linux-kernel - Re: [PATCH] ocfs2: Avoid NULL pointer dereference in dx_dir_lookup

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Message-ID: <d6c72ab8-d600-4cc2-9609-8b749b61703d@linux.alibaba.com>
Date: Mon, 30 Jun 2025 09:26:40 +0800
From: Joseph Qi <joseph.qi@...ux.alibaba.com>
To: Ivan Pravdin <ipravdin.official@...il.com>, mark@...heh.com,
 jlbec@...lplan.org, ocfs2-devel@...ts.linux.dev, linux-kernel@...r.kernel.org
Cc: syzbot+20282c1b2184a857ac4c@...kaller.appspotmail.com
Subject: Re: [PATCH] ocfs2: Avoid NULL pointer dereference in
 dx_dir_lookup_rec()

Hi,


On 2025/6/27 10:38, Ivan Pravdin wrote:
> When a directory entry is not found, ocfs2_dx_dir_lookup_rec() prints an
> error message that unconditionally dereferences the 'rec' pointer.
> However, if 'rec' is NULL, this leads to a NULL pointer dereference and
> a kernel panic.
> 

This looks possible, but syzbot reports slab-out-of-bounds Read in
ocfs2_dx_dir_lookup_rec(), not NULL pointer dereference.

So I think it is because it construct a malicious image and set a wrong
l_recs, then access this damaged l_recs.

Thanks,
Joseph


> Add an explicit check for a NULL 'rec' and use an alternate error
> message in that case to avoid unsafe access.
> 
> Reported-by: syzbot+20282c1b2184a857ac4c@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/all/67483b75.050a0220.253251.007c.GAE@google.com/T/
> Signed-off-by: Ivan Pravdin <ipravdin.official@...il.com>
> ---
>  fs/ocfs2/dir.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ocfs2/dir.c b/fs/ocfs2/dir.c
> index 7799f4d16ce9..dccf0349e523 100644
> --- a/fs/ocfs2/dir.c
> +++ b/fs/ocfs2/dir.c
> @@ -809,11 +809,17 @@ static int ocfs2_dx_dir_lookup_rec(struct inode *inode,
>  	}
>  
>  	if (!found) {
> -		ret = ocfs2_error(inode->i_sb,
> -				  "Inode %lu has bad extent record (%u, %u, 0) in btree\n",
> -				  inode->i_ino,
> -				  le32_to_cpu(rec->e_cpos),
> -				  ocfs2_rec_clusters(el, rec));
> +		if (rec) {
> +			ret = ocfs2_error(inode->i_sb,
> +					"Inode %lu has bad extent record (%u, %u, 0) in btree\n",
> +					inode->i_ino,
> +					le32_to_cpu(rec->e_cpos),
> +					ocfs2_rec_clusters(el, rec));
> +		} else {
> +			ret = ocfs2_error(inode->i_sb,
> +					"Inode %lu has no extent records in btree\n",
> +					inode->i_ino);
> +		}
>  		goto out;
>  	}
>