linux-kernel - Re: [PATCH] genetlink: Prevent memory leak when krealloc fail

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Message-ID: <20231118120235.GA30289@breakpoint.cc>
Date:   Sat, 18 Nov 2023 13:02:35 +0100
From:   Florian Westphal <fw@...len.de>
To:     Kamil Duljas <kamil.duljas@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, Jiri Pirko <jiri@...nulli.us>,
        Johannes Berg <johannes@...solutions.net>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] genetlink: Prevent memory leak when krealloc fail

Kamil Duljas <kamil.duljas@...il.com> wrote:
> genl_allocate_reserve_groups() allocs new memory in while loop
> but if krealloc fail, the memory allocated by kzalloc is not freed.
> It seems allocated memory is unnecessary when the function
> returns -ENOMEM

Why should it be free'd?  mc_groups is not a local variable.

>  				new_groups = krealloc(mc_groups, nlen,
>  						      GFP_KERNEL);
> -				if (!new_groups)
> +				if (!new_groups) {
> +					kfree(mc_groups);
>  					return -ENOMEM;
> +				}

How did you test this?  AFAICS this results in use-after-free for every
access to mc_groups after this error path is taken.

Existing code looks correct, we can't grow mc_groups and return an
error.