Re: lockdep + kasan bug?

[Mark Rutland]

On Tue, Nov 21, 2023 at 11:36:14AM +0100, Peter Zijlstra wrote:
> On Mon, Nov 20, 2023 at 06:36:59PM -0500, Kent Overstreet wrote:
> > I've been seeing a lot of reports like the following in a lot of my
> > lockdep + kasan tests.
> 
> I'm not aware of any such issues, then again, I rarely run with KASAN
> enabled myself, I mostly leave that to the robots, who are far more
> patient than me with slow kernels.
> 
> > Some lockdep patches are in my tree: they don't touch this code path
> > (except I do have to increase MAX_LOCK_DEPTH from 48 to 63, perhaps that
> > has unintended side effects?)
> > 
> > https://evilpiepirate.org/git/bcachefs.git/log/?id=2f42f415f7573001b4f4887b785d8a8747b3757f
> 
> yeah, don't see anything weird there. I mean, sad about the no-recursion
> thing, esp. after you did those custom order bits.
> 
> > bcachefs does take a _large_ number of locks for lockdep to track, also
> > possibly relevant
> > 
> > Have not dug into the lockdep hash table of outstanding locks code yet
> > but happy to test patches...
> > 
> > 04752 ========= TEST   tiering_variable_buckets_replicas
> > 04752 
> > 04752 WATCHDOG 3600
> > 04753 bcachefs (ea667958-8bbd-451b-9043-9132a2fd2fa4): mounting version 1.3: rebalance_work opts=metadata_replicas=2,data_replicas=2,foreground_target=ssd,background_target=hdd,promote_target=ssd,fsck
> > 04753 bcachefs (ea667958-8bbd-451b-9043-9132a2fd2fa4): initializing new filesystem
> > 04753 bcachefs (ea667958-8bbd-451b-9043-9132a2fd2fa4): going read-write
> > 04753 bcachefs (ea667958-8bbd-451b-9043-9132a2fd2fa4): marking superblocks
> > 04753 bcachefs (ea667958-8bbd-451b-9043-9132a2fd2fa4): initializing freespace
> > 04753 bcachefs (ea667958-8bbd-451b-9043-9132a2fd2fa4): done initializing freespace
> > 04753 bcachefs (ea667958-8bbd-451b-9043-9132a2fd2fa4): reading snapshots table
> > 04753 bcachefs (ea667958-8bbd-451b-9043-9132a2fd2fa4): reading snapshots done
> > 04753 WATCHDOG 3600
> > 04753 randrw: (g=0): rw=randrw, bs=(R) 4096B-1024KiB, (W) 4096B-1024KiB, (T) 4096B-1024KiB, ioengine=libaio, iodepth=64
> > 04753 fio-3.33
> > 04753 Starting 1 process
> > 04753 randrw: Laying out IO file (1 file / 3500MiB)
> > 05117 Jobs: 1 (f=1)
> > 05117 BUG: KASAN: global-out-of-bounds in add_chain_block+0x44/0x288
> > 05117 Read of size 4 at addr ffffffc081b7a8bc by task fio/120528
> > 05117 
> > 05117 CPU: 11 PID: 120528 Comm: fio Tainted: G             L     6.6.0-ktest-gc18b7260ddd3 #8209
> > 05117 Hardware name: linux,dummy-virt (DT)
> > 05117 Call trace:
> > 05117  dump_backtrace+0xa8/0xe8
> > 05117  show_stack+0x1c/0x30
> > 05117  dump_stack_lvl+0x5c/0xa0
> > 05117  print_report+0x1e4/0x5a0
> > 05117  kasan_report+0x80/0xc0
> > 05117  __asan_load4+0x90/0xb0
> > 05117  add_chain_block+0x44/0x288
> > 05117  __lock_acquire+0x1104/0x24f8
> > 05117  lock_acquire+0x1e0/0x470
> > 05117  _raw_spin_lock_nested+0x54/0x78
> > 05117  raw_spin_rq_lock_nested+0x30/0x50
> > 05117  try_to_wake_up+0x3b4/0x1050
> > 05117  wake_up_process+0x1c/0x30
> > 05117  kick_pool+0x104/0x1b0
> > 05117  __queue_work+0x350/0xa58
> > 05117  queue_work_on+0x98/0xd0
> > 05117  __bch2_btree_node_write+0xec0/0x10a0
> > 05117  bch2_btree_node_write+0x88/0x138
> > 05117  btree_split+0x744/0x14a0
> > 05117  bch2_btree_split_leaf+0x94/0x258
> > 05117  bch2_trans_commit_error.isra.0+0x234/0x7d0
> > 05117  __bch2_trans_commit+0x1128/0x3010
> > 05117  bch2_extent_update+0x410/0x570
> > 05117  bch2_write_index_default+0x404/0x598
> > 05117  __bch2_write_index+0xb0/0x3b0
> > 05117  __bch2_write+0x6f0/0x928
> > 05117  bch2_write+0x368/0x8e0
> > 05117  bch2_direct_write+0xaa8/0x12c0
> > 05117  bch2_write_iter+0x2e4/0x1050
> > 05117  aio_write.constprop.0+0x19c/0x420
> > 05117  io_submit_one.constprop.0+0xf30/0x17a0
> > 05117  __arm64_sys_io_submit+0x244/0x388
> > 05117  invoke_syscall.constprop.0+0x64/0x138
> > 05117  do_el0_svc+0x7c/0x120
> > 05117  el0_svc+0x34/0x80
> > 05117  el0t_64_sync_handler+0xb8/0xc0
> > 05117  el0t_64_sync+0x14c/0x150
> > 05117 
> > 05117 The buggy address belongs to the variable:
> > 05117  nr_large_chain_blocks+0x3c/0x40
> 
> This is weird, nr_lage_chain_blocks is a single variable, if the
> compiler keeps layout according to the source file, this would be
> chaing_block_bucket[14] or something weird like that.

I think the size here is bogus; IIUC that's determined form the start of the
next symbol, which happens to be 64 bytes away from the start of
nr_lage_chain_blocks.

>From the memory state dump, there's padding/redzone between two global objects,
and I think we're accessing a negative offset from the next object. More on
that below.

> Perhaps figure out what it things the @size argument to
> add_chain_block() would be?
> 
> > 05117 
> > 05117 The buggy address belongs to the virtual mapping at
> > 05117  [ffffffc081710000, ffffffc088861000) created by:
> > 05117  paging_init+0x260/0x820
> > 05117 
> > 05117 The buggy address belongs to the physical page:
> > 05117 page:00000000ce625900 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x41d7a
> > 05117 flags: 0x4000(reserved|zone=0)
> > 05117 page_type: 0xffffffff()
> > 05117 raw: 0000000000004000 fffffffe00075e88 fffffffe00075e88 0000000000000000
> > 05117 raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000
> > 05117 page dumped because: kasan: bad access detected
> > 05117 
> > 05117 Memory state around the buggy address:
> > 05117  ffffffc081b7a780: 00 f9 f9 f9 f9 f9 f9 f9 00 f9 f9 f9 f9 f9 f9 f9
> > 05117  ffffffc081b7a800: 00 f9 f9 f9 f9 f9 f9 f9 04 f9 f9 f9 f9 f9 f9 f9
> > 05117 >ffffffc081b7a880: 04 f9 f9 f9 f9 f9 f9 f9 00 00 00 00 00 00 00 00
> > 05117                                         ^

In this dump:

* '00' means all 8 bytes of an 8-byte region areaccessible
* '04' means the first 4 bytes on an 8-byte region are accessible
* 'f9' means KASAN_GLOBAL_REDZONE / padding between objects

So at 0xffffffc081b7a880 we have a 4-byte object, 60 bytes of padding, then a
64-byte object.

I think the 4-byte object at 0xffffffc081b7a880 is nr_large_chain_blocks, and
the later 64-byte object is chain_block_buckets[].

I suspect the dodgy access is to chain_block_buckets[-1], which hits the last 4
bytes of the redzone and gets (incorrectly/misleadingly) attributed to
nr_large_chain_blocks.

Mark.

> > 05117  ffffffc081b7a900: f9 f9 f9 f9 04 f9 f9 f9 f9 f9 f9 f9 04 f9 f9 f9
> > 05117  ffffffc081b7a980: f9 f9 f9 f9 04 f9 f9 f9 f9 f9 f9 f9 00 f9 f9 f9
> > 05117 ==================================================================
> > 05117 Kernel panic - not syncing: kasan.fault=panic set ...
> > 05117 CPU: 11 PID: 120528 Comm: fio Tainted: G             L     6.6.0-ktest-gc18b7260ddd3 #8209
> > 05117 Hardware name: linux,dummy-virt (DT)
> > 05117 Call trace:
> > 05117  dump_backtrace+0xa8/0xe8
> > 05117  show_stack+0x1c/0x30
> > 05117  dump_stack_lvl+0x5c/0xa0
> > 05117  dump_stack+0x18/0x20
> > 05117  panic+0x3ac/0x408
> > 05117  kasan_report_invalid_free+0x0/0x90
> > 05117  kasan_report+0x90/0xc0
> > 05117  __asan_load4+0x90/0xb0
> > 05117  add_chain_block+0x44/0x288
> > 05117  __lock_acquire+0x1104/0x24f8
> > 05117  lock_acquire+0x1e0/0x470
> > 05117  _raw_spin_lock_nested+0x54/0x78
> > 05117  raw_spin_rq_lock_nested+0x30/0x50
> > 05117  try_to_wake_up+0x3b4/0x1050
> > 05117  wake_up_process+0x1c/0x30
> > 05117  kick_pool+0x104/0x1b0
> > 05117  __queue_work+0x350/0xa58
> > 05117  queue_work_on+0x98/0xd0
> > 05117  __bch2_btree_node_write+0xec0/0x10a0
> > 05117  bch2_btree_node_write+0x88/0x138
> > 05117  btree_split+0x744/0x14a0
> > 05117  bch2_btree_split_leaf+0x94/0x258
> > 05117  bch2_trans_commit_error.isra.0+0x234/0x7d0
> > 05117  __bch2_trans_commit+0x1128/0x3010
> > 05117  bch2_extent_update+0x410/0x570
> > 05117  bch2_write_index_default+0x404/0x598
> > 05117  __bch2_write_index+0xb0/0x3b0
> > 05117  __bch2_write+0x6f0/0x928
> > 05117  bch2_write+0x368/0x8e0
> > 05117  bch2_direct_write+0xaa8/0x12c0
> > 05117  bch2_write_iter+0x2e4/0x1050
> > 05117  aio_write.constprop.0+0x19c/0x420
> > 05117  io_submit_one.constprop.0+0xf30/0x17a0
> > 05117  __arm64_sys_io_submit+0x244/0x388
> > 05117  invoke_syscall.constprop.0+0x64/0x138
> > 05117  do_el0_svc+0x7c/0x120
> > 05117  el0_svc+0x34/0x80
> > 05117  el0t_64_sync_handler+0xb8/0xc0
> > 05117  el0t_64_sync+0x14c/0x150
> > 05117 SMP: stopping secondary CPUs
> > 05117 Kernel Offset: disabled
> > 05117 CPU features: 0x0,00000000,70000001,1040500b
> > 05117 Memory Limit: none
> > 05117 ---[ end Kernel panic - not syncing: kasan.fault=panic set ... ]---
> > 05122 ========= FAILED TIMEOUT tiering_variable_buckets_replicas in 3600s