linux-kernel - Re: [PATCH v2 1/2] of: overlay: Fix memory leak in of_overlay

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Date:   Mon, 4 Dec 2017 21:07:25 -0500
From:   Frank Rowand <frowand.list@...il.com>
To:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Rob Herring <robh+dt@...nel.org>
Cc:     Colin King <colin.king@...onical.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] of: overlay: Fix memory leak in of_overlay_apply()
 error path

Hi Geert,

Thanks for finding the issues and for the fixes.

Comments in line.


On 12/04/17 10:47, Geert Uytterhoeven wrote:
> If of_resolve_phandles() fails, free_overlay_changeset() is called in
> the error path.  However, that function returns early if the list hasn't
> been initialized yet, before freeing the object.
> 
> Explicitly calling kfree() instead would solve that issue. However, that
> complicates matter, by having to consider which of two different methods
> to use to dispose of the same object.
> 
> Hence make free_overlay_changeset() consider initialization state of the
> different parts of the object, making it always safe to call (once!) to
> dispose of a (partially) initialized overlay_changeset:
>   - Only destroy the changeset if the list was initialized,
>   - Ignore uninitialized IDs (zero).
> 
> Reported-by: Colin King <colin.king@...onical.com>
> Fixes: f948d6d8b792bb90 ("of: overlay: avoid race condition between applying multiple overlays")
> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
> ---
>  drivers/of/overlay.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
> index 3b7a3980ff50d6bf..312cd658bec0083b 100644
> --- a/drivers/of/overlay.c
> +++ b/drivers/of/overlay.c
> @@ -630,11 +630,10 @@ static void free_overlay_changeset(struct overlay_changeset *ovcs)
>  {
>  	int i;
>  
> -	if (!ovcs->cset.entries.next)
> -		return;
> -	of_changeset_destroy(&ovcs->cset);
> +	if (ovcs->cset.entries.next)
> +		of_changeset_destroy(&ovcs->cset);
>  

OK

> -	if (ovcs->id)
> +	if (ovcs->id > 0)

Instead of this change, could you please make a change in init_overlay_changeset()?

Current init_overlay_changeset():

        ovcs->id = idr_alloc(&ovcs_idr, ovcs, 1, 0, GFP_KERNEL);
        if (ovcs->id <= 0)
                return ovcs->id;

My proposed version:

        ret = idr_alloc(&ovcs_idr, ovcs, 1, 0, GFP_KERNEL);
        if (ret <= 0)
                return ret;
        ovcs->id = ret;


>  		idr_remove(&ovcs_idr, ovcs->id);
>  
>  	for (i = 0; i < ovcs->count; i++) {
> 

Also, the previous version of the patch, and the discussion around the resulting
bug make me think that I should not have moved 'kfree(ovcs)' into
free_overlay_changeset(), because that kfree is then not very visible in the
error path of of_overlay_apply().  Could you remove 'kfree(ovcs)' from
free_overlay_changeset(), and instead call it immediately after each call
to free_overlay_changeset()?

-Frank