0xea/0x180 fs/fs_context.c:592
   vfs_get_tree+0x88/0x270 fs/super.c:1547
   fc_mount fs/namespace.c:978 [inline]
   vfs_kern_mount+0xc9/0x160 fs/namespace.c:1008
   btrfs_mount+0x33c/0xae0 fs/btrfs/super.c:1732
   legacy_get_tree+0xea/0x180 fs/fs_context.c:592
   vfs_get_tree+0x88/0x270 fs/super.c:1547
   do_new_mount fs/namespace.c:2875 [inline]
   path_mount+0x179d/0x29e0 fs/namespace.c:3192
   do_mount fs/namespace.c:3205 [inline]
   __do_sys_mount fs/namespace.c:3413 [inline]
   __se_sys_mount+0x126/0x180 fs/namespace.c:3390
   do_syscall_64+0x31/0x70 arch/x86/entry/common.c:46
   entry_SYSCALL_64_after_hwframe+0x44/0xa9

  The buggy address belongs to the object at ffff8880878e0000
   which belongs to the cache kmalloc-16k of size 16384
  The buggy address is located 1704 bytes inside of
   16384-byte region [ffff8880878e0000, ffff8880878e4000)
  The buggy address belongs to the page:
  page:0000000060704f30 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x878e0
  head:0000000060704f30 order:3 compound_mapcount:0 compound_pincount:0
  flags: 0xfffe0000010200(slab|head)
  raw: 00fffe0000010200 ffffea00028e9a08 ffffea00021e3608 ffff8880aa440b00
  raw: 0000000000000000 ffff8880878e0000 0000000100000001 0000000000000000
  page dumped because: kasan: bad access detected

  Memory state around the buggy address:
   ffff8880878e0580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
   ffff8880878e0600: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
  >ffff8880878e0680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
				    ^
   ffff8880878e0700: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
   ffff8880878e0780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
  ==================================================================

The syzkaller reproducer for this use-after-free crafts a filesystem image
and loop mounts it twice in a loop. The mount will fail as the crafted
image has an invalid chunk tree. When this happens btrfs_mount_root() will
call deactivate_locked_super(), which then cleans up fs_info and
fs_info::sb. If a second thread now adds the same block-device to the
filesystem, it will get detected as a duplicate device and
device_list_add() will reject the duplicate and print a warning. But as
the fs_info pointer passed in is non-NULL this will result in a
use-after-free.

Instead of printing possibly uninitialized or already freed memory in
btrfs_printk(), explicitly pass in a NULL fs_info so the printing of the
device name will be skipped altogether.

There was a slightly different approach discussed in
https://lore.kernel.org/linux-btrfs/20200114060920.4527-1-anand.jain@oracle.com/t/#u

Link: https://lore.kernel.org/linux-btrfs/000000000000c9e14b05afcc41ba@google.com
Reported-by: syzbot+582e66e5edf36a22c7b0@syzkaller.appspotmail.com
CC: stable@vger.kernel.org # 4.19+
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>