Date: Tue, 30 Nov 2021 13:32:17 +0100 From: Stanislaw Adaszewski <s.adaszewski@gmail.com> To: Warner Losh <imp@bsdimp.com> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: TPM2 Support in bootloader / kernel in order to retrieve GELI passphrase Message-ID: <CADxsEsTDfzJS0e0M8Cqyd_P%2BjRwiU5NJg8VOEd0mgOV=kLuqgQ@mail.gmail.com> In-Reply-To: <CADxsEsQ0hrPSfEXE4Dt28AWt9mWHu-qieYo0jpnMie=VeRZHnA@mail.gmail.com> References: <CADxsEsRbt6xj1TOHVMMC3jhT%2BCfqZqX479JvdNyM31eAQh1%2BtA@mail.gmail.com> <CANCZdfphx9ZwL4j1deR9LLMBTatqVH%2B_PtkGp8ReQtWzp6T24Q@mail.gmail.com> <CADxsEsQKWrt5w%2B-Xo11QnDb_z5j%2BXkxC5q09%2BZnbMX1VLwAwYA@mail.gmail.com> <CADxsEsSiWF2W9axytK_QXyS-_rEABuOoCJCAku19kF%2B-GxAmBw@mail.gmail.com> <CADxsEsQ0hrPSfEXE4Dt28AWt9mWHu-qieYo0jpnMie=VeRZHnA@mail.gmail.com>
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I have submitted a PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D= 260138 On Mon, 29 Nov 2021 at 17:12, Stanislaw Adaszewski <s.adaszewski@gmail.com> wrote: > > Made a handful of other changes: > > 1) Prevent interaction (exit) if autoboot fails (stand/common/interp.c) > > 2) Wipe env vars and GELI keys before panic()'ing the kernel in case the > passphrase marker does not match - probably does not really help much > but I guess the idea to narrow the time window any secrets are in memory > a good rule of thumb, > > 3) Call Tpm2NvReadLock() right after retrieving the passphrase from TPM > in the bootloader. Therefore, it cannot be read again from the TPM until = the > next restart. Thus, it remains unavailable for example in the booted OS. > > On Sun, 28 Nov 2021 at 12:19, Stanislaw Adaszewski > <s.adaszewski@gmail.com> wrote: > > > > I have made further modifications: > > > > 1) As per suggestion - use SYSINIT() and EVENTHANDLER_REGISTER() > > in a separate file (tpm2cpm.c) rather than hardcoding in init_main.c. > > > > 2) Make the functionality configurable / optional both for the kernel > > (option TPM2_PASSPHRASE) and the bootloader > > (WITH_LOADER_TPM2_PASSPHRASE). The one for the bootloader > > is enabled by default and the one for the kernel is enabled by default > > in the GENERIC amd64 kernel. > > > > Best regards, > > > > -- > > Stanislaw > > > > On Sat, 27 Nov 2021 at 21:35, Stanislaw Adaszewski > > <s.adaszewski@gmail.com> wrote: > > > > > > Hi Warner, > > > > > > > > > Thanks a lot for the quick reaction - that helps. Accordingly, > > > I have taken several actions (below). If you have any more tips > > > how to push this forward, please let me know. Like I don't know > > > is there a person formally responsible for this kind of > > > contributions? Let's say when you are happy with the general > > > architecture of the solution and the quality of the code > > > (it still requires some polishing) - is that enough to pull > > > the changes into the codebase? How does that work? > > > Thank you in advance! > > > > > > > > > 1. I have rebased my changeset on top of the tip of the > > > FreeBSD's main branch [1] > > > > > > > > > 2. I have changed the /.passphrase_marker convention to hold > > > (instead of the passphrase) a human-readable lower-case digest > > > of SHA256(salt | passphrase) where salt is a new (optional) > > > parameter which can be passed using another EFI variable: > > > KernGeomEliPassphraseFromTpm2Salt. > > > > > > I think it is more for the peace of mind than anything else, > > > as anyone having access to /.passphrase_marker would normally > > > have to be the root user. Nevertheless it is perhaps nicer not > > > to keep raw passphrases laying around in files. > > > > > > We still pass the passphrase to the kernel via a kernel variable > > > kern.geom.eli.passphrase.from_tpm2.passphrase which is unset > > > before the system becomes interactive. I could be easily > > > passing the hash computed by the bootloader instead - what do > > > you think? > > > > > > One thing to keep in mind is that another kernel variable - > > > kern.geom.eli.passphrase has been passed around like this > > > as well and it is being unset precisely in init_main.c. > > > > > > But even more importantly, one has to keep in mind that > > > geli_export_key_buffer() passes all GELI keys to the kernel > > > anyway, so access to the encrypted drives is already possible > > > by that alone. Not to mention that the root user can simply > > > read the driver's master key with a simple geli show. > > > > > > > > > 3) As an explanation, also to @Ka Ho Ng, the /.passphrase_marker > > > serves only as a tag to allow to reliably pair a boot filesystem > > > to the keyphrase retrieved from the TPM. If we were to just > > > retrieve the passphrase and pass it to any boot environment then > > > one would simply boot another OS with another root password and > > > could read all our secrets. The same goes for the root filesystem > > > that is mounted in turn by the kernel. If one would for example > > > remove the boot drive - that would cause the kernel to drop out > > > to interactive mountfrom> prompt and then one could for example > > > boot from another drive. That is unacceptable. Kernel is by default > > > very "boot happy" - it tries really hard to boot SOMETHING rather > > > than accept a strict specification of what is allowed to boot. > > > > > > > > > 4) The code is fully functional and I have tested it quite a bit > > > on a Zotac CI622 mini-PC with the latest BIOS update which enables > > > the TPM functionality on the Intel 10110U process in there. If you > > > have a TPM-capable BIOS and CPU, I would encourage you to try, like > > > this you will understand better how it works and that the design is > > > necessary like it is with the /.passphrase_marker. I am not 100% sure > > > if there are no ways left to fool the system to boot or run some > > > arbitrary code. Such attacks would generally consist of causing some > > > kind of errors on the "trusted" UFS-on-GELI or ZFS-on-GELI systems an= d > > > making the system drop out into some kind of interactive prompts. I > > > hope that does not happen. For example if one removes the drive durin= g init. > > > I would certainly expect that no process drops out to interactive pro= mpts > > > on a system with a root password set bit this kind of scares is > > > a tradeoff of not using full VERIEXEC. It is however very convenient > > > to say - just trust everything on the XXX-on-GELI since this was encr= ypted > > > and therefore tampering-proof. More tests are necessary but also - VE= RIEXEC > > > can be enabled in addition to that to ensure that such weird scenario= s do not > > > happen. > > > > > > > > > 5) @Ka Ho Ng what you said is taking place clearly, the code relies o= n a PCR > > > policy of user's choice and uses that to read the passphrase. > > > > > > > > > 6) Regarding ZFS encryption I am not sure if that is supported in the= EFI > > > bootloader - at first glance I would say that it isn't. With that sai= d, > > > the code can be used to further modify the loader to read any kind of > > > values stored in the TPM and put them in kernel variables for later u= se > > > in the boot process. Just a big WARNING, kenv seems to let even luser= s to read > > > the variables. So whatever one would do with them, it would have to b= e done > > > quickly and the variables would need to be discarded before letting > > > the lusers log in. > > > > > > > > > [1] https://github.com/sadaszewski/freebsd-src/compare/main...main-ch= erry-pick-tpm-support-in-stand > > > > > > > > > Kind regards, > > > > > > -- > > > Stanislaw > > > > > > On Sat, 27 Nov 2021 at 18:00, Warner Losh <imp@bsdimp.com> wrote: > > > > > > > > > > > > > > > > On Sat, Nov 27, 2021, 7:36 AM Stanislaw Adaszewski <s.adaszewski@gm= ail.com> wrote: > > > >> > > > >> Dear All, > > > >> > > > >> Could you please guide me so that we can together integrate > > > >> the following piece of work into the FreeBSD base system? > > > >> Thank you for your time and consideration. > > > > > > > > > > > > See below for some advice. > > > > > > > >> I have created the following bundle of work [1]. The referenced > > > >> patch implements on top of releng/13.0, the support for TPM2 > > > >> in the EFI bootloader and in the kernel in order to allow for > > > >> storage and retrievel of a GELI passphrase in a TPM2 module, > > > >> secured with a PCR policy. > > > >> > > > >> The way the bootloader behavior is modified is the following: > > > >> > > > >> 1) before calling efipart_inithandles() an attempt to retrieve the > > > >> passphrase from a TPM2 module might be performed - > > > >> how this is achieved is described later on. > > > >> 2) if a passphrase is indeed retrieved, then after determining > > > >> currdev, the currdev is checked for the presence of a > > > >> /.passphrase_marker file which must contain the same passphrase > > > >> as retrieved from the TPM. This is supposed to ensure that we > > > >> do not end up booting an environment not on the device we just > > > >> unlocked with the passphrase. > > > >> 3a) If all is go, the autoboot_delay is set to -1 in order to prev= ent > > > >> any interaction and continue the boot process of the "safe" enviro= nment, > > > >> a 'kern.geom.eli.passphrase.from_tpm2.passphrase' variable is set > > > >> to the passphrase from TPM in order for kernel use later, as well = as a > > > >> kern.geom.eli.passphrase.from_tpm2.was_retrieved'=3D'1' variable. > > > >> 3b) If the passphrase marker does not match, the bootloader cleans= up > > > >> GELI keys, the TPM passphrase and kern.geom.eli.passphrase and exi= ts. > > > > > > > > > > > > I worry about information disclosure having the pass phrase availab= le on the running system with this setup. Can you explain why that design p= oint was selected? Usually there is something signed with a private key tha= t the public key can be used to verify instead of a direct comparison like = this to prevent disclosure of key material. I've not looked at the code yet= , so it may already do this... > > > > > > > >> The way the kernel behavior is modified is the following: > > > >> > > > >> 1) In init_main.c, after vfs_mountroot() a check is added > > > >> 2a) If kern.geom.eli.passphrase.from_tpm2.was_retrieved is not > > > >> set to 1, then we do nothing and continue the boot process > > > >> 2b) If the was_retrieved variable is set to '1' then we check for = the > > > >> same passphrase marker as the bootloader, its content compared > > > >> against the 'kern.geom.eli.passphrase.from_tpm2.passphrase' > > > >> variable. > > > >> 3a) If all is go, the passphrase variable is unset and the boot pr= ocess > > > >> continues, > > > >> 3c) If the passphrase marker does not match, we panic. > > > > > > > > > > > > I'm sure that main_init should not know about geom or geli. This is= usually done with a handler of some sort so the mountroot code can just ca= ll the generic handler. Can your code be restructured such that this is pos= sible? The reason I ask is that ZFS supports encryption too and it would b= e nice to use that instead of geli. > > > > > > > >> The configuration of the bootloader for this procedure looks the f= ollowing: > > > >> > > > >> 1) We set an efivar KernGeomEliPassphraseFromTpm2NvIndex > > > >> to contain the TPM2 NV Index we store our passphrase in, e.g. 0x10= 00001 > > > >> 2) We set an efivar KernGeomEfiPassphraseFromTpm2PolicyPcr > > > >> to contain the PCR policy used to secure the passphrase, e.g. > > > >> sha256:0,2,4,7 > > > >> 3a) If both are set, the bootloader will attempt to retrieve the p= assphrase > > > >> and behave in the modified way described above > > > >> 3b) Otherwise, it behaves as the vanilla version and will ask for = GELI > > > >> passphrases if necessary > > > >> > > > >> The configuration of the TPM and the passphrase marker looks the f= ollowing: > > > >> > > > >> 1) echo -n "passphrase" >/.passphrase_marker > > > >> 2) chmod 600 /.passphrase_marker > > > >> 3) tpm2_createpolicy -L policy.digest --policy-pcr -l sha256:0,2,4= ,7 > > > >> 4) tpm2_nvdefine -Q 0x1000001 -s `wc -c /.passphrase_marker` -L > > > >> policy.digest -A "policyread|policywrite" > > > >> 5) tpm2_nvwrite -Q 0x1000001 -i /.passphrase_marker -P pcr:sha256:= 0,2,4,7 > > > >> > > > >> [1] https://github.com/sadaszewski/freebsd-src/compare/releng/13.0= ...tpm-support-in-stand > > > > > > > > > > > > This sounds cool. Any chance you can rebase this to the tip of the = main branch? All code goes into FreeBSD that way and 13.0 is about a year o= ld already. > > > > > > > > Thanks for sharing this. Despite some reservations expressed above,= I think this has potential to be quite cool. > > > > > > > > Warner > > > > > > > >> > > > >> Kind regards,
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