On Tue, 2018-12-18 at 18:00 -0500, Mimi Zohar wrote: > Hi Ignaz, > > On Tue, 2018-12-18 at 20:49 +0100, Ignaz Forster wrote: > > Hi, > > > > as a follow up to my attempts to use overlayfs on an IMA protected > > system[1] I've now tried to also enable EVM. From what I understand this > > should - at least in theory - be possible: EVM will call > > d_backing_inode(dentry), which I thought would get the inode of the > > underlying file system[2], and use that for HMAC verification. > > > > In practice simply trying to access an existing file will fail with > > "Permission denied" already. In the corresponding audit log I can see > > the file access (failed with "invalid-HMAC"), but with an inode number > > unknown to me - stat returns a completely different number for the file > > in the lower and target dir. > > > > For testing purposes I added a new hashing algorithm to > > evm_ima_xattr_type which will not add the file system specific > > attributes (inode number, generation, file system uuid) to the hash - > > just like EVM_XATTR_PORTABLE_DIGSIG, but with the hashes generated by > > the kernel. Files created with this signature can be read correctly, > > though writing the files will still fail. > > > > Unfortunately I'm out of ideas what is happening here. If anybody wants > > to have a look at this: Any help would be appreciated. > > > > Kind Regards, > > Ignaz > > > > [1] https://www.spinics.net/lists/linux-integrity/msg03593.html > > [2] https://www.kernel.org/doc/htmldocs/filesystems/API-d-backing-inode.html > > > > After creating a file on the overlay, I wasn't able to access it from > the overlay, but was able to access it from "upper". Both "stat" and > "getfattr -m ^security" returned exactly the same things for both > pathnames. However, the ino in the audit log was different. > > After modifying evm_calc_hmac_or_hash(), replacing d_backing_inode() > with d_real_inode(), the hmac properly calculated for both the overlay > and the upper pathnames. > > Something must have changed in d_backing_inode(). Confirmed, in linux-4.18.y d_backing_inode returns the real i_ino, but newer kernels do not. This is a problem for EVM as the i_ino is included in the HMAC calculation. Mimi
