On Wed, Jun 25, 2025 at 07:43:07PM +0300, Jarkko Sakkinen wrote:
On Sun, Jun 22, 2025 at 09:52:58PM +0100, Jonathan McDowell wrote:
On Fri, Jun 20, 2025 at 06:08:31PM +0000, Orlov, Ivan wrote:
> The current implementation of timeout detection works in the following
> way:
>
> 1. Read completion status. If completed, return the data
> 2. Sleep for some time (usleep_range)
> 3. Check for timeout using current jiffies value. Return an error if
> timed out
> 4. Goto 1
>
> usleep_range doesn't guarantee it's always going to wake up strictly in
> (min, max) range, so such a situation is possible:
>
> 1. Driver reads completion status. No completion yet
> 2. Process sleeps indefinitely. In the meantime, TPM responds
> 3. We check for timeout without checking for the completion again.
> Result is lost.
>
> Such a situation also happens for the guest VMs: if vCPU goes to sleep
> and doesn't get scheduled for some time, the guest TPM driver will
> timeout instantly after waking up without checking for the completion
> (which may already be in place).
>
> Perform the completion check once again after exiting the busy loop in
> order to give the device the last chance to send us some data.
>
> Since now we check for completion in two places, extract this check into
> a separate function.
>
> Signed-off-by: Ivan Orlov <ior...@amazon.com>
> ---
> V1 -> V2:
> - Exclude the jiffies -> ktime change from the patch
> - Instead of recording the time before checking for completion, check
> for completion once again after leaving the loop
>
> drivers/char/tpm/tpm-interface.c | 17 +++++++++++++++--
> 1 file changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm-interface.c
b/drivers/char/tpm/tpm-interface.c
> index 8d7e4da6ed53..6960ee2798e1 100644
> --- a/drivers/char/tpm/tpm-interface.c
> +++ b/drivers/char/tpm/tpm-interface.c
> @@ -82,6 +82,13 @@ static bool tpm_chip_req_canceled(struct tpm_chip *chip,
u8 status)
> return chip->ops->req_canceled(chip, status);
> }
>
> +static bool tpm_transmit_completed(struct tpm_chip *chip)
> +{
> + u8 status_masked = tpm_chip_status(chip) & chip->ops->req_complete_mask;
> +
> + return status_masked == chip->ops->req_complete_val;
> +}
> +
> static ssize_t tpm_try_transmit(struct tpm_chip *chip, void *buf, size_t
bufsiz)
> {
> struct tpm_header *header = buf;
> @@ -129,8 +136,7 @@ static ssize_t tpm_try_transmit(struct tpm_chip *chip,
void *buf, size_t bufsiz)
> stop = jiffies + tpm_calc_ordinal_duration(chip, ordinal);
> do {
> u8 status = tpm_chip_status(chip);
> - if ((status & chip->ops->req_complete_mask) ==
> - chip->ops->req_complete_val)
> + if (tpm_transmit_completed(chip))
> goto out_recv;
The only thing I'd point out here is we end up doing a double status read
one after the other (once here, once in tpm_transmit_completed), and I'm
pretty sure I've seen instances where that caused a problem.
It would be easy to to prevent at least double reads after completion
e.g., in tpm_chip_status():
Or just take the simple approach and make the check after the while
loop:
if ((tpm_chip_status(chip) & chip->ops->req_complete_mask) ==
chip->ops->req_complete_val)
goto out_recv;
There might be potential for a longer term cleanup using chip->status to
cache things, but I'm little concerned that's going to open paths where
we might not correctly update it, so I think it should be a separate
piece.
(I'm motivated by the fact we've started to see the "Operation Canceled"
error and I'd like us to close on the best way to fix it. :) )
J.
--
I am afraid of the dark.
