linux-ext4 - [PATCH] ext4: fix off by one issue in alloc_flex

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Message-Id: <20240927063620.2630898-1-libaokun@huaweicloud.com>
Date: Fri, 27 Sep 2024 14:36:20 +0800
From: libaokun@...weicloud.com
To: linux-ext4@...r.kernel.org
Cc: tytso@....edu,
	adilger.kernel@...ger.ca,
	jack@...e.cz,
	linux-kernel@...r.kernel.org,
	yi.zhang@...wei.com,
	yangerkun@...wei.com,
	libaokun@...weicloud.com,
	Baokun Li <libaokun1@...wei.com>,
	Wesley Hershberger <wesley.hershberger@...onical.com>,
	Stéphane Graber <stgraber@...raber.org>,
	Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>,
	Eric Sandeen <sandeen@...hat.com>,
	stable@...r.kernel.org
Subject: [PATCH] ext4: fix off by one issue in alloc_flex_gd()

From: Baokun Li <libaokun1@...wei.com>

Wesley reported an issue:

==================================================================
EXT4-fs (dm-5): resizing filesystem from 7168 to 786432 blocks
------------[ cut here ]------------
kernel BUG at fs/ext4/resize.c:324!
CPU: 9 UID: 0 PID: 3576 Comm: resize2fs Not tainted 6.11.0+ #27
RIP: 0010:ext4_resize_fs+0x1212/0x12d0
Call Trace:
 __ext4_ioctl+0x4e0/0x1800
 ext4_ioctl+0x12/0x20
 __x64_sys_ioctl+0x99/0xd0
 x64_sys_call+0x1206/0x20d0
 do_syscall_64+0x72/0x110
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
==================================================================

While reviewing the patch, Honza found that when adjusting resize_bg in
alloc_flex_gd(), it was possible for flex_gd->resize_bg to be bigger than
flexbg_size.

The reproduction of the problem requires the following:

 o_group = flexbg_size * 2 * n;
 o_size = (o_group + 1) * group_size;
 n_group: [o_group + flexbg_size, o_group + flexbg_size * 2)
 o_size = (n_group + 1) * group_size;

Take n=0,flexbg_size=16 as an example:

              last:15
|o---------------|--------------n-|
o_group:0    resize to      n_group:30

The corresponding reproducer is:

img=test.img
truncate -s 600M $img
mkfs.ext4 -F $img -b 1024 -G 16 8M
dev=`losetup -f --show $img`
mkdir -p /tmp/test
mount $dev /tmp/test
resize2fs $dev 248M

Delete the problematic plus 1 to fix the issue, and add a WARN_ON_ONCE()
to prevent the issue from happening again.

Reported-by: Wesley Hershberger <wesley.hershberger@...onical.com>
Closes: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2081231
Reported-by: Stéphane Graber <stgraber@...raber.org>
Closes: https://lore.kernel.org/all/20240925143325.518508-1-aleksandr.mikhalitsyn@canonical.com/
Tested-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
Tested-by: Eric Sandeen <sandeen@...hat.com>
Fixes: 665d3e0af4d3 ("ext4: reduce unnecessary memory allocation in alloc_flex_gd()")
Cc: stable@...r.kernel.org
Signed-off-by: Baokun Li <libaokun1@...wei.com>
---
 fs/ext4/resize.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
index e04eb08b9060..397970121d43 100644
--- a/fs/ext4/resize.c
+++ b/fs/ext4/resize.c
@@ -253,9 +253,9 @@ static struct ext4_new_flex_group_data *alloc_flex_gd(unsigned int flexbg_size,
 	/* Avoid allocating large 'groups' array if not needed */
 	last_group = o_group | (flex_gd->resize_bg - 1);
 	if (n_group <= last_group)
-		flex_gd->resize_bg = 1 << fls(n_group - o_group + 1);
+		flex_gd->resize_bg = 1 << fls(n_group - o_group);
 	else if (n_group - last_group < flex_gd->resize_bg)
-		flex_gd->resize_bg = 1 << max(fls(last_group - o_group + 1),
+		flex_gd->resize_bg = 1 << max(fls(last_group - o_group),
 					      fls(n_group - last_group));
 
 	flex_gd->groups = kmalloc_array(flex_gd->resize_bg,
-- 
2.46.0