[PATCH] drm/amdgpu/pm: Fix potential Spectre v1

[Alex Deucher]

On Mon, Jul 23, 2018 at 12:32 PM, Gustavo A. R. Silva
<gustavo at embeddedor.com> wrote:
> idx can be indirectly controlled by user-space, hence leading to a
> potential exploitation of the Spectre variant 1 vulnerability.
>
> This issue was detected with the help of Smatch:
>
> drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c:408 amdgpu_set_pp_force_state()
> warn: potential spectre issue 'data.states'
>
> Fix this by sanitizing idx before using it to index data.states

Is this actually necessary?  We already check that idx is valid a few
lines before:
        if (ret || idx >= ARRAY_SIZE(data.states)) {
                        count = -EINVAL;
                        goto fail;
                }

Alex


>
> Notice that given that speculation windows are large, the policy is
> to kill the speculation on the first load and not worry if it can be
> completed with a dependent load/store [1].
>
> [1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2
>
> Cc: stable at vger.kernel.org
> Signed-off-by: Gustavo A. R. Silva <gustavo at embeddedor.com>
> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c
> index 15a1192..a446c7c 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c
> @@ -31,7 +31,7 @@
>  #include <linux/power_supply.h>
>  #include <linux/hwmon.h>
>  #include <linux/hwmon-sysfs.h>
> -
> +#include <linux/nospec.h>
>
>  static int amdgpu_debugfs_pm_init(struct amdgpu_device *adev);
>
> @@ -403,6 +403,7 @@ static ssize_t amdgpu_set_pp_force_state(struct device *dev,
>                         count = -EINVAL;
>                         goto fail;
>                 }
> +               idx = array_index_nospec(idx, ARRAY_SIZE(data.states));
>
>                 amdgpu_dpm_get_pp_num_states(adev, &data);
>                 state = data.states[idx];
> --
> 2.7.4
>
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