Re: [PATCH] ext4: xattr: fix wrong search.here in clone_block

[Jinchao Wang]

On Tue, Dec 16, 2025 at 07:34:55PM +0800, Jinchao Wang wrote:
> syzbot reported a KASAN out-of-bounds Read in ext4_xattr_set_entry()[1].
> 
> When xattr_find_entry() returns -ENODATA, search.here still points to the
> position after the last valid entry. ext4_xattr_block_set() clones the xattr
> block because the original block maybe shared and must not be modified in
> place.
> 
> In the clone_block, search.here is recomputed unconditionally from the old
> offset, which may place it past search.first. This results in a negative
> reset size and an out-of-bounds memmove() in ext4_xattr_set_entry().
> 
> Fix this by initializing search.here correctly when search.not_found is set.
> 
> [1] https://syzkaller.appspot.com/bug?extid=f792df426ff0f5ceb8d1
> 
> Fixes: fd48e9acdf2 (ext4: Unindent codeblock in ext4_xattr_block_set)
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: syzbot+f792df426ff0f5ceb8d1@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Jinchao Wang <wangjinchao600@xxxxxxxxx>
> ---
>  fs/ext4/xattr.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
> index 2e02efbddaac..cc30abeb7f30 100644
> --- a/fs/ext4/xattr.c
> +++ b/fs/ext4/xattr.c
> @@ -1980,7 +1980,10 @@ ext4_xattr_block_set(handle_t *handle, struct inode *inode,
>  			goto cleanup;
>  		s->first = ENTRY(header(s->base)+1);
>  		header(s->base)->h_refcount = cpu_to_le32(1);
> -		s->here = ENTRY(s->base + offset);
> +		if (s->not_found)
> +			s->here = s->first;
> +		else
> +			s->here = ENTRY(s->base + offset);
>  		s->end = s->base + bs->bh->b_size;
>  
>  		/*
> -- 
> 2.43.0
> 

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