linux-kernel - Re: [PATCH] driver/ata: fix potential null pointer dereference on pointer last

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Message-ID: <81a99dd1-0ef6-9bd0-31c0-b0934023746b@opensource.wdc.com>
Date:   Tue, 26 Oct 2021 13:09:41 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     Chengfeng Ye <cyeaa@...nect.ust.hk>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] driver/ata: fix potential null pointer dereference on
 pointer last_sge

On 2021/10/25 22:13, Chengfeng Ye wrote:
> The pointer cs_desc could be null if the loop is not
> executed, so there is a potential NULL-PTR dereference
> issue. Fix this by adding a null check before dereference.
> 
> Signed-off-by: Chengfeng Ye <cyeaa@...nect.ust.hk>
> ---
>  drivers/ata/sata_sil24.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/ata/sata_sil24.c b/drivers/ata/sata_sil24.c
> index 06a1e27c4f84..2bd595da799f 100644
> --- a/drivers/ata/sata_sil24.c
> +++ b/drivers/ata/sata_sil24.c
> @@ -785,6 +785,8 @@ static inline void sil24_fill_sg(struct ata_queued_cmd *qc,
>  		sge++;
>  	}
>  
> +	if (!last_sge)
> +		return;
>  	last_sge->flags = cpu_to_le32(SGE_TRM);
>  }

I do not think that this fix is necessary: sil24_fill_sg() is called only if the
qc has ATA_QCFLAG_DMAMAP set (see sil24_qc_prep()) and that in turn is only set
if ata_sg_setup() sees at least one sg entry. So the loop in sil24_fill_sg()
will always be executed for qc marked with ATA_QCFLAG_DMAMAP and so last_sg
cannot be NULL.

-- 
Damien Le Moal
Western Digital Research