linux-kernel - Re: [PATCH] media: vidtv: fix read after free

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Date:   Wed, 2 Dec 2020 13:17:21 +0100
From:   Mauro Carvalho Chehab <mchehab@...nel.org>
To:     Cengiz Can <cengiz@...nel.wtf>
Cc:     "Daniel W . S . Almeida" <dwlsalmeida@...il.com>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: vidtv: fix read after free

Hi Cengiz,

Em Mon, 30 Nov 2020 12:28:26 +0300
Cengiz Can <cengiz@...nel.wtf> escreveu:

> `vidtv_channel_si_destroy` function has a call to
>     `vidtv_psi_pat_table_destroy` that frees Program Association Table.
> 
> However it is followed by a loop that iterates over the count of Program
> Map Tables. This obviously accesses an invalid memory.
> 
> Eliminate this by making a copy of num_pmt before free'ing Program
> Association Table and loop on it instead.
> 
> Signed-off-by: Cengiz Can <cengiz@...nel.wtf>

Collin sent me a similar fix. I'm applying his because:
- it was sent first;
- it uses a better approach: it just moves the 
  vidtv_psi_pat_table_destroy() call to happen after freeing
  the PMT tables. So, it doesn't need an aux var.

Regards,
Mauro

> ---
>  drivers/media/test-drivers/vidtv/vidtv_channel.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/test-drivers/vidtv/vidtv_channel.c b/drivers/media/test-drivers/vidtv/vidtv_channel.c
> index 8ad6c0744d36..4af39a8786a7 100644
> --- a/drivers/media/test-drivers/vidtv/vidtv_channel.c
> +++ b/drivers/media/test-drivers/vidtv/vidtv_channel.c
> @@ -503,10 +503,13 @@ int vidtv_channel_si_init(struct vidtv_mux *m)
>  void vidtv_channel_si_destroy(struct vidtv_mux *m)
>  {
>  	u32 i;
> +	u16 num_pmt;
> +
> +	num_pmt = m->si.pat->num_pmt;
>  
>  	vidtv_psi_pat_table_destroy(m->si.pat);
>  
> -	for (i = 0; i < m->si.pat->num_pmt; ++i)
> +	for (i = 0; i < num_pmt; ++i)
>  		vidtv_psi_pmt_table_destroy(m->si.pmt_secs[i]);
>  
>  	kfree(m->si.pmt_secs);



Thanks,
Mauro