How can this fix prevent information from leaking to user space and prevent the kernel from crashing?

[Lizhi Xu]

Hi, Anirudh Rayabharam

I verify this patch in the following environment, using reproducer: https://syzkaller.appspot.com/x/repro.c?x=122870ff800000

1. kernel version:  kernel version 5.15.72 
2. gcc 11.3
3. libc 2.35

Because the kernel version 5.15.72 already contains this patch ce3aba43599f0b50adbebff133df8d08a3d5fffe, 
So I deleted this patch to make a kernel image to reproduce the problem,
On the other hand, I reserve this patch to verify that the problem has been fixed,
The result of the experiment is that no matter whether this patch is applied or not, 
this problem cannot be reproduced in kernel version 5.15.72.

In addition, I am also very confused. There are three places to initialize "eh_generation". 
There is no other reference to the parameter "eh_generation" in all the source code of the kernel,
At the same time, there is no indirect operation on the parameter "eh_generation" in reproducer,
How can this fix prevent information from leaking to user space and prevent the kernel from crashing?

> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 77c84d6f1af6..677d4821bcc1 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -825,6 +825,7 @@ void ext4_ext_tree_init(handle_t *handle, struct inode *inode)
>  	eh->eh_entries = 0;
>  	eh->eh_magic = EXT4_EXT_MAGIC;
>  	eh->eh_max = cpu_to_le16(ext4_ext_space_root(inode, 0));
> +	eh->eh_generation = 0;
>  	ext4_mark_inode_dirty(handle, inode);
>  }
>  
> @@ -1090,6 +1091,7 @@ static int ext4_ext_split(handle_t *handle, struct inode *inode,
>  	neh->eh_max = cpu_to_le16(ext4_ext_space_block(inode, 0));
>  	neh->eh_magic = EXT4_EXT_MAGIC;
>  	neh->eh_depth = 0;
> +	neh->eh_generation = 0;
>  
>  	/* move remainder of path[depth] to the new leaf */
>  	if (unlikely(path[depth].p_hdr->eh_entries !=
> @@ -1167,6 +1169,7 @@ static int ext4_ext_split(handle_t *handle, struct inode *inode,
>  		neh->eh_magic = EXT4_EXT_MAGIC;
>  		neh->eh_max = cpu_to_le16(ext4_ext_space_block_idx(inode, 0));
>  		neh->eh_depth = cpu_to_le16(depth - i);
> +		neh->eh_generation = 0;
>  		fidx = EXT_FIRST_INDEX(neh);
>  		fidx->ei_block = border;
>  		ext4_idx_store_pblock(fidx, oldblock);
> -- 
> 2.26.2