On Mon, Sep 15, 2025 at 10:05 PM Jarkko Sakkinen <jar...@kernel.org> wrote:
>
> On Mon, Sep 15, 2025 at 05:08:29PM -0400, Denis Aleksandrov wrote:
> > Reads on tpm/tpm0/ppi/*operations can become very long on
> > misconfigured systems. Reading the TPM is a blocking operation,
> > thus a user could effectively trigger a DOS.
> >
> > Resolve this by caching the results and avoiding the blocking
> > operations after the first read.
> >
> > Reported-by: Jan Stancek <jstan...@redhat.com>
> > Signed-off-by: Denis Aleksandrov <dalek...@redhat.com>
> > Suggested-by: Jarkko Sakkinen <jar...@kernel.org>
> > Reviewed-by: Paul Menzel <pmen...@molgen.mpg.de>
> > Reviewed-by: Jarkko Sakkinen <jar...@kernel.org>
> > ---
> >
> > Changes in v5:
> > - Unlocks the tpm_ppi_lock if cache_ppi_operations() returns and
> > error.
> >
> > drivers/char/tpm/tpm_ppi.c | 89 ++++++++++++++++++++++++++++----------
> > 1 file changed, 66 insertions(+), 23 deletions(-)
> >
> > diff --git a/drivers/char/tpm/tpm_ppi.c b/drivers/char/tpm/tpm_ppi.c
> > index d53fce1c9d6f..47655407fea5 100644
> > --- a/drivers/char/tpm/tpm_ppi.c
> > +++ b/drivers/char/tpm/tpm_ppi.c
> > @@ -33,6 +33,20 @@ static const guid_t tpm_ppi_guid =
> > GUID_INIT(0x3DDDFAA6, 0x361B, 0x4EB4,
> > 0xA4, 0x24, 0x8D, 0x10, 0x08, 0x9D, 0x16, 0x53);
> >
> > +static const char * const tpm_ppi_info[] = {
> > + "Not implemented",
> > + "BIOS only",
> > + "Blocked for OS by system firmware",
> > + "User required",
> > + "User not required",
> > +};
> > +
> > +/* A spinlock to protect access to the cache from concurrent reads */
> > +static DEFINE_SPINLOCK(tpm_ppi_lock);
> > +
> > +static u32 ppi_operations_cache[PPI_VS_REQ_END + 1];
> > +static bool ppi_cache_populated;
> > +
> > static bool tpm_ppi_req_has_parameter(u64 req)
> > {
> > return req == 23;
> > @@ -277,8 +291,7 @@ static ssize_t tpm_show_ppi_response(struct device *dev,
> > return status;
> > }
> >
> > -static ssize_t show_ppi_operations(acpi_handle dev_handle, char *buf, u32
> > start,
> > - u32 end)
> > +static ssize_t cache_ppi_operations(acpi_handle dev_handle, char *buf)
> > {
> > int i;
> > u32 ret;
> > @@ -286,34 +299,22 @@ static ssize_t show_ppi_operations(acpi_handle
> > dev_handle, char *buf, u32 start,
> > union acpi_object *obj, tmp;
> > union acpi_object argv = ACPI_INIT_DSM_ARGV4(1, &tmp);
> >
> > - static char *info[] = {
> > - "Not implemented",
> > - "BIOS only",
> > - "Blocked for OS by BIOS",
> > - "User required",
> > - "User not required",
> > - };
> > -
> > if (!acpi_check_dsm(dev_handle, &tpm_ppi_guid, TPM_PPI_REVISION_ID_1,
> > 1 << TPM_PPI_FN_GETOPR))
> > return -EPERM;
> >
> > tmp.integer.type = ACPI_TYPE_INTEGER;
> > - for (i = start; i <= end; i++) {
> > + for (i = 0; i <= PPI_VS_REQ_END; i++) {
> > tmp.integer.value = i;
> > obj = tpm_eval_dsm(dev_handle, TPM_PPI_FN_GETOPR,
> > ACPI_TYPE_INTEGER, &argv,
> > TPM_PPI_REVISION_ID_1);
> > - if (!obj) {
> > + if (!obj)
> > return -ENOMEM;
> > - } else {
> > - ret = obj->integer.value;
> > - ACPI_FREE(obj);
> > - }
> >
> > - if (ret > 0 && ret < ARRAY_SIZE(info))
> > - len += sysfs_emit_at(buf, len, "%d %d: %s\n",
> > - i, ret, info[ret]);
> > + ret = obj->integer.value;
> > + ppi_operations_cache[i] = ret;
> > + ACPI_FREE(obj);
> > }
> >
> > return len;
> > @@ -324,9 +325,30 @@ static ssize_t tpm_show_ppi_tcg_operations(struct
> > device *dev,
> > char *buf)
> > {
> > struct tpm_chip *chip = to_tpm_chip(dev);
> > + ssize_t len = 0;
> > + u32 ret;
> > + int i;
> > +
> > + spin_lock(&tpm_ppi_lock);
> > + if (!ppi_cache_populated) {
> > + len = cache_ppi_operations(chip->acpi_dev_handle, buf);
> > + if (len < 0) {
> > + spin_unlock(&tpm_ppi_lock);
> > + return len;
> > + }
> >
> > - return show_ppi_operations(chip->acpi_dev_handle, buf, 0,
> > - PPI_TPM_REQ_MAX);
> > + ppi_cache_populated = true;
> > + }
> > +
> > + for (i = 0; i <= PPI_TPM_REQ_MAX; i++) {
> > + ret = ppi_operations_cache[i];
> > + if (ret >= 0 && ret < ARRAY_SIZE(tpm_ppi_info))
> > + len += sysfs_emit_at(buf, len, "%d %d: %s\n",
> > + i, ret,
> > tpm_ppi_info[ret]);
> > + }
> > + spin_unlock(&tpm_ppi_lock);
> > +
> > + return len;
> > }
> >
> > static ssize_t tpm_show_ppi_vs_operations(struct device *dev,
> > @@ -334,9 +356,30 @@ static ssize_t tpm_show_ppi_vs_operations(struct
> > device *dev,
> > char *buf)
> > {
> > struct tpm_chip *chip = to_tpm_chip(dev);
> > + ssize_t len = 0;
> > + u32 ret;
> > + int i;
> >
> > - return show_ppi_operations(chip->acpi_dev_handle, buf,
> > PPI_VS_REQ_START,
> > - PPI_VS_REQ_END);
> > + spin_lock(&tpm_ppi_lock);
> > + if (!ppi_cache_populated) {
> > + len = cache_ppi_operations(chip->acpi_dev_handle, buf);
> > + if (len < 0) {
> > + spin_unlock(&tpm_ppi_lock);
> > + return len;
> > + }
> > +
> > + ppi_cache_populated = true;
> > + }
> > +
> > + for (i = PPI_VS_REQ_START; i <= PPI_VS_REQ_END; i++) {
> > + ret = ppi_operations_cache[i];
> > + if (ret >= 0 && ret < ARRAY_SIZE(tpm_ppi_info))
> > + len += sysfs_emit_at(buf, len, "%d %d: %s\n",
> > + i, ret,
> > tpm_ppi_info[ret]);
> > + }
> > + spin_unlock(&tpm_ppi_lock);
> > +
> > + return len;
> > }
> >
> > static DEVICE_ATTR(version, S_IRUGO, tpm_show_ppi_version, NULL);
> > --
> > 2.48.1
> >
>
> Thank you. 'next' has been updated.
Great, thank you for your patience. I'm excited to see the patch upstream :)
>
> BR, Jarkko
>
Cheers,
Denis
