Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> writes: >> > > On the flip side when it really is a trusted mounter, and it is in a >> > > configuration that IMA has a reasonable expectation of seeing all of >> > > the changes it would be nice if we can say please trust this mount. >> > >> > IMA has no way of detecting file change. This was one of the reasons >> > for the original patch set's not using the cached IMA results. >> > >> > Even in the case of a trusted mounter and not using IMA cached >> > results, there are no guarantees that the data read to calculate the >> > file hash, will be the same as what is subsequently read. In some >> > environments this might be an acceptable risk, while in others not. >> >> So for the cases where it's not, there should be an IMA option or policy >> to say any SB_I_IMA_UNVERIFIABLE_SIGNATURES mounts should be not >> trusted, with the default being both SB_I_IMA_UNVERIFIABLE_SIGNATURES and >> SB_I_UNTRUSTED_MOUNTER must be true to not trust, right? > > Right. To summarize, we've identified 3 scenarios: > 1. Fail signature verification on unprivileged non-init root mounted > file systems. > > flags: SB_I_IMA_UNVERIFIABLE_SIGNATURES and SB_I_UNTRUSTED_MOUNTER > (always enabled) > > 2. Permit signature verification on privileged file system mounts in a > secure system environment. Willing to accept the risk. Does not rely > on cached integrity results, but forces re-evaluation. > > flags: SB_I_IMA_UNVERIFIABLE_SIGNATURES, not SB_I_UNTRUSTED_MOUNTER or > IMA_FAIL_UNVERIFICABLE_SIGNATURES (default behavior) > > 3. Fail signature verification also on privileged file system mounts. > Fail safe, unwilling to accept the risk. > > flags: > SB_I_IMA_UNVERIFIABLE_SIGNATURES and IMA_FAIL_UNVERIFIABLE_SIGNATURES > > Enabled by specifying "ima_policy=unverifiable_sigs" on the boot > command line. There is another scenaro. 4. Permit signature verification on out of kernel but otherwise fully capable and trusted filesystems. Fuse has a mode where it appears to be cache coherent, and guaranteed to be local. AKA when fuse block is used and FUSE_WRITEBACK_CACHE is set. That configuratioin plus the the allow_other mount option appear to signal a fuse mount that can be reasonably be trusted as much as an in-kernel block based filesystem. That is a mode someone might use to mount exFat or ntfs-3g. As all writes come from the kernel, and it is safe to have a write-back cache I believe ima can reasonably verify signatures. There may be something technical like the need to verify i_version in this case, but for purposes of argument let's say fuse has implemented all of the necessary technical details. In that case we have a case where it is reasonable to say that SB_I_IMA_UNVERIFIABLE_SIGNATURES would be incorrect to set on a fuse filesystem. Mimi do you agree or am I missing something? Eric
