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SubjectRe: [PATCH v5 09/12] evm: Allow setxattr() and setattr() for unmodified metadata
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On Mon, 2021-05-03 at 15:11 +0000, Roberto Sassu wrote:
> > From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> > Sent: Monday, May 3, 2021 3:00 PM
> > On Wed, 2021-04-07 at 12:52 +0200, Roberto Sassu wrote:
> >
> > > diff --git a/security/integrity/evm/evm_main.c
> > b/security/integrity/evm/evm_main.c
> > > @@ -389,6 +473,11 @@ static int evm_protect_xattr(struct
> > user_namespace *mnt_userns,
> > > if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
> > > return 0;
> > >
> > > + if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
> > > + !evm_xattr_change(mnt_userns, dentry, xattr_name, xattr_value,
> > > + xattr_value_len))
> > > + return 0;
> > > +
> >
> > If the purpose of evm_protect_xattr() is to prevent allowing an invalid
> > security.evm xattr from being re-calculated and updated, making it
> > valid, INTEGRITY_PASS_IMMUTABLE shouldn't need to be conditional. Any
> > time there is an attr or xattr change, including setting it to the
> > existing value, the status flag should be reset.
> >
> > I'm wondering if making INTEGRITY_PASS_IMMUTABLE conditional would
> > prevent the file from being resigned.
> >
> > > if (evm_status != INTEGRITY_PASS)
> > > integrity_audit_msg(AUDIT_INTEGRITY_METADATA,
> > d_backing_inode(dentry),
> > > dentry->d_name.name,
> > "appraise_metadata",
> >
> > This would then be updated to if not INTEGRITY_PASS or
> > INTEGRITY_PASS_IMMUTABLE. The subsequent "return" would need to be
> > updated as well.
>
> I agree on the first suggestion, to reduce the number of log messages.
> For the second, if you meant that we should return 0 if the status is
> INTEGRITY_PASS_IMMUTABLE, I thought we wanted to deny xattr
> changes when there is an EVM portable signature.

Why? I must be missing something. As long as we're not relying on the
cached status, allowing the file metadata to be updated shouldn't be an
issue.

Mimi

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