Re: [PATCH v3 5/5] soc: qcom: cmd-db: Map with read-only mappings

[Evan Green]

On Tue, Sep 10, 2019 at 9:09 AM Stephen Boyd <swboyd@xxxxxxxxxxxx> wrote:
>
> The command DB is read-only already to the kernel because everything is
> const marked once we map it. Let's go one step further and try to map
> the memory as read-only in the page tables. This should make it harder
> for random code to corrupt the database and change the contents.
>
> Cc: Evan Green <evgreen@xxxxxxxxxxxx>
> Cc: Rob Herring <robh+dt@xxxxxxxxxx>
> Cc: Bjorn Andersson <bjorn.andersson@xxxxxxxxxx>
> Cc: Andy Gross <agross@xxxxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Reviewed-by: Bjorn Andersson <bjorn.andersson@xxxxxxxxxx>
> Signed-off-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
> ---
>  drivers/soc/qcom/cmd-db.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/soc/qcom/cmd-db.c b/drivers/soc/qcom/cmd-db.c
> index 10a34d26b753..6365e8260282 100644
> --- a/drivers/soc/qcom/cmd-db.c
> +++ b/drivers/soc/qcom/cmd-db.c
> @@ -240,7 +240,8 @@ static int cmd_db_dev_probe(struct platform_device *pdev)
>  {
>         int ret = 0;
>
> -       cmd_db_header = devm_memremap_reserved_mem(&pdev->dev, MEMREMAP_WB);
> +       cmd_db_header = devm_memremap_reserved_mem(&pdev->dev,
> +                                                  MEMREMAP_RO | MEMREMAP_WB);

It seems weird to have both flags, like: "It's read-only, but if it
ever did get written to somehow, make it writeback".

>         if (IS_ERR(cmd_db_header)) {
>                 ret = PTR_ERR(cmd_db_header);
>                 cmd_db_header = NULL;
> --
> Sent by a computer through tubes
>