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References: <20231118113357.1999-1-kamil.duljas@gmail.com> <20231118120235.GA30289@breakpoint.cc>
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From:   Kamil Duljas <kamil.duljas@gmail.com>
Date:   Sat, 18 Nov 2023 14:27:56 +0100
Message-ID: <CAFR=A7nkyx_Lf=p0BS-S68_vxQL97rUoLMZpo4kxHjKykAgTRw@mail.gmail.com>
Subject: Re: [PATCH] genetlink: Prevent memory leak when krealloc fail
To:     Florian Westphal <fw@strlen.de>
Cc:     Jakub Kicinski <kuba@kernel.org>,
        "David S . Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Paolo Abeni <pabeni@redhat.com>, Jiri Pirko <jiri@resnulli.us>,
        Johannes Berg <johannes@sipsolutions.net>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
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Precedence: bulk

Yes, you're right. I did not think about it. So if we have a static
pointer that may be resued, should not restore the pointer as at the
beginning?
static unsigned long *mc_groups =3D &mc_group_start;

At this moment we don't know how much memory is allocated. What do you
think about this?

>                               new_groups =3D krealloc(mc_groups, nlen,
>                                                     GFP_KERNEL);
> -                             if (!new_groups)
> +                             if (!new_groups) {
> +                                     kfree(mc_groups);
> +                                     mc_groups =3D &mc_group_start;
>                                       return -ENOMEM;
> +                             }


sob., 18 lis 2023 o 13:02 Florian Westphal <fw@strlen.de> napisa=C5=82(a):
>
> Kamil Duljas <kamil.duljas@gmail.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 =3D 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.


--=20
Pozdrawiam,
Kamil Duljas