On Wed, 2018-02-21 at 16:46 -0600, Eric W. Biederman wrote: > 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? This simply sounds like a performance improvement to the second scenario, where instead of *always* forcing re-validation, it checks the i_version. Perhaps based on a different flag. Mimi
