[PATCH v2 1/1] drm/mediatek: Fix potential KP on 0 bytes nvmem cell read

[LIANKUN YANG (杨连坤)]

On Wed, 2024-10-09 at 08:41 -0700, Guillaume Ranquet wrote:
>  	 
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
>  On Fri, 27 Sep 2024 11:03, Liankun Yang <liankun.yang at mediatek.com>
> wrote:
> >If the len is 0, kernel crash will occur when performing operations.
> >Add the len check conditions to prevent kernel crash.
> >
> >Fixes: f70ac097a2cf ("drm/mediatek: Add MT8195 Embedded DisplayPort
> driver")
> >Signed-off-by: Liankun Yang <liankun.yang at mediatek.com>
> >---
> >Changes in V2:
> >- Fix the commit title.
> >- Remove blank line between the Fixes and Signe-off-by.
> >- Modify the judgment writing method.
> >Per suggestion from the previous thread:
> >
> https://patchwork.kernel.org/project/linux-mediatek/patch/20240925084116.28848-1-liankun.yang@mediatek.com/
> >---
> > drivers/gpu/drm/mediatek/mtk_dp.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/drivers/gpu/drm/mediatek/mtk_dp.c
> b/drivers/gpu/drm/mediatek/mtk_dp.c
> >index d8796a904eca..9ecdf62398ee 100644
> >--- a/drivers/gpu/drm/mediatek/mtk_dp.c
> >+++ b/drivers/gpu/drm/mediatek/mtk_dp.c
> >@@ -1082,7 +1082,7 @@ static void mtk_dp_get_calibration_data(struct
> mtk_dp *mtk_dp)
> > buf = (u32 *)nvmem_cell_read(cell, &len);
> > nvmem_cell_put(cell);
> >
> >-if (IS_ERR(buf) || ((len / sizeof(u32)) != 4)) {
> >+if (IS_ERR(buf) || !len || ((len / sizeof(u32)) != 4)) {
> > dev_warn(dev, "Failed to read nvmem_cell_read\n");
> 
> Hello Liankun,
> Would you be able to describe the crash with more details?
> 
> I'm afraid I don't understand the fix?
> 
> if len is 0, dividing 0 by sizeof(u32) is 0 and thus != 4
> So to me, checking for len != 0 is redundant?
> 
> Thx,
> Guillaume.

Hello Guillaume,

Thank you for your correction.

The check for len != 0 is redundant.

I did not review this patch carefully, and will review it again next
time.

Thx,
Liankun.

> >
> > if (!IS_ERR(buf))
> >--
> >2.45.2