linux-ext4 - Re: [PATCH] e2fsck: Fix access after free for dx

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Date:   Fri, 3 Nov 2017 14:00:10 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Artem Blagodarenko <artem.blagodarenko@...il.com>
Cc:     linux-ext4@...r.kernel.org, adilger.kernel@...ger.ca
Subject: Re: [PATCH] e2fsck: Fix access after free for dx_db structure

> On Oct 30, 2017, at 7:32 AM, Artem Blagodarenko <artem.blagodarenko@...il.com> wrote:
> 
> dx_db structure is freed after fixing of PR_2_HTREE_BAD_ROOT
> problem. Next code block use this structure to unerstand if leaf
> is beeng processed.
> 
> If dx_db is freed, then root block is being processed and if_leaf
> need to be set to 0.
> 
> Signed-off-by: Artem Blagodarenko <artem.blagodarenko@...il.com>
> ---
> e2fsck/pass2.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/e2fsck/pass2.c b/e2fsck/pass2.c
> index 09c16179..7bda37a7 100644
> --- a/e2fsck/pass2.c
> +++ b/e2fsck/pass2.c
> @@ -1109,7 +1109,7 @@ inline_read_fail:
> 			    ((fs->blocksize - (8 + dx_csum_size)) /
> 			     sizeof(struct ext2_dx_entry))))
> 			dx_db->type = DX_DIRBLOCK_NODE;
> -		is_leaf = (dx_db->type == DX_DIRBLOCK_LEAF);
> +		is_leaf = dx_db ? (dx_db->type == DX_DIRBLOCK_LEAF) : 0;

Ah, this code is a bit confusing.  From looking at this patch, and even
looking at the code directly, I thought "the previous line is also
accessing dx_db, so what is the problem?"  However, there really IS a
problem, because the previous line is part of a different if/else block.

It would be good to add braces around the "else" part of the condition
to make it clear that this is a separate block of code, which is why we
typically ask for matching braces for both legs of an if/else block.  It
would also be good to set "dx_db = NULL" instead of setting it to "0".

Cheers, Andreas

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