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

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Message-ID: <CAEivzxej-DiXpkcQeYrVVPXbXXnCf=4d3EWyhw8euwBjuB8S9w@mail.gmail.com>
Date: Fri, 27 Sep 2024 10:22:23 +0200
From: Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To: libaokun@...weicloud.com
Cc: linux-ext4@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca, 
	jack@...e.cz, linux-kernel@...r.kernel.org, yi.zhang@...wei.com, 
	yangerkun@...wei.com, Baokun Li <libaokun1@...wei.com>, 
	Wesley Hershberger <wesley.hershberger@...onical.com>, 
	Stéphane Graber <stgraber@...raber.org>, 
	Eric Sandeen <sandeen@...hat.com>, stable@...r.kernel.org
Subject: Re: [PATCH] ext4: fix off by one issue in alloc_flex_gd()

On Fri, Sep 27, 2024 at 8:39 AM <libaokun@...weicloud.com> wrote:
>
> 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>

Thanks, Baokun!

JFYI, I'm on the way to submit a test to xfstests suite.

Kind regards,
Alex

> ---
>  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
>