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Thu, 17 Sep 2020 12:01:15 GMT Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E4D2011C05E; Thu, 17 Sep 2020 12:01:15 +0000 (GMT) Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 3160611C06F; Thu, 17 Sep 2020 12:01:14 +0000 (GMT) Received: from sig-9-65-208-105.ibm.com (unknown [9.65.208.105]) by d06av25.portsmouth.uk.ibm.com (Postfix) with ESMTP; Thu, 17 Sep 2020 12:01:14 +0000 (GMT) Message-ID: <5bbf2169cfa38bb7a3d696e582c1de954a82d5c6.camel@linux.ibm.com> Subject: Re: [PATCH v2 07/12] evm: Introduce EVM_RESET_STATUS atomic flag From: Mimi Zohar To: Roberto Sassu , mjg59@google.com, John Johansen Cc: linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org, silviu.vlasceanu@huawei.com, stable@vger.kernel.org Date: Thu, 17 Sep 2020 08:01:13 -0400 In-Reply-To: <20200904092643.20013-3-roberto.sassu@huawei.com> References: <20200904092339.19598-1-roberto.sassu@huawei.com> <20200904092643.20013-3-roberto.sassu@huawei.com> Content-Type: text/plain; charset="ISO-8859-15" X-Mailer: Evolution 3.28.5 (3.28.5-12.el8) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235,18.0.687 definitions=2020-09-17_08:2020-09-16,2020-09-17 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 clxscore=1015 mlxlogscore=999 bulkscore=0 spamscore=0 malwarescore=0 impostorscore=0 adultscore=0 suspectscore=3 mlxscore=0 lowpriorityscore=0 priorityscore=1501 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2009170085 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org [Cc'ing John Johansen] Hi Roberto, On Fri, 2020-09-04 at 11:26 +0200, Roberto Sassu wrote: > When EVM_ALLOW_METADATA_WRITES is set, EVM allows any operation on > 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 > 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 > 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") > Signed-off-by: Roberto Sassu > --- > 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 > 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, > evm_status = (rc == -ENODATA) ? > INTEGRITY_NOXATTRS : INTEGRITY_FAIL; > out: > - if (iint) > + if (iint) { > + /* > + * EVM_RESET_STATUS can be cleared only by evm_verifyxattr() > + * when EVM_ALLOW_METADATA_WRITES is set. This guarantees that > + * IMA sees the EVM_RESET_STATUS flag set before it is cleared. > + */ > + 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. Mimi > + } > 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) > 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); > }