Re: EVM: Permission denied with overlayfs

[Mimi Zohar <zohar@linux.ibm.com>]

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