[RFC][PATCH] overlayfs: Redirect xattr ops on security.evm to security.evm_overlayfs

[Christian Brauner]

On Fri, Dec 08, 2023 at 11:55:19PM +0200, Amir Goldstein wrote:
> On Fri, Dec 8, 2023 at 7:25 PM Roberto Sassu
> <roberto.sassu at huaweicloud.com> wrote:
> >
> > From: Roberto Sassu <roberto.sassu at huawei.com>
> >
> > EVM updates the HMAC in security.evm whenever there is a setxattr or
> > removexattr operation on one of its protected xattrs (e.g. security.ima).
> >
> > Unfortunately, since overlayfs redirects those xattrs operations on the
> > lower filesystem, the EVM HMAC cannot be calculated reliably, since lower
> > inode attributes on which the HMAC is calculated are different from upper
> > inode attributes (for example i_generation and s_uuid).
> >
> > Although maybe it is possible to align such attributes between the lower
> > and the upper inode, another idea is to map security.evm to another name
> > (security.evm_overlayfs)
> 
> If we were to accept this solution, this will need to be trusted.overlay.evm
> to properly support private overlay xattr escaping.
> 
> > during an xattr operation, so that it does not
> > collide with security.evm set by the lower filesystem.
> 
> You are using wrong terminology and it is very confusing to me.

Same.

> see the overlay mount command has lowerdir= and upperdir=.
> Seems that you are using lower filesystem to refer to the upper fs
> and upper filesystem to refer to overlayfs.
> 
> >
> > Whenever overlayfs wants to set security.evm, it is actually setting
> > security.evm_overlayfs calculated with the upper inode attributes. The
> > lower filesystem continues to update security.evm.
> >
> 
> I understand why that works, but I am having a hard time swallowing
> the solution, mainly because I feel that there are other issues on the
> intersection of overlayfs and IMA and I don't feel confident that this
> addresses them all.
> 
> If you want to try to convince me, please try to write a complete
> model of how IMA/EVM works with overlayfs, using the section
> "Permission model" in Documentation/filesystems/overlayfs.rst
> as a reference.

I want us to go the other way. Make the overlayfs layer completely
irrelevant for EVM and IMA. See a related discussion here:

Subject: Re: [PATCH 09/16] fs: add vfs_set_fscaps()
https://lore.kernel.org/r/ZXHZ8uNEg1IK5WMW@do-x1extreme

Amir, if you have some time I'd appreciate a comment on that.