This patch set depends on: https://lore.kernel.org/linux-integrity/20210409114313.4073-1-roberto.sassu@xxxxxxxxxx/ https://lore.kernel.org/linux-integrity/20210407105252.30721-1-roberto.sassu@xxxxxxxxxx/ One of the challenges that must be tackled to move IMA and EVM to the LSM infrastructure is to ensure that EVM is capable to correctly handle multiple stacked LSMs providing an xattr at file creation. At the moment, there are few issues that would prevent a correct integration. This patch set aims at solving them. >From the LSM infrastructure side, the LSM stacking feature added the possibility of registering multiple implementations of the security hooks, that are called sequentially whenever someone calls the corresponding security hook. However, security_inode_init_security() and security_old_inode_init_security() are currently limited to support one xattr provided by LSM and one by EVM. In addition, using the call_int_hook() macro causes some issues. According to the documentation in include/linux/lsm_hooks.h, it is a legitimate case that an LSM returns -EOPNOTSUPP when it does not want to provide an xattr. However, the loop defined in the macro would stop calling subsequent LSMs if that happens. In the case of security_old_inode_init_security(), using the macro would also cause a memory leak due to replacing the *value pointer, if multiple LSMs provide an xattr. >From EVM side, the first operation to be done is to change the definition of evm_inode_init_security() to be compatible with the security hook definition. Unfortunately, the current definition does not provide enough information for EVM, as it must have visibility of all xattrs provided by LSMs to correctly calculate the HMAC. This patch set changes the security hook definition by adding the full array of xattr as a parameter. Secondly, EVM must know how many elements are in the xattr array. It seems that it is not necessary to add another parameter, as all filesystems that define an initxattr function, expect that the last element of the array is one with the name field set to NULL. EVM reuses the same assumption. This patch set has been tested by introducing several instances of a TestLSM (some providing an xattr, some not, one with a wrong implementation to see how the LSM infrastructure handles it). The patch is not included in this set but it is available here: https://github.com/robertosassu/linux/commit/0370ff0fbc16e5d63489836a958e65d697f956db The test, added to ima-evm-utils, is available here: https://github.com/robertosassu/ima-evm-utils/blob/evm-multiple-lsms-v1-devel-v1/tests/evm_multiple_lsms.test The test takes a UML kernel built by Travis and launches it several times, each time with a different combination of LSMs. After boot, it first checks that there is an xattr for each LSM providing it, and then calculates the HMAC in user space and compares it with the HMAC calculated by EVM in kernel space. A test report can be obtained here: https://www.travis-ci.com/github/robertosassu/ima-evm-utils/jobs/498699540 Lastly, running the test on reiserfs to check security_old_inode_init_security(), some issues have been discovered: a free of xattr->name which is not correct after commit 9548906b2bb7 ('xattr: Constify ->name member of "struct xattr"'), and a misalignment with security_inode_init_security() (the old version expects the full xattr name with the security. prefix, the new version just the suffix). The last issue has not been fixed yet. Roberto Sassu (5): xattr: Complete constify ->name member of "struct xattr" security: Support multiple LSMs implementing the inode_init_security hook security: Pass xattrs allocated by LSMs to the inode_init_security hook evm: Align evm_inode_init_security() definition with LSM infrastructure evm: Support multiple LSMs providing an xattr fs/reiserfs/xattr_security.c | 2 - include/linux/evm.h | 21 ++++--- include/linux/lsm_hook_defs.h | 2 +- include/linux/lsm_hooks.h | 5 +- security/integrity/evm/evm.h | 2 + security/integrity/evm/evm_crypto.c | 9 ++- security/integrity/evm/evm_main.c | 35 +++++++---- security/security.c | 95 +++++++++++++++++++++++------ security/selinux/hooks.c | 3 +- security/smack/smack_lsm.c | 4 +- 10 files changed, 135 insertions(+), 43 deletions(-) -- 2.26.2
