On Wed, 2018-02-21 at 17:12 -0600, Eric W. Biederman wrote: > Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> writes: > > > 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. > > As I understand the second scenario SB_I_IMA_UNVERIFIABLE_SIGNATURES > is set, which implies that the filesystem is lacking something for IMA > to reliably know when a file has changed. AKA a technical deficiency. > > The fourth scenario is the case when SB_I_IMA_UNVERIFIABLE_SIGNATURES > can be legitimately be cleared, because the filesystem provides all > of the necessary support for IMA to reliably know when a file has > changed. The information might be there, but IMA currently detects a file change and resets the flags only when the last writer calls __fput(). Any other time, new support would be needed. Mimi > My point is that cases exists or it is straight forward to implemented > in fuse. > > > I add the fourth case so that we can get a solid definition of > SB_I_IMA_UNVERIFIABLE_SIGNATURES. > > Eric >
