linux-kernel - Re: [PATCH v3 03/26] ext4: correct the hole length returned by ext4_map

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Date: Thu, 09 May 2024 16:16:34 +0100
From: Luis Henriques <luis.henriques@...ux.dev>
To: Zhang Yi <yi.zhang@...weicloud.com>
Cc: linux-ext4@...r.kernel.org,  linux-fsdevel@...r.kernel.org,
  linux-mm@...ck.org,  linux-kernel@...r.kernel.org,  tytso@....edu,
  adilger.kernel@...ger.ca,  jack@...e.cz,  ritesh.list@...il.com,
  hch@...radead.org,  djwong@...nel.org,  willy@...radead.org,
  zokeefe@...gle.com,  yi.zhang@...wei.com,  chengzhihao1@...wei.com,
  yukuai3@...wei.com,  wangkefeng.wang@...wei.com
Subject: Re: [PATCH v3 03/26] ext4: correct the hole length returned by
 ext4_map_blocks()

On Sat 27 Jan 2024 09:58:02 AM +08, Zhang Yi wrote;
<...>
> +static ext4_lblk_t ext4_ext_determine_insert_hole(struct inode *inode,
> +						  struct ext4_ext_path *path,
> +						  ext4_lblk_t lblk)
> +{
> +	ext4_lblk_t hole_start, len;
> +	struct extent_status es;
> +
> +	hole_start = lblk;
> +	len = ext4_ext_find_hole(inode, path, &hole_start);
> +again:
> +	ext4_es_find_extent_range(inode, &ext4_es_is_delayed, hole_start,
> +				  hole_start + len - 1, &es);
> +	if (!es.es_len)
> +		goto insert_hole;
> +
> +	/*
> +	 * There's a delalloc extent in the hole, handle it if the delalloc
> +	 * extent is in front of, behind and straddle the queried range.
> +	 */
> +	if (lblk >= es.es_lblk + es.es_len) {
> +		/*
> +		 * The delalloc extent is in front of the queried range,
> +		 * find again from the queried start block.
> +		 */
> +		len -= lblk - hole_start;
> +		hole_start = lblk;
> +		goto again;

It's looks like it's easy to trigger an infinite loop here using fstest
generic/039.  If I understand it correctly (which doesn't happen as often
as I'd like), this is due to an integer overflow in the 'if' condition,
and should be fixed with the patch below.

>From 3117af2f8dacad37a2722850421f31075ae9e88d Mon Sep 17 00:00:00 2001
From: "Luis Henriques (SUSE)" <luis.henriques@...ux.dev>
Date: Thu, 9 May 2024 15:53:01 +0100
Subject: [PATCH] ext4: fix infinite loop caused by integer overflow

An integer overflow will happen if the extent_status len is set to
EXT_MAX_BLOCKS (0xffffffff).  This may cause an infinite loop in function
ext4_ext_determine_insert_hole(), easily reproducible using fstest
generic/039.

Fixes: 6430dea07e85 ("ext4: correct the hole length returned by ext4_map_blocks()")
Signed-off-by: Luis Henriques (SUSE) <luis.henriques@...ux.dev>
---
 fs/ext4/extents.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index e57054bdc5fd..193121b394f9 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -4064,7 +4064,7 @@ static ext4_lblk_t ext4_ext_determine_insert_hole(struct inode *inode,
 	 * There's a delalloc extent in the hole, handle it if the delalloc
 	 * extent is in front of, behind and straddle the queried range.
 	 */
-	if (lblk >= es.es_lblk + es.es_len) {
+	if (lblk >= ((__u64) es.es_lblk) + es.es_len) {
 		/*
 		 * The delalloc extent is in front of the queried range,
 		 * find again from the queried start block.