On Wed, Jun 25, 2025 at 07:43:11PM +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():
>
> /*
> * Read the chip status bitmask. After completion, the returned will mask will
> * return value cached at the point of completion up until the next transmit.
> */
> static u8 tpm_chip_status(struct tpm_chip *chip)
> {
> u8 status_masked = chip->status & chip->ops_req_complete_mask;
>
> if (status_masked == chip->ops->req_complete_val)
> return chip->status;
>
> chip->status = tpm_chip_status(chip);
OOPS:
chip->status = chip->ops->status(chip);
Couple of additional constraints:
1. tpm_chip_alloc() should initialize chip->status to
chip->ops->req_complete_mask.
2. tpm_try_transmit() should reset the value to chip->ops->req_complete_mask on
failure paths.
With these constraints there is framework where chip->ops->status() is
only invoked when it actually makes sense i.e., only during the course
of tpm_try_trasmit().
BR, Jarkko
