On Aug 1, 2016 4:30 PM, "Jason Cooper" wrote: > > On Mon, Aug 01, 2016 at 04:13:50PM -0700, Andy Lutomirski wrote: > > On Aug 1, 2016 4:04 PM, "Jason Cooper" wrote: > > > > > > On Mon, Aug 01, 2016 at 03:36:51PM -0700, Andy Lutomirski wrote: > > > > On Mon, Aug 1, 2016 at 3:21 PM, Mimi Zohar > > wrote: > > > > > On Mo, 2016-08-01 at 10:59 -0700, Andy Lutomirski wrote: > > > > > > > > > >> Mimi, I'm curious: I don't fully understand what is covered by IMA > > > > >> policy. How does the IMA kernel_read_file stuff deal with symlinks? > > > > >> For example, if I symlink /lib/firmware/iwlwifi-8265-21.ucode to > > > > >> /home/badguy/iwlwifi-8265-21.ucode, what happens? What if I symlink > > > > >> /lib/firmware/iwlwifi-8265-21.ucode to /home/badguy/something_else? > > > > >> Or even /lib/modules/kernel/foo/bar.ko to /home/badguy/evil.ko? The > > > > >> interesting case is where the "badguy" user is duly authorized to > > > > >> write to /home/badguy and holds whatever keys may be needed. > > > > > > > > > > Lets step back a second. In order for a key to be added to the IMA > > > > > keyring, the key must be signed by a key on the builtin keyring. The > > > > > key on the builtin keyring can be compiled into the kernel image or > > > > > added post build using Mehmet Kayaalp's patches. > > > > > > > > > > True, any key on the IMA keyring could be used to verify file > > signatures > > > > > (in IMA terminology appraise the file's integrity). The enumeration > > is > > > > > a first step to making sure that only properly signed code is read by > > > > > the kernel. The next step requires finer grain key management. In > > > > > general, pathname based policies are not a good idea. Whatever method > > > > > is defined, it should not be limited to just firmware or files read by > > > > > the kernel, but to all files. > > > > > > > > > > > > > Unless I'm mistaken (which is quite possible), IMA is primarily > > > > intended to appraise the content of POSIX filesystems. So, if IMA is > > > > in use, then doing: > > > > > > > > $ cat /foo/bar > > > > > > > > should only succeed if /foo/bar is signed according to loaded policy. > > > > It's the system administrator's decision what filesystem is actually > > > > mounted at /foo, and root can presumably mess around with application > > > > expectations by, say, bind-mounting something over /foo. > > > > > > > > Modules and firmware are special: even root should not be able to > > > > avoid the full signature policy. This means that, for example: > > > > > > > > # mount --bind /evil /lib/firmware > > > > > > > > should not result in violating policy. So the pathname should not be > > > > used as such. However, firmware is a bit special in that the driver > > > > chooses the pathname to request, and it really does uniquely identify > > > > the intended firmware. So, when a driver asks for: > > > > > > > > "iwlwifi-whatever.ucode" > > > > > > > > and the driver core tries to read "/lib/firmware/iwlwifi-whatever.ucode" > > > > > > > > it's entirely possible that we'll follow a symlink and end up > > > > elsewhere (Fedora, for example, does exactly this), but the file > > > > that's loaded should be appraised (or verified using a non-IMA means, > > > > etc.) to verify that whatever blob gets found is actually signed by > > > > the holder of an authorized key for the purpose of being used as > > > > "iwlwifi-whatever.ucode". > > > > > > Assuming Andy's lightweight signature scheme, it would probably be best > > > to do a lookup based on the sha256 hash of the file. Then pathnames > > > don't matter, and bad files don't even get to the signature checking > > > code. > > > > > > > I'm not sure I understand what you mean. What table would we look the hash > > up in? What are we finding in that table? > > From the other subthread: > > > Then, to verify a signature, the kernel hashes the blob, generates its > > own linux_blob_signed_data, memcmps it to the one that Kyle signed > > (and rejects if they differ *at all*), and then verifies the > > signature. (Do not try to be clever and parse the supplied > > linux_blob_signed_data -- there is a long and storied history of > > equivalent ideas being implemented incorrectly, and I can dig out > > literature references if you really want. Just generate your own and > > memcmp it, which leaves no room for ambiguity.) > > > > So, I'm suggesting that when "the kernel hashes the blob", it use that > hash to locate *which* "Kyle-signed" linux_blob_signed_data it needs to > compare against. That's all, just removing the filename from the > equation. :-) So Kyle would generate a list of signatures indexed by the blob's hash instead of generating things like "iwlwifi-whatever.ucode.sig"? Seems okay. It'll keep the existing hooks working, I think. Of course, we still need to check the "iwlwifi-whatever.ucode" bit to confirm that it matches Kyle's signed data.