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

From: Nikolay Borisov <nborisov@suse.com>
To: Qu Wenruo <wqu@suse.com>, linux-btrfs@vger.kernel.org
Cc: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>,
	David Sterba <dsterba@suse.com>
Subject: Re: [PATCH v2] btrfs: relocation: Fix KASAN reports caused by extended reloc tree lifespan
Date: Wed, 8 Jan 2020 14:28:07 +0200	[thread overview]
Message-ID: <5cfff64e-0843-12ae-1ffc-37016552073d@suse.com> (raw)
In-Reply-To: <20200108051200.8909-1-wqu@suse.com>

On 8.01.20 г. 7:12 ч., 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

what do you mean by "root->reloc_root can undergo btrfs_drop_snapshot" ?

> based protection to reloc roots, we can reach already dropped reloc
> root, triggering KASAN.
what's the relationship between not having a refcount protection and
reaching reloc roots, perhaps you could expand the explanation?

> 
> [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

 ^^ Perhaps you meant: Every caller of have_reloc_root or
test_bit(DED_RELOC_ROOT) holds a transaction handle which ensures
modifications in those function are limited to a single transaction?

> 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;
> +	if (!root->reloc_root)
> +		return false;
> +	return true;
> +}
>  
>  static int should_ignore_root(struct btrfs_root *root)
>  {
> @@ -525,6 +541,10 @@ static int should_ignore_root(struct btrfs_root *root)
>  	if (!test_bit(BTRFS_ROOT_REF_COWS, &root->state))
>  		return 0;
>  
> +	/* This root has been merged with its reloc tree, we can ignore it */
> +	if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state))
> +		return 1;
> +
>  	reloc_root = root->reloc_root;
>  	if (!reloc_root)
>  		return 0;
> @@ -1478,8 +1498,7 @@ int btrfs_update_reloc_root(struct btrfs_trans_handle *trans,
>  	struct btrfs_root_item *root_item;
>  	int ret;
>  
> -	if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state) ||
> -	    !root->reloc_root)
> +	if (!have_reloc_root(root))
>  		goto out;
>  
>  	reloc_root = root->reloc_root;
> @@ -2201,6 +2220,11 @@ static int clean_dirty_subvols(struct reloc_control *rc)
>  				if (ret2 < 0 && !ret)
>  					ret = ret2;
>  			}
> +			/*
> +			 * Need barrier to ensure clear_bit() only happens after
> +			 * root->reloc_root = NULL.
> +			 */
> +			smp_mb__before_atomic();
>  			clear_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state);
>  			btrfs_put_fs_root(root);
>  		} else {
> @@ -4717,7 +4741,7 @@ void btrfs_reloc_pre_snapshot(struct btrfs_pending_snapshot *pending,
>  	struct btrfs_root *root = pending->root;
>  	struct reloc_control *rc = root->fs_info->reloc_ctl;
>  
> -	if (!root->reloc_root || !rc)
> +	if (!rc || !have_reloc_root(root))
>  		return;
>  
>  	if (!rc->merge_reloc_tree)
> @@ -4751,7 +4775,7 @@ int btrfs_reloc_post_snapshot(struct btrfs_trans_handle *trans,
>  	struct reloc_control *rc = root->fs_info->reloc_ctl;
>  	int ret;
>  
> -	if (!root->reloc_root || !rc)
> +	if (!rc || !have_reloc_root(root))
>  		return 0;
>  
>  	rc = root->fs_info->reloc_ctl;
> 