Re: [PATCH v2 07/12] evm: Introduce EVM_RESET_STATUS atomic flag
From: Mimi Zohar <zohar@linux.ibm.com>
Date: 2020-09-17 13:08:56
Also in:
linux-integrity, lkml, stable
[Cc'ing John Johansen] Hi Roberto, On Fri, 2020-09-04 at 11:26 +0200, Roberto Sassu wrote:
quoted hunk ↗ jump to hunk
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 <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 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
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+ } 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); }