Re: [PATCH 1/2] hw/gpio/aspeed: Don't let guests modify input pins

[Joel Stanley]

On Thu, 7 Jul 2022 at 07:17, Peter Delevoryas <peter@pjd.dev> wrote:
>
> It seems that aspeed_gpio_update is allowing the value for input pins to be
> modified through register writes and QOM property modification.
>
> The QOM property modification is fine, but modifying the value through
> register writes from the guest OS seems wrong if the pin's direction is set
> to input.
>
> The datasheet specifies that "0" bits in the direction register select input
> mode, and "1" selects output mode.
>
> OpenBMC userspace code is accidentally writing 0's to the GPIO data
> registers somewhere (or perhaps the driver is doing it through a reset or
> something), and this is overwriting GPIO FRU information (board ID, slot
> presence pins) that is initialized in Aspeed machine reset code (see
> fby35_reset() in hw/arm/aspeed.c).
>
> Signed-off-by: Peter Delevoryas <peter@pjd.dev>
> Fixes: 4b7f956862dc ("hw/gpio: Add basic Aspeed GPIO model for AST2400 and 
> AST2500")
> ---
>  hw/gpio/aspeed_gpio.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
>
> diff --git a/hw/gpio/aspeed_gpio.c b/hw/gpio/aspeed_gpio.c
> index a62a673857..2eae427201 100644
> --- a/hw/gpio/aspeed_gpio.c
> +++ b/hw/gpio/aspeed_gpio.c
> @@ -268,7 +268,7 @@ static ptrdiff_t aspeed_gpio_set_idx(AspeedGPIOState *s, 
> GPIOSets *regs)
>  }
>
>  static void aspeed_gpio_update(AspeedGPIOState *s, GPIOSets *regs,
> -                               uint32_t value)
> +                               uint32_t value, bool force)
>  {
>      uint32_t input_mask = regs->input_mask;
>      uint32_t direction = regs->direction;
> @@ -293,10 +293,12 @@ static void aspeed_gpio_update(AspeedGPIOState *s, 
> GPIOSets *regs,
>              }
>

Reading this model hurts my head a little. Perhaps we also need to add
a test for this case to make it clear what's going on.

The test above the code you've changed does this:

>            /* ...and we're output or not input-masked... */
>            if (!(direction & mask) && (input_mask & mask)) {
>                continue;
>            }

For clarity, !(direction & mask) means "is input".

The comment confuses me because it says "or", but the code has "and".

input_mask doesn't seem to feature in the Linux driver, so that will
always be zero. The test will be X && 0, which is always 0.

If you changed it to || then we would do the test that the comment
suggests. When the pin is input, we will skip updating the value.

This will solve the bug you had with your input pins being reset. It
won't fix the QOM case, but we could consider handling that separately
without confusing the logic in this function.


>              /* ...then update the state. */
> -            if (mask & new) {
> -                regs->data_value |= mask;
> -            } else {
> -                regs->data_value &= ~mask;
> +            if (direction & mask || force) {
> +                if (mask & new) {
> +                    regs->data_value |= mask;
> +                } else {
> +                    regs->data_value &= ~mask;
> +                }
>              }
>
>              /* If the gpio is set to output... */
> @@ -339,7 +341,7 @@ static void aspeed_gpio_set_pin_level(AspeedGPIOState *s, 
> uint32_t set_idx,
>          value &= ~pin_mask;
>      }
>
> -    aspeed_gpio_update(s, &s->sets[set_idx], value);
> +    aspeed_gpio_update(s, &s->sets[set_idx], value, true);
>  }
>
>  /*
> @@ -653,7 +655,7 @@ static void aspeed_gpio_write_index_mode(void *opaque, 
> hwaddr offset,
>          reg_value = update_value_control_source(set, set->data_value,
>                                                  reg_value);
>          set->data_read = reg_value;
> -        aspeed_gpio_update(s, set, reg_value);
> +        aspeed_gpio_update(s, set, reg_value, false);
>          return;
>      case gpio_reg_idx_direction:
>          reg_value = set->direction;
> @@ -753,7 +755,7 @@ static void aspeed_gpio_write_index_mode(void *opaque, 
> hwaddr offset,
>              __func__, offset, data, reg_idx_type);
>          return;
>      }
> -    aspeed_gpio_update(s, set, set->data_value);
> +    aspeed_gpio_update(s, set, set->data_value, false);
>      return;
>  }
>
> @@ -799,7 +801,7 @@ static void aspeed_gpio_write(void *opaque, hwaddr 
> offset, uint64_t data,
>          data &= props->output;
>          data = update_value_control_source(set, set->data_value, data);
>          set->data_read = data;
> -        aspeed_gpio_update(s, set, data);
> +        aspeed_gpio_update(s, set, data, false);
>          return;
>      case gpio_reg_direction:
>          /*
> @@ -875,7 +877,7 @@ static void aspeed_gpio_write(void *opaque, hwaddr 
> offset, uint64_t data,
>                        PRIx64"\n", __func__, offset);
>          return;
>      }
> -    aspeed_gpio_update(s, set, set->data_value);
> +    aspeed_gpio_update(s, set, set->data_value, false);
>      return;
>  }
>
> --
> 2.36.1
>