First of all: NO system is 100% safe, but using TPM is better than no TPM at all. TPM Chip is just some kind of encrypted storage, that resides on the motherboard of computers that support Trusted Platform Environment, and have BIOSes prepared to handle it.
PCR are registers with specific functions that are handled through the
TPM_Extend operation. They cannot be "set", only extended(new_hash = [old_hash||new_measurement]).
TPM have Static Root of Trust for Measurements (SRTM) and Dynamic Root of Trust for Measurements (DRTM), and the combination of both creates the Secure Environment. This guy explains very well how this is done. It is a chain of trust between fixed and dynamic items.
Back to PCRs, they are platform independent registers, and the most common ones are:
PCR 0 to 3 for the BIOS, ROMS...
PCR 4 - MBR information and stage1
PCR 8 - bootloader information stage2 part1
PCR 9 - bootloader information stage2 part2
PCR 12 - all commandline arguments from menu.lst and those entered in the shell
PCR 13 - all files checked via the checkfile-routine
PCR 14 - all files which are actually loaded (e.g., Linux kernel, initramfs, modules...)
PCR 15 to 23 are not used
Intel based notebooks commonly use the first 16 registers, but it could be extended to other softwares/uses.
While writing information (sealing) to TPM, you can add a Storage Root Key (SRK) that is somehow a "Management key" and is used to add other keys to this storage. As per manpages, using
-z will set
TSS_WELL_KNOWN_SECRET (20 zero bytes).
Use TSS_WELL_KNOWN_SECRET (20 zero bytes) as the SRK password.
You will not be prompted for the SRK password with this option.
So, having this SRK set to the default secret (
TSS_WELL_KNOWN_SECRET) will not be enough to attack someone since TPM can only be unsealed if the current PCRs match those used to seal the data. Also, some of the PCR handling happen at boot time(BIOS) and it's very hard to manipulate those and thus create "fake" PCRs. BIOS is the only place where PCRs are seen as zeros before the rest of the process takes place.
The only FEASABLE attack is the one who aims to MITM communications between BIOS and PCR to zeroes PCR without restarting the machine to put the system on "trusted" state. This attack is known as TPM Reset Attack.
So, given everything we've seen above, it should be very difficult to
fake a trusted boot process, as long as the BIOS takes the first few
measurements. The critical assumption here is that the PCRs cannot be
easily reset without restarting the whole platform that the TPM
resides on. If an attacker is capable of monitoring the measurements
sent to the PCRs by the BIOS (with, for instance a logic analyzer, see
this paper), and capable of zeroing out the PCRs without restarting
the machine, then she could take a platform in any configuration and
put it into a 'trusted' state. So, the difficult part is getting the
TPM to reset without bringing down the whole machine. It is worth
mentioning that we've also looked at interposing memory and other such
things to change the running system after its been measured, but due
to the speed of the busses that memory and hard drives sit on, this is
a tricky endeavor. Attacking a slower bus is much easier.
TPMs typically reside on the Low Pin Count (LPC) bus. The LPC bus
supports a ground driven reset line. This means that when this
particular line on the bus is driven to ground every device on this
bus is supposed to reset. Other devices attached to this bus include
the BIOS, and legacy keyboard and mouse controllers. The video below
demonstrates that driving this line is indeed possible, and fairly
easy to do. Please note that in the video, we are accessing the
computer in question via a remote ssh session. This is because the
keyboard and mouse controller get reset when we drive the reset pin,
but the network card does not. More details of this attack (and
others!) can be seen in my senior honors thesis: A Security Assessment
of Trusted Platform Modules, Dartmouth College Computer Science
Technical Report TR2007-597.
Note that this is an oversimplified version of ALL THINGS that involve Trusted Computing. Please take a look at the Architecture Document of TPMv2 to get more information about all operations that happens between bios, hardware and software during the setup of a trusted environment.
tl;dr: Using the default Storage Root Key(20 zero bytes) is not enough to create an insecure system.