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

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Date:   Wed, 18 Sep 2019 12:40:52 -0700
From:   Evan Green <evgreen@...omium.org>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Rob Herring <robh+dt@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Andy Gross <agross@...nel.org>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH v3 5/5] soc: qcom: cmd-db: Map with read-only mappings

On Tue, Sep 10, 2019 at 9:09 AM Stephen Boyd <swboyd@...omium.org> 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@...omium.org>
> Cc: Rob Herring <robh+dt@...nel.org>
> Cc: Bjorn Andersson <bjorn.andersson@...aro.org>
> Cc: Andy Gross <agross@...nel.org>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Reviewed-by: Bjorn Andersson <bjorn.andersson@...aro.org>
> Signed-off-by: Stephen Boyd <swboyd@...omium.org>
> ---
>  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
>