Re: [PATCH 4/4] habanalabs: never copy_from_user inside spinlock

[Oded Gabbay <oded.gabbay@gmail.com>]

On Fri, Aug 20, 2021 at 6:09 PM Oded Gabbay <ogabbay@kernel.org> wrote:
>
> copy_from_user might sleep so we can never call it when we have
> a spinlock.
>
> Moreover, it is not necessary in waiting for user interrupt, because
> if multiple threads will call this function on the same interrupt,
> each one will have it's own fence object inside the driver. The
> user address might be the same, but it doesn't really matter to us,
> as we only read from it.
>
> Signed-off-by: Oded Gabbay <ogabbay@kernel.org>
> ---
>  .../habanalabs/common/command_submission.c    | 35 +++++++------------
>  1 file changed, 12 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/misc/habanalabs/common/command_submission.c b/drivers/misc/habanalabs/common/command_submission.c
> index a97bb27ebb90..7b0516cf808b 100644
> --- a/drivers/misc/habanalabs/common/command_submission.c
> +++ b/drivers/misc/habanalabs/common/command_submission.c
> @@ -2740,14 +2740,10 @@ static int _hl_interrupt_wait_ioctl(struct hl_device *hdev, struct hl_ctx *ctx,
>         else
>                 interrupt = &hdev->user_interrupt[interrupt_offset];
>
> -       spin_lock_irqsave(&interrupt->wait_list_lock, flags);
> -
> -       if (copy_from_user(&completion_value, u64_to_user_ptr(user_address),
> -                                                                       4)) {
> -               dev_err(hdev->dev,
> -                       "Failed to copy completion value from user\n");
> +       if (copy_from_user(&completion_value, u64_to_user_ptr(user_address), 4)) {
> +               dev_err(hdev->dev, "Failed to copy completion value from user\n");
>                 rc = -EFAULT;
> -               goto unlock_and_free_fence;
> +               goto free_fence;
>         }
>
>         if (completion_value >= target_value)
> @@ -2756,42 +2752,35 @@ static int _hl_interrupt_wait_ioctl(struct hl_device *hdev, struct hl_ctx *ctx,
>                 *status = CS_WAIT_STATUS_BUSY;
>
>         if (!timeout_us || (*status == CS_WAIT_STATUS_COMPLETED))
> -               goto unlock_and_free_fence;
> +               goto free_fence;
>
>         /* Add pending user interrupt to relevant list for the interrupt
>          * handler to monitor
>          */
> +       spin_lock_irqsave(&interrupt->wait_list_lock, flags);
>         list_add_tail(&pend->wait_list_node, &interrupt->wait_list_head);
>         spin_unlock_irqrestore(&interrupt->wait_list_lock, flags);
>
>  wait_again:
>         /* Wait for interrupt handler to signal completion */
> -       completion_rc =
> -               wait_for_completion_interruptible_timeout(
> -                               &pend->fence.completion, timeout);
> +       completion_rc = wait_for_completion_interruptible_timeout(&pend->fence.completion,
> +                                                                               timeout);
>
>         /* If timeout did not expire we need to perform the comparison.
>          * If comparison fails, keep waiting until timeout expires
>          */
>         if (completion_rc > 0) {
> -               spin_lock_irqsave(&interrupt->wait_list_lock, flags);
> -
> -               if (copy_from_user(&completion_value,
> -                               u64_to_user_ptr(user_address), 4)) {
> -
> -                       spin_unlock_irqrestore(&interrupt->wait_list_lock, flags);
> -
> -                       dev_err(hdev->dev,
> -                               "Failed to copy completion value from user\n");
> +               if (copy_from_user(&completion_value, u64_to_user_ptr(user_address), 4)) {
> +                       dev_err(hdev->dev, "Failed to copy completion value from user\n");
>                         rc = -EFAULT;
>
>                         goto remove_pending_user_interrupt;
>                 }
>
>                 if (completion_value >= target_value) {
> -                       spin_unlock_irqrestore(&interrupt->wait_list_lock, flags);
>                         *status = CS_WAIT_STATUS_COMPLETED;
>                 } else {
> +                       spin_lock_irqsave(&interrupt->wait_list_lock, flags);
>                         reinit_completion(&pend->fence.completion);
>                         timeout = completion_rc;
>
> @@ -2811,9 +2800,9 @@ static int _hl_interrupt_wait_ioctl(struct hl_device *hdev, struct hl_ctx *ctx,
>  remove_pending_user_interrupt:
>         spin_lock_irqsave(&interrupt->wait_list_lock, flags);
>         list_del(&pend->wait_list_node);
> -
> -unlock_and_free_fence:
>         spin_unlock_irqrestore(&interrupt->wait_list_lock, flags);
> +
> +free_fence:
>         kfree(pend);
>         hl_ctx_put(ctx);
>
> --
> 2.17.1
>

Hi Greg,
Thanks for pointing this issue out. It slipped my CR (my bad).
I believe this fixes the problem and I've gone over the entire driver
and didn't see any other occurrence of this bug.

Oded