RE: [PATCH v2 07/12] evm: Introduce EVM_RESET_STATUS atomic flag

From: Mimi Zohar [mailto:zohar@linux.ibm.com]
Sent: Thursday, September 17, 2020 2:01 PM
[Cc'ing John Johansen]

Hi Roberto,

On Fri, 2020-09-04 at 11:26 +0200, Roberto Sassu wrote:

quoted

When EVM_ALLOW_METADATA_WRITES is set, EVM allows any operation

on

quoted

metadata. Its main purpose is to allow users to freely set metadata when
they are protected by a portable signature, until the HMAC key is loaded.

However, IMA is not notified about metadata changes and, after the first
successful appraisal, always allows access to the files without checking
metadata again.

This patch introduces the new atomic flag EVM_RESET_STATUS in
integrity_iint_cache that is set in the EVM post hooks and cleared in
evm_verify_hmac(). IMA checks the new flag in process_measurement()

and if

quoted

it is set, it clears the appraisal flags.

Although the flag could be cleared also by evm_inode_setxattr() and
evm_inode_setattr() before IMA sees it, this does not happen if
EVM_ALLOW_METADATA_WRITES is set. Since the only remaining caller is
evm_verifyxattr(), this ensures that IMA always sees the flag set before it
is cleared.

This patch also adds a call to evm_reset_status() in
evm_inode_post_setattr() so that EVM won't return the cached status

the

quoted

next time appraisal is performed.

Cc: stable@vger.kernel.org # 4.16.x
Fixes: ae1ba1676b88e ("EVM: Allow userland to permit modification of

EVM-protected metadata")

quoted

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
---
 security/integrity/evm/evm_main.c | 17 +++++++++++++++--
 security/integrity/ima/ima_main.c |  8 ++++++--
 security/integrity/integrity.h    |  1 +
 3 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/security/integrity/evm/evm_main.c

b/security/integrity/evm/evm_main.c

quoted

index 4e9f5e8b21d5..05be1ad3e6f3 100644

--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c

@@ -221,8 +221,15 @@ static enum integrity_status

evm_verify_hmac(struct dentry *dentry,

quoted

 		evm_status = (rc == -ENODATA) ?
 				INTEGRITY_NOXATTRS : INTEGRITY_FAIL;
 out:
-	if (iint)
+	if (iint) {
+		/*
+		 * EVM_RESET_STATUS can be cleared only by

evm_verifyxattr()

quoted

+		 * when EVM_ALLOW_METADATA_WRITES is set. This

guarantees that

quoted

+		 * IMA sees the EVM_RESET_STATUS flag set before it is

cleared.

quoted

+		 */
+		clear_bit(EVM_RESET_STATUS, &iint->atomic_flags);
 		iint->evm_status = evm_status;

True IMA is currently the only caller of evm_verifyxattr() in the
upstreamed kernel, but it is an exported function, which may be called
from elsewhere.  The previous version crossed the boundary between EVM
& IMA with EVM modifying the IMA flag directly.  This version assumes
that IMA will be the only caller.  Otherwise, I like this version.

Ok, I think it is better, as you suggested, to export a new EVM function
that tells if evm_reset_status() will be executed in the EVM post hooks, and
to call this function from IMA. IMA would then call ima_reset_appraise_flags()
also depending on the result of the new EVM function.

ima_reset_appraise_flags() should be called in a post hook in IMA.
Should I introduce it?

Thanks

Roberto

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

Mimi

quoted

+	}
 	kfree(xattr_data);
 	return evm_status;
 }

@@ -418,8 +425,12 @@ static void evm_reset_status(struct inode *inode)
 	struct integrity_iint_cache *iint;

 	iint = integrity_iint_find(inode);
-	if (iint)
+	if (iint) {
+		if (evm_initialized & EVM_ALLOW_METADATA_WRITES)
+			set_bit(EVM_RESET_STATUS, &iint->atomic_flags);
+
 		iint->evm_status = INTEGRITY_UNKNOWN;
+	}
 }

 /**

@@ -513,6 +524,8 @@ void evm_inode_post_setattr(struct dentry

*dentry, int ia_valid)

quoted

 	if (!evm_key_loaded())
 		return;

+	evm_reset_status(dentry->d_inode);
+
 	if (ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID))
 		evm_update_evmxattr(dentry, NULL, NULL, 0);
 }