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Date:   Mon, 30 Dec 2019 12:06:28 -0500
From:   "Theodore Y. Ts'o" <tytso@mit.edu>
To:     Wang Shilong <wangshilong1991@gmail.com>
Cc:     linux-ext4@vger.kernel.org, adilger@dilger.ca, lixi@ddn.com,
        wshilong@ddn.com
Subject: Re: [PATCH 2/2] e2fsck: fix use after free in calculate_tree()
Message-ID: <20191230170628.GB125106@mit.edu>
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 <1574759039-7429-2-git-send-email-wangshilong1991@gmail.com>
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On Tue, Nov 26, 2019 at 06:03:59PM +0900, Wang Shilong wrote:
> @@ -725,12 +728,18 @@ static errcode_t calculate_tree(ext2_filsys fs,
>  					return retval;
>  			}
>  			if (c3 == 0) {
> +				int delta1 = int_offset;;
> +				int delta2 = (char *)root - outdir->buf;
> +
>  				retval = alloc_blocks(fs, &limit, &int_ent,
>  						      &dx_ent, &int_offset,
>  						      NULL, outdir, i, &c2,
>  						      &c3);
>  				if (retval)
>  					return retval;
> +				/* outdir->buf might be reallocated */
> +				int_limit = (struct ext2_dx_countlimit *)(outdir->buf + delta1);
> +				root = (struct ext2_dx_entry *)(outdir->buf + delta2);
>  
>  			}
>  			dx_ent->block = ext2fs_cpu_to_le32(i);

Um, are you sure

				int delta1 = int_offset;;

is correct?  I would think
				int delta1 = (char *)int_limit - outdir->buf;

is what is needed; it's certainly much more clear.

   	   	   		       	    - Ted