Re: [PATCH] btrfs: fix invalid memory access with journal_info

[Omar Sandoval <osandov@osandov.com>]

On Wed, May 09, 2018 at 06:35:25PM +0800, robbieko wrote:
> From: Robbie Ko <robbieko@synology.com>
> 
> When send process requires memory allocation, shrinker may be triggered
> due to insufficient memory.
> Then evict_inode gets called when inode is freed, and this function
> may need to start transaction.
> However, the journal_info is already points to BTRFS_SEND_TRANS_STUB, it
> passed the if condition,
> and the following use yields illegal memory access.
> 
>   if (current->journal_info) {
>       WARN_ON(type & TRANS_EXTWRITERS);
>       h = current->journal_info;
>       refcount_inc(&h->use_count);
>       WARN_ON(refcount_read(&h->use_count) > 2);
>       h->orig_rsv = h->block_rsv;
>       h->block_rsv = NULL;
>       goto got_it;
>   }

start_transaction() has

    ASSERT(current->journal_info != BTRFS_SEND_TRANS_STUB);

Are you saying that's wrong? Are there other cases where the shrinker
can end up starting a transaction?

> Direct IO has a similar problem, journal_info will store btrfs_dio_data,
> which will lead to illegal memory access.

I have patches getting rid of this for direct I/O here:
https://github.com/osandov/linux/tree/btrfs-journal-info-abuse

I originally did that for btrfs swapfile support, but if it actually
fixes an existing bug it should be easy to get merged.

> We fixed the problem by save the journal_info and restore afterwards.
> 
> CallTrace looks like this:
>  BUG: unable to handle kernel NULL pointer dereference at 0000000000000021
>  IP: [<ffffffffa086f2d4>] start_transaction+0x64/0x450 [btrfs]
>  PGD 8fea4b067 PUD a33bea067 PMD 0
>  Oops: 0000 [#1] SMP
>  CPU: 3 PID: 12681 Comm: btrfs Tainted: P C O 3.10.102 #15266
>  RIP: 0010:[<ffffffffa086f2d4>] start_transaction+0x64/0x450 [btrfs]
>  Call Trace:
>  [<ffffffffa087a838>] ? btrfs_evict_inode+0x3d8/0x580 [btrfs]
>  [<ffffffff81115932>] ? evict+0xa2/0x1a0
>  [<ffffffff81112888>] ? shrink_dentry_list+0x308/0x3d0
>  [<ffffffff811137f3>] ? prune_dcache_sb+0x133/0x160
>  [<ffffffff810fa51f>] ? prune_super+0xcf/0x1a0
>  [<ffffffff810bf6bf>] ? shrink_slab+0x11f/0x1d0
>  [<ffffffff810c19f2>] ? do_try_to_free_pages+0x452/0x560
>  [<ffffffff810bf054>] ? throttle_direct_reclaim+0x74/0x240
>  [<ffffffff810c1bae>] ? try_to_free_pages+0xae/0xc0
>  [<ffffffff810ba16b>] ? __alloc_pages_nodemask+0x53b/0x9f0
>  [<ffffffff810bc89c>] ? __do_page_cache_readahead+0xec/0x270
>  [<ffffffff810bcb2b>] ? ondemand_readahead+0xbb/0x220
>  [<ffffffffa08d7c43>] ? fill_read_buf+0x2b3/0x3a0 [btrfs]
>  [<ffffffffa08dbf5e>] ? send_extent_data+0x10e/0x300 [btrfs]
>  [<ffffffffa08dc34b>] ? process_extent+0x1fb/0x1310 [btrfs]
>  [<ffffffffa08d8300>] ? iterate_dir_item.isra.28+0x1b0/0x250 [btrfs]
>  [<ffffffffa08dd500>] ? send_set_xattr+0xa0/0xa0 [btrfs]
>  [<ffffffffa08de565>] ? changed_cb+0xd5/0xc40 [btrfs]
>  [<ffffffffa08df1c2>] ? full_send_tree+0xf2/0x1a0 [btrfs]
>  [<ffffffffa08e022b>] ? btrfs_ioctl_send+0xfbb/0x1040 [btrfs]
>  [<ffffffffa08a9864>] ? btrfs_ioctl+0x1084/0x32a0 [btrfs]
> 
> Signed-off-by: Robbie Ko <robbieko@synology.com>
> ---
>  fs/btrfs/inode.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index f534701..77aec8d 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -5295,6 +5295,7 @@ void btrfs_evict_inode(struct inode *inode)
>  	int steal_from_global = 0;
>  	u64 min_size;
>  	int ret;
> +	void *journal_info = NULL;

This initialization isn't necessary.

>  	trace_btrfs_inode_evict(inode);
>  
> @@ -5303,6 +5304,16 @@ void btrfs_evict_inode(struct inode *inode)
>  		return;
>  	}
>  
> +	/*
> +	 * Send or Direct IO may store information in journal_info.
> +	 * However, this function may use start_transaction and
> +	 * start_transaction will use journal_info.
> +	 * To avoid accessing invalid memory, we can save the journal_info
> +	 * and restore it later.
> +	 */
> +	journal_info = current->journal_info;
> +	current->journal_info = NULL;
> +
>  	min_size = btrfs_calc_trunc_metadata_size(fs_info, 1);
>  
>  	evict_inode_truncate_pages(inode);
> @@ -5462,6 +5473,7 @@ void btrfs_evict_inode(struct inode *inode)
>  no_delete:
>  	btrfs_remove_delayed_node(BTRFS_I(inode));
>  	clear_inode(inode);
> +	current->journal_info = journal_info;
>  }
>  
>  /*
> -- 
> 1.9.1
> 
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