Re: [PATCH v2] btrfs: relocation: Fix KASAN reports caused by extended reloc tree lifespan

[Nikolay Borisov <nborisov@suse.com>]

On 8.01.20 г. 16:55 ч., Josef Bacik wrote:
> On 1/8/20 12:12 AM, Qu Wenruo wrote:
>> [BUG]
>> There are several different KASAN reports for balance + snapshot
>> workloads.
>> Involved call paths include:
>>
>>     should_ignore_root+0x54/0xb0 [btrfs]
>>     build_backref_tree+0x11af/0x2280 [btrfs]
>>     relocate_tree_blocks+0x391/0xb80 [btrfs]
>>     relocate_block_group+0x3e5/0xa00 [btrfs]
>>     btrfs_relocate_block_group+0x240/0x4d0 [btrfs]
>>     btrfs_relocate_chunk+0x53/0xf0 [btrfs]
>>     btrfs_balance+0xc91/0x1840 [btrfs]
>>     btrfs_ioctl_balance+0x416/0x4e0 [btrfs]
>>     btrfs_ioctl+0x8af/0x3e60 [btrfs]
>>     do_vfs_ioctl+0x831/0xb10
>>     ksys_ioctl+0x67/0x90
>>     __x64_sys_ioctl+0x43/0x50
>>     do_syscall_64+0x79/0xe0
>>     entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>>     create_reloc_root+0x9f/0x460 [btrfs]
>>     btrfs_reloc_post_snapshot+0xff/0x6c0 [btrfs]
>>     create_pending_snapshot+0xa9b/0x15f0 [btrfs]
>>     create_pending_snapshots+0x111/0x140 [btrfs]
>>     btrfs_commit_transaction+0x7a6/0x1360 [btrfs]
>>     btrfs_mksubvol+0x915/0x960 [btrfs]
>>     btrfs_ioctl_snap_create_transid+0x1d5/0x1e0 [btrfs]
>>     btrfs_ioctl_snap_create_v2+0x1d3/0x270 [btrfs]
>>     btrfs_ioctl+0x241b/0x3e60 [btrfs]
>>     do_vfs_ioctl+0x831/0xb10
>>     ksys_ioctl+0x67/0x90
>>     __x64_sys_ioctl+0x43/0x50
>>     do_syscall_64+0x79/0xe0
>>     entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>>     btrfs_reloc_pre_snapshot+0x85/0xc0 [btrfs]
>>     create_pending_snapshot+0x209/0x15f0 [btrfs]
>>     create_pending_snapshots+0x111/0x140 [btrfs]
>>     btrfs_commit_transaction+0x7a6/0x1360 [btrfs]
>>     btrfs_mksubvol+0x915/0x960 [btrfs]
>>     btrfs_ioctl_snap_create_transid+0x1d5/0x1e0 [btrfs]
>>     btrfs_ioctl_snap_create_v2+0x1d3/0x270 [btrfs]
>>     btrfs_ioctl+0x241b/0x3e60 [btrfs]
>>     do_vfs_ioctl+0x831/0xb10
>>     ksys_ioctl+0x67/0x90
>>     __x64_sys_ioctl+0x43/0x50
>>     do_syscall_64+0x79/0xe0
>>     entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> [CAUSE]
>> All these call sites are only relying on root->reloc_root, which can
>> undergo btrfs_drop_snapshot(), and since we don't have real refcount
>> based protection to reloc roots, we can reach already dropped reloc
>> root, triggering KASAN.
>>
>> [FIX]
>> To avoid such access to unstable root->reloc_root, we should check
>> BTRFS_ROOT_DEAD_RELOC_TREE bit first.
>>
>> This patch introduces a new wrapper, have_reloc_root(), to do the proper
>> check for most callers who don't distinguish merged reloc tree and no
>> reloc tree.
>>
>> The only exception is should_ignore_root(), as merged reloc tree can be
>> ignored, while no reloc tree shouldn't.
>>
>> [CRITICAL SECTION ANALYSE]
>> Although test_bit()/set_bit()/clear_bit() doesn't imply a barrier, the
>> DEAD_RELOC_TREE bit has extra help from transaction as a higher level
>> barrier, the lifespan of root::reloc_root and DEAD_RELOC_TREE bit are:
>>
>>     NULL: reloc_root is NULL    PTR: reloc_root is not NULL
>>     0: DEAD_RELOC_ROOT bit not set    DEAD: DEAD_RELOC_ROOT bit set
>>
>>     (NULL, 0)    Initial state         __
>>       |                     /\ Section A
>>          btrfs_init_reloc_root()             \/
>>       |                      __
>>     (PTR, 0)     reloc_root initialized      /\
>>            |                     |
>>     btrfs_update_reloc_root()         |  Section B
>>            |                     |
>>     (PTR, DEAD)  reloc_root has been merged  \/
>>            |                     __
>>     === btrfs_commit_transaction() ====================
>>       |                     /\
>>     clean_dirty_subvols()             |
>>       |                     |  Section C
>>     (NULL, DEAD) reloc_root cleanup starts   \/
>>            |                     __
>>     btrfs_drop_snapshot()             /\
>>       |                     |  Section D
>>     (NULL, 0)    Back to initial state     \/
>>
>> Very have_reloc_root() or test_bit(DEAD_RELOC_ROOT) caller has hold a
>> transaction handler, so none of such caller can cross transaction
>> boundary.
>>
>> In Section A, every caller just found no DEAD bit, and grab reloc_root.
>>
>> In the cross section A-B, caller may get no DEAD bit, but since
>> reloc_root is still completely valid thus accessing reloc_root is
>> completely safe.
>>
>> No test_bit() caller can cross the boundary of Section B and Section C.
>>
>> In Section C, every caller found the DEAD bit, so no one will access
>> reloc_root.
>>
>> In the cross section C-D, either caller gets the DEAD bit set, avoiding
>> access reloc_root no matter if it's safe or not.
>> Or caller get the DEAD bit cleared, then access reloc_root, which is
>> already NULL, nothing will be wrong.
>>
>> Here we need extra memory barrier in cross section C-D, to ensure
>> proper memory order between reloc_root and clear_bit().
>>
>> In Section D, since no DEAD bit and no reloc_root, it's back to initial
>> state.
>>
>> With this lifespan, it should be clear only one memory barrier is
>> needed, between setting reloc_root to NULL and clearing DEAD_RELOC_ROOT
>> bit.
>>
>> Reported-by: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
>> Fixes: d2311e698578 ("btrfs: relocation: Delay reloc tree deletion
>> after merge_reloc_roots")
>> Suggested-by: David Sterba <dsterba@suse.com>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>> Changelog:
>> v2:
>> - Add the [CRITICAL SECTION ANALYSE] part
>>    This gets me into the rabbit hole of memory ordering, but thanks for
>>    the help from David (initially mentioning the mb hell) and Nikolay
>>    (for the proper doc), finally I could explain clearly why only
>>    one mb is needed.
>> - Add comment for the only needed memory barrier.
>> ---
>>   fs/btrfs/relocation.c | 32 ++++++++++++++++++++++++++++----
>>   1 file changed, 28 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
>> index d897a8e5e430..17a2484f76a5 100644
>> --- a/fs/btrfs/relocation.c
>> +++ b/fs/btrfs/relocation.c
>> @@ -517,6 +517,22 @@ static int update_backref_cache(struct
>> btrfs_trans_handle *trans,
>>       return 1;
>>   }
>>   +/*
>> + * Check if this subvolume tree has valid reloc(*) tree.
>> + *
>> + * *: Reloc tree after swap is considered dead, thus not considered
>> as valid.
>> + *    This is enough for most callers, as they don't distinguish dead
>> reloc
>> + *    root from no reloc root.
>> + *    But should_ignore_root() below is a special case.
>> + */
>> +static bool have_reloc_root(struct btrfs_root *root)
>> +{
>> +    if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state))
>> +        return false;
> 
> You still need a smp_mb__after_atomic() here, test_bit is unordered. 

Nope, that won't do anything, since smp_mb__(After|before)_atomic only
orders RMW operations and test_bit is not an RMW operation. From
atomic_bitops.txt:


Non-RMW ops:



  test_bit()

Furthermore from atomic_t.txt:

The barriers:



  smp_mb__{before,after}_atomic()



only apply to the RMW atomic ops and can be used to augment/upgrade the

ordering inherent to the op.

> Thanks,
> 
> Josef