From: Mimi Zohar [mailto:zohar@linux.ibm.com]
Sent: Friday, December 4, 2020 2:05 PM
On Fri, 2020-12-04 at 08:05 +0000, Roberto Sassu wrote:

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From: Mimi Zohar [mailto:zohar@linux.ibm.com]
Sent: Thursday, December 3, 2020 9:43 PM
Hi Roberto,

On Wed, 2020-11-11 at 10:22 +0100, Roberto Sassu wrote:

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When a file is being created, LSMs can set the initial label with the
inode_init_security hook. If no HMAC key is loaded, the new file will

have

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LSM xattrs but not the HMAC.

Unfortunately, EVM will deny any further metadata operation on new

files,

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as evm_protect_xattr() will always return the INTEGRITY_NOLABEL

error.

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This

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would limit the usability of EVM when only a public key is loaded, as
commands such as cp or tar with the option to preserve xattrs won't

work.

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Ignoring this error won't be an issue if no HMAC key is loaded, as the
inode is locked until the post hook, and EVM won't calculate the HMAC

on

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metadata that wasn't previously verified. Thus this patch checks if an
HMAC key is loaded and if not, ignores INTEGRITY_NOLABEL.

I'm not sure what problem this patch is trying to solve.
evm_protect_xattr() is only called by evm_inode_setxattr() and
evm_inode_removexattr(), which first checks whether
EVM_ALLOW_METADATA_WRITES is enabled.

The idea is to also support EVM verification when only a public key
is loaded. An advantage to do that is that for example we can prevent
accidental metadata changes when the signature is portable.

Right, there are a couple of  scenarios.  Let's be more specific as to
which scenario this patch is addressing.

- a public key is loaded and EVM_ALLOW_METADATA_WRITES is enabled,
- a public key is loaded and EVM_ALLOW_METADATA_WRITES is disabled,
- an HMAC key is loaded

For the first and last case, this patch shouldn't be necessary.  Only
the second case, with EVM_ALLOW_METADATA_WRITES disabled, probably
does
not work.  I would claim that is working as designed.

If there is no HMAC key loaded and a file is created, I think EVM should
not expect an HMAC and return an error. If we do metadata verification
only when an HMAC key is loaded, we miss a functionality that could be
useful also when only a public key is loaded.

Roberto

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