linux-kernel - Re: [PATCH] hfs: fix deadlock in hfs_extend

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Date: Wed, 3 Jan 2024 12:03:31 +0300
From: Viacheslav Dubeyko <slava@...eyko.com>
To: Edward Adam Davis <eadavis@...com>
Cc: syzbot+41a88b825a315aac2254@...kaller.appspotmail.com,
 Linux FS Devel <linux-fsdevel@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>,
 syzkaller-bugs@...glegroups.com,
 willy@...radead.org
Subject: Re: [PATCH] hfs: fix deadlock in hfs_extend_file



> On Jan 2, 2024, at 3:36 PM, Edward Adam Davis <eadavis@...com> 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.
> 
> Reported-and-tested-by: syzbot+41a88b825a315aac2254@...kaller.appspotmail.com
> Signed-off-by: Edward Adam Davis <eadavis@...com>
> ---
> 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;
> +

I don’t think that fix can be so simple. It looks like the code requires significant
refactoring. Because, currently, it looks like bad recursion: hfs_extend_file() finally
calls hfs_extend_file(). And it smells really badly. Also, from the logical point of view,
hfs_ext_read_extent() method calls __hfs_ext_write_extent() that sounds like not
good logic. I believe we need more serious refactoring of hfs_extend_file() logic.

Potentially, hfs_extend_file() can check that we have HFS_FLG_EXT_DIRTY and
execute logic of write extent without calling himself again. But I haven’t clear picture
of necessary refactoring efforts yet.

Thanks,
Slava.