linux-ext4 - Re: [PATCH] ext4: add reserved GDT blocks check

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Date:   Sat, 28 May 2022 20:31:11 +0530
From:   Ritesh Harjani <ritesh.list@...il.com>
To:     Zhang Yi <yi.zhang@...wei.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, jack@...e.cz, yukuai3@...wei.com,
        lilingfeng3@...wei.com
Subject: Re: [PATCH] ext4: add reserved GDT blocks check

On 22/05/26 03:32PM, Zhang Yi wrote:
> We capture a NULL pointer issue when resizing a corrupt ext4 image which
> is freshly clear resize_inode feature (not run e2fsck). It could be
> simply reproduced by following steps. The problem is because of the
> resize_inode feature was cleared, and it will convert the filesystem to
> meta_bg mode in ext4_resize_fs(), but the es->s_reserved_gdt_blocks was
> not reduced to zero, so could we mistakenly call reserve_backup_gdb()
> and passing an uninitialized resize_inode to it when adding new group
> descriptors.
>
>  mkfs.ext4 /dev/sda 3G
>  tune2fs -O ^resize_inode /dev/sda #forget to run requested e2fsck
>  mount /dev/sda /mnt
>  resize2fs /dev/sda 8G
>
>  ========
>  BUG: kernel NULL pointer dereference, address: 0000000000000028
>  CPU: 19 PID: 3243 Comm: resize2fs Not tainted 5.18.0-rc7-00001-gfde086c5ebfd #748
>  ...
>  RIP: 0010:ext4_flex_group_add+0xe08/0x2570
>  ...
>  Call Trace:
>   <TASK>
>   ext4_resize_fs+0xbec/0x1660
>   __ext4_ioctl+0x1749/0x24e0
>   ext4_ioctl+0x12/0x20
>   __x64_sys_ioctl+0xa6/0x110
>   do_syscall_64+0x3b/0x90
>   entry_SYSCALL_64_after_hwframe+0x44/0xae
>  RIP: 0033:0x7f2dd739617b
>  ========
>
> The fix is simple, add a check in ext4_resize_fs() to make sure that the
> es->s_reserved_gdt_blocks is zero when the resize_inode feature is
> disabled.

Your reasoning looks correct to me.

>
> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
> ---
>  fs/ext4/resize.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
> index 90a941d20dff..5791eb7c0761 100644
> --- a/fs/ext4/resize.c
> +++ b/fs/ext4/resize.c
> @@ -2031,6 +2031,9 @@ int ext4_resize_fs(struct super_block *sb, ext4_fsblk_t n_blocks_count)
>  			ext4_warning(sb, "Error opening resize inode");
>  			return PTR_ERR(resize_inode);
>  		}
> +	} else if (es->s_reserved_gdt_blocks) {
> +		ext4_error(sb, "resize_inode disabled but reserved GDT blocks non-zero");
> +		return -EFSCORRUPTED;
>  	}

I think we should do this check in ext4_resize_begin(), i.e.
if ext4_has_feature_resize_inode() is false and es->s_reserved_gdt_blocks is
non-zero, then we should straight away mark and return error.

Later (not as part of this patch/fix) maybe if we detect this problem, we could
use helpers like ext4_update_super() to fix this mismatch problem in kernel
during mount itself. But I think this is not absolutely necessary,
as kernel already during mount outputs a warning and ask for running e2fsck.

Thougts?

-ritesh

>
>  	if ((!resize_inode && !meta_bg) || n_blocks_count == o_blocks_count) {
> --
> 2.31.1
>