On Wed, Sep 10, 2025 at 07:15:19AM -0400, Mimi Zohar wrote:
On Wed, 2025-09-10 at 09:20 +0800, Coiby Xu wrote:
On Tue, Sep 09, 2025 at 11:31:20AM -0400, Mimi Zohar wrote:
> On Tue, 2025-09-09 at 12:19 +0800, Coiby Xu wrote:
> > When both IMA and EVM fix modes are enabled, accessing a file with IMA
> > signature won't cause security.evm to be fixed. But this doesn't happen
> > to a file with correct IMA hash already set because accessing it will
> > cause setting security.ima again which triggers fixing security.evm
> > thanks to security_inode_post_setxattr->evm_update_evmxattr.
> >
> > Let's use the same mechanism to fix security.evm for a file with IMA
> > signature.
> >
> > Signed-off-by: Coiby Xu <[email protected]>
>
> Agreed, re-writing the file signature stored as security.ima would force
> security.evm to be updated.
>
> Unfortunately, I'm missing something. ima_appraise_measurement() first
verifies
> the existing security.evm xattr, before verifying the security.ima xattr. If
> the EVM HMAC fails to verify, it immediately exits ima_appraise_measurement().
> security.ima in this case is never verified.
>
> This patch seems to address the case where the existing security.evm is valid,
> but the file signature stored in security.ima is invalid. (To get to the new
> code, the "status" flag is not INTEGRITY_PASS.) Re-writing the same invalid
> file signature would solve an invalid security.evm, but not an invalid IMA
file
> signature. What am I missing?
Hi, Mimi,
Thanks for raising the question! This patch is to address the case where
IMA signature is already added but security.evm doesn't yet exist. So
EVM HMAC fails to verify but there is no exiting
ima_appraise_measurement immediately.
And you are right that re-writing an invalid IMA file won't fix an
invalid IMA file signature. And even when IMA signature is valid, the
verification may fail because the key is missing from .ima keyring. This
happens because we need to turn off secure boot to enable fix mode. As a
result, CA keys won't be loaded into .machine keyring.
Btw, if I'm not
mistaken, current IMA code assumes we are not supposed to fix IMA file
signature.
Right, unlike file hashes, new or the same file signature can be written, but
cannot be "fixed" in the literal sense, as that would require calculating the
file hash and signing it with a private key.
Thanks for the confirmation! I also added some code comments to explain
the IMA iint cache atomic_flags including IMA_DIGSIG in v2.
This patch triggers "fixing" the EVM HMAC by re-writing the existing IMA file
signature. I assume the same result could be achieved by simply re-installing
the file signature. In both cases, the EVM HMAC key needs to exist and be
loaded.
--
Best regards,
Coiby
