Re: [PATCH] hfs: fix deadlock in hfs_extend_file

[Matthew Wilcox]

On Tue, Jan 02, 2024 at 08:36:51PM +0800, Edward Adam Davis wrote:
> [syz report]
> syz-executor279/5059 is trying to acquire lock:
> ffff888079c100f8 (&HFS_I(tree->inode)->extents_lock){+.+.}-{3:3}, at: hfs_extend_file+0xa2/0xb10 fs/hfs/extent.c:397
> 
> but task is already holding lock:
> ffff888079c10778 (&HFS_I(tree->inode)->extents_lock){+.+.}-{3:3}, at: hfs_extend_file+0xa2/0xb10 fs/hfs/extent.c:397
> 
> other info that might help us debug this:
>  Possible unsafe locking scenario:
> 
>        CPU0
>        ----
>   lock(&HFS_I(tree->inode)->extents_lock);
>   lock(&HFS_I(tree->inode)->extents_lock);
> 
>  *** DEADLOCK ***
> [Analysis] 
>  hfs_extend_file()->
>    hfs_ext_read_extent()->
>      __hfs_ext_cache_extent()->
>        __hfs_ext_write_extent()->
>          hfs_bmap_reserve()->
>            hfs_extend_file()->
> 
> When an inode has both the HFS_FLG_EXT_DIRTY and HFS_FLG_EXT_NEW flags, it will
> enter the above loop and trigger a deadlock.
> 
> [Fix]
> In hfs_ext_read_extent(), check if the above two flags exist simultaneously, 
> and exit the subsequent process when the conditions are met.

Why is this the correct fix?  Seems to me that returning -ENOENT here is
going to lead to an error being reported to the user when the user has
done nothing wrong?

> Reported-and-tested-by: syzbot+41a88b825a315aac2254@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
> ---
>  fs/hfs/extent.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/hfs/extent.c b/fs/hfs/extent.c
> index 6d1878b99b30..1b02c7b6a10c 100644
> --- a/fs/hfs/extent.c
> +++ b/fs/hfs/extent.c
> @@ -197,6 +197,10 @@ static int hfs_ext_read_extent(struct inode *inode, u16 block)
>  	    block < HFS_I(inode)->cached_start + HFS_I(inode)->cached_blocks)
>  		return 0;
>  
> +	if (HFS_I(inode)->flags & HFS_FLG_EXT_DIRTY && 
> +	    HFS_I(inode)->flags & HFS_FLG_EXT_NEW) 
> +		return -ENOENT;
> +
>  	res = hfs_find_init(HFS_SB(inode->i_sb)->ext_tree, &fd);
>  	if (!res) {
>  		res = __hfs_ext_cache_extent(&fd, inode, block);
> -- 
> 2.43.0
> 
>