Re: [PATCH] media: bttv: Fix a NULL pointer dereference in bttv_overlay()

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Jan 25, 2022 at 01:26:24AM +0800, Zhou Qingyang wrote:
> In bttv_overlay(), the return value of videobuf_sg_alloc() is assigned
> to variable new and there is a dereference of it after that. the return
> value of videobuf_sg_alloc() could be NULL on failure of allocation,
> which could lead to a NULL pointer dereference.
> 
> Fix this bug by adding a NULL check of new.
> 
> This bug was found by a static analyzer.
> 
> Builds with 'make allyesconfig' show no new warnings,
> and our static analyzer no longer warns about this code.
> 
> Fixes: 7c018804c090 ("V4L/DVB (7197): bttv: Fix overlay divide error")
> Signed-off-by: Zhou Qingyang <zhou1615@umn.edu>
> ---
> The analysis employs differential checking to identify inconsistent 
> security operations (e.g., checks or kfrees) between two code paths 
> and confirms that the inconsistent operations are not recovered in the
> current function or the callers, so they constitute bugs. 
> 
> Note that, as a bug found by static analysis, it can be a false
> positive or hard to trigger. Multiple researchers have cross-reviewed
> the bug.
> 
>  drivers/media/pci/bt8xx/bttv-driver.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/media/pci/bt8xx/bttv-driver.c b/drivers/media/pci/bt8xx/bttv-driver.c
> index 6338f98d845d..5aa46593ddc6 100644
> --- a/drivers/media/pci/bt8xx/bttv-driver.c
> +++ b/drivers/media/pci/bt8xx/bttv-driver.c
> @@ -2552,6 +2552,8 @@ static int bttv_overlay(struct file *file, void *f, unsigned int on)
>  	if (on) {
>  		fh->ov.tvnorm = btv->tvnorm;
>  		new = videobuf_sg_alloc(sizeof(*new));
> +		if (!new)
> +			return -ENOMEM;
>  		new->crop = btv->crop[!!fh->do_crop].rect;
>  		bttv_overlay_risc(btv, &fh->ov, fh->ovfmt, new);
>  	} else {
> -- 
> 2.25.1
> 

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