Re: [PATCH for-5.10] spi: gpio: Don't leak SPI master in probe error path

[Andrey Smirnov <andrew.smirnov@xxxxxxxxx>]

On Mon, Nov 16, 2020 at 12:44 AM Lukas Wunner <lukas@xxxxxxxxx> wrote:
>
> If the call to devm_spi_register_master() fails on probe of the GPIO SPI
> driver, the spi_master struct is erroneously not freed:
>
> After allocating the spi_master, its reference count is 1.  The driver
> unconditionally decrements the reference count on unbind using a devm
> action.  Before calling devm_spi_register_master(), the driver
> unconditionally increments the reference count because on success,
> that function will decrement the reference count on unbind.  However on
> failure, devm_spi_register_master() does *not* decrement the reference
> count, so the spi_master is leaked.

Not sure I fully understand this. On failure
devm_spi_register_master() will return a negative error code which
should result in probe failure and release of devres resource, right?

>
> The issue was introduced by commits 8b797490b4db ("spi: gpio: Make sure
> spi_master_put() is called in every error path") and 79567c1a321e ("spi:
> gpio: Use devm_spi_register_master()"), which sought to plug leaks
> introduced by 9b00bc7b901f ("spi: spi-gpio: Rewrite to use GPIO
> descriptors") but missed this remaining leak.
>

That extra spi_master_get() that might be problematic was present in
the code before 8b797490b4db ("spi: gpio: Make sure spi_master_put()
is called in every error path") and I think was first introduced in

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers?h=v5.9-rc4&id=702a4879ec337463f858c8ab467482cce260bf18

Or am I missing something?

Not really questioning the validity of this fix, just trying to
understand what I missed in 8b797490b4db ("spi: gpio: Make sure
spi_master_put() is called in every error path")

> The situation was later aggravated by commit d3b0ffa1d75d ("spi: gpio:
> prevent memory leak in spi_gpio_probe"), which introduced a
> use-after-free because it releases a reference on the spi_master if
> devm_add_action_or_reset() fails even though the function already
> does that.
>
> Fix by switching over to the new devm_spi_alloc_master() helper.
>
> Fixes: 9b00bc7b901f ("spi: spi-gpio: Rewrite to use GPIO descriptors")
> Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # v4.17+: 5e844cc37a5c: spi: Introduce device-managed SPI controller allocation
> Cc: <stable@xxxxxxxxxxxxxxx> # v5.1-: 8b797490b4db: spi: gpio: Make sure spi_master_put() is called in every error path
> Cc: <stable@xxxxxxxxxxxxxxx> # v5.1-: 45beec351998: spi: bitbang: Introduce spi_bitbang_init()
> Cc: <stable@xxxxxxxxxxxxxxx> # v5.1-: 79567c1a321e: spi: gpio: Use devm_spi_register_master()
> Cc: <stable@xxxxxxxxxxxxxxx> # v5.4-: d3b0ffa1d75d: spi: gpio: prevent memory leak in spi_gpio_probe
> Cc: <stable@xxxxxxxxxxxxxxx> # v4.17+
> Cc: Navid Emamdoost <navid.emamdoost@xxxxxxxxx>
> Cc: Andrey Smirnov <andrew.smirnov@xxxxxxxxx>
> Cc: Linus Walleij <linus.walleij@xxxxxxxxxx>
> ---
>  drivers/spi/spi-gpio.c | 15 ++-------------
>  1 file changed, 2 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/spi/spi-gpio.c b/drivers/spi/spi-gpio.c
> index 7ceb0ba27b75..0584f4d2fde2 100644
> --- a/drivers/spi/spi-gpio.c
> +++ b/drivers/spi/spi-gpio.c
> @@ -350,11 +350,6 @@ static int spi_gpio_probe_pdata(struct platform_device *pdev,
>         return 0;
>  }
>
> -static void spi_gpio_put(void *data)
> -{
> -       spi_master_put(data);
> -}
> -
>  static int spi_gpio_probe(struct platform_device *pdev)
>  {
>         int                             status;
> @@ -363,16 +358,10 @@ static int spi_gpio_probe(struct platform_device *pdev)
>         struct device                   *dev = &pdev->dev;
>         struct spi_bitbang              *bb;
>
> -       master = spi_alloc_master(dev, sizeof(*spi_gpio));
> +       master = devm_spi_alloc_master(dev, sizeof(*spi_gpio));
>         if (!master)
>                 return -ENOMEM;
>
> -       status = devm_add_action_or_reset(&pdev->dev, spi_gpio_put, master);
> -       if (status) {
> -               spi_master_put(master);
> -               return status;
> -       }
> -
>         if (pdev->dev.of_node)
>                 status = spi_gpio_probe_dt(pdev, master);
>         else
> @@ -432,7 +421,7 @@ static int spi_gpio_probe(struct platform_device *pdev)
>         if (status)
>                 return status;
>
> -       return devm_spi_register_master(&pdev->dev, spi_master_get(master));
> +       return devm_spi_register_master(&pdev->dev, master);
>  }
>
>  MODULE_ALIAS("platform:" DRIVER_NAME);
> --
> 2.28.0
>