On Fri, Jul 10, 2020 at 02:29:55PM -0500, Tyler Hicks wrote: > Require that the TCG_PCR_EVENT2.digests.count value strictly matches the > value of TCG_EfiSpecIdEvent.numberOfAlgorithms in the event field of the > TCG_PCClientPCREvent event log header. Also require that > TCG_EfiSpecIdEvent.numberOfAlgorithms is non-zero. > > The TCG PC Client Platform Firmware Profile Specification section 9.1 > (Family "2.0", Level 00 Revision 1.04) states: > > For each Hash algorithm enumerated in the TCG_PCClientPCREvent entry, > there SHALL be a corresponding digest in all TCG_PCR_EVENT2 structures. > Note: This includes EV_NO_ACTION events which do not extend the PCR. > > Section 9.4.5.1 provides this description of > TCG_EfiSpecIdEvent.numberOfAlgorithms: > > The number of Hash algorithms in the digestSizes field. This field MUST > be set to a value of 0x01 or greater. > > Enforce these restrictions, as required by the above specification, in > order to better identify and ignore invalid sequences of bytes at the > end of an otherwise valid TPM2 event log. Firmware doesn't always have > the means necessary to inform the kernel of the actual event log size so > the kernel's event log parsing code should be stringent when parsing the > event log for resiliency against firmware bugs. This is true, for > example, when firmware passes the event log to the kernel via a reserved > memory region described in device tree. > > POWER and some ARM systems use the "linux,sml-base" and "linux,sml-size" > device tree properties to describe the memory region used to pass the > event log from firmware to the kernel. Unfortunately, the > "linux,sml-size" property describes the size of the entire reserved > memory region rather than the size of the event long within the memory > region and the event log format does not include information describing > the size of the event log. > > tpm_read_log_of(), in drivers/char/tpm/eventlog/of.c, is where the > "linux,sml-size" property is used. At the end of that function, > log->bios_event_log_end is pointing at the end of the reserved memory > region. That's typically 0x10000 bytes offset from "linux,sml-base", > depending on what's defined in the device tree source. > > The firmware event log only fills a portion of those 0x10000 bytes and > the rest of the memory region should be zeroed out by firmware. Even in > the case of a properly zeroed bytes in the remainder of the memory > region, the only thing allowing the kernel's event log parser to detect > the end of the event log is the following conditional in > __calc_tpm2_event_size(): > > if (event_type == 0 && event_field->event_size == 0) > size = 0; > > If that wasn't there, __calc_tpm2_event_size() would think that a 16 > byte sequence of zeroes, following an otherwise valid event log, was > a valid event. > > However, problems can occur if a single bit is set in the offset > corresponding to either the TCG_PCR_EVENT2.eventType or > TCG_PCR_EVENT2.eventSize fields, after the last valid event log entry. > This could confuse the parser into thinking that an additional entry is > present in the event log and exposing this invalid entry to userspace in > the /sys/kernel/security/tpm0/binary_bios_measurements file. Such > problems have been seen if firmware does not fully zero the memory > region upon a warm reboot. > > This patch significantly raises the bar on how difficult it is for > stale/invalid memory to confuse the kernel's event log parser but > there's still, ultimately, a reliance on firmware to properly initialize > the remainder of the memory region reserved for the event log as the > parser cannot be expected to detect a stale but otherwise properly > formatted firmware event log entry. > > Fixes: fd5c78694f3f ("tpm: fix handling of the TPM 2.0 event logs") > Signed-off-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxxxxxxxx> > --- > > * v2 > - Rebase the patch on top of the TPM next branch, commit 786a2aa281f4 > ("Revert commit e918e570415c ("tpm_tis: Remove the HID IFX0102")") > - Expand on the technical reasoning for needing strict event > validation in the commit message > - Improve the inline comment explaining the need for detecting > malformed events > > include/linux/tpm_eventlog.h | 11 +++++++++-- > 1 file changed, 9 insertions(+), 2 deletions(-) > > diff --git a/include/linux/tpm_eventlog.h b/include/linux/tpm_eventlog.h > index 64356b199e94..739ba9a03ec1 100644 > --- a/include/linux/tpm_eventlog.h > +++ b/include/linux/tpm_eventlog.h > @@ -211,9 +211,16 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event, > > efispecid = (struct tcg_efi_specid_event_head *)event_header->event; > > - /* Check if event is malformed. */ > + /* > + * Perform validation of the event in order to identify malformed > + * events. This function may be asked to parse arbitrary byte sequences > + * immediately following a valid event log. The caller expects this > + * function to recognize that the byte sequence is not a valid event > + * and to return an event size of 0. > + */ > if (memcmp(efispecid->signature, TCG_SPECID_SIG, > - sizeof(TCG_SPECID_SIG)) || count > efispecid->num_algs) { > + sizeof(TCG_SPECID_SIG)) || > + !efispecid->num_algs || count != efispecid->num_algs) { > size = 0; > goto out; > } > -- > 2.25.1 > Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx> /Jarkko