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SubjectRE: [PATCH v5 09/12] evm: Allow setxattr() and setattr() for unmodified metadata
Date
> 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.

The status is always reset if evm_protect_xattr() returns 0. This does not
change.

Not making INTEGRITY_PASS_IMMUTABLE conditional would cause issues.
Suppose that the status is INTEGRITY_FAIL. Writing the same xattr would
cause evm_protect_xattr() to return 0 and the HMAC to be updated.

> I'm wondering if making INTEGRITY_PASS_IMMUTABLE conditional would
> prevent the file from being resigned.

INTEGRITY_FAIL_IMMUTABLE should be enough to continue the
operation.

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli

> > 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.
>
> thanks,
>
> Mimi

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