Re: [PATCH] btrfs: relocation: fix reloc_root lifespan and access

[Josef Bacik <josef@toxicpanda.com>]

On 1/13/20 11:16 AM, David Sterba wrote:
> From: Qu Wenruo <wqu@suse.com>
> 
> This is what I'm going to commit, but as this has a long discussion
> behind I'm sending it to the mailinglist.
> 
> * there are 2 helpers to avoid using raw barriers in the tests where there
>    are at least 2 places
> * each barrier is commented
> * subject and changelog have been updated to reflect the changes
> 
> https://lore.kernel.org/linux-btrfs/20200108051200.8909-1-wqu@suse.com/
> 
> ---
> 
> [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
> 
>     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
> 
>     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
> 
> [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 wrappers that provide the correct way to check the
> bit with memory barriers protection.
> 
> Most callers 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 ANALYSIS]
> 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	 \/
> 
> Every have_reloc_root() or test_bit(DEAD_RELOC_ROOT) caller holds
> transaction handle, 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.
> 
> The memory write barriers are between the reloc_root updates and bit
> set/clear, the pairing read side is before test_bit.
> 
> Reported-by: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
> Fixes: d2311e698578 ("btrfs: relocation: Delay reloc tree deletion after merge_reloc_roots")
> CC: stable@vger.kernel.org # 5.4+
> Suggested-by: David Sterba <dsterba@suse.com>
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> [ barriers ]
> Signed-off-by: David Sterba <dsterba@suse.com>

Let's just get this fixed, the root refcounting stuff will solve this so we just 
need something for now.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef