| Message ID | 20230314081720.4158676-1-roberto.sassu@huaweicloud.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | evm: Do HMAC of multiple per LSM xattrs for new inodes | expand |
On Tue, 2023-03-14 at 09:17 +0100, Roberto Sassu wrote: > From: Roberto Sassu <roberto.sassu@huawei.com> > > One of the major goals of LSM stacking is to run multiple LSMs side by side > without interfering with each other. The ultimate decision will depend on > individual LSM decision. > > Several changes need to be made to the LSM infrastructure to be able to > support that. This patch set tackles one of them: gives to each LSM the > ability to specify one or multiple xattrs to be set at inode creation > time and, at the same time, gives to EVM the ability to access all those > xattrs and calculate the HMAC on them. > > The first problem that this patch set addresses is to make the > inode_init_security hook definition suitable to use with EVM which, unlike > other LSMs, needs to have visibility of all xattrs and not only the one > that the LSM infrastructure passes to the LSM to be set. > > The solution is to replace in the inode_init_security definition the > name/value/len parameters with the beginning of the array containing all > xattrs set by LSMs. Due to security_old_inode_init_security() API > limitation of setting only one xattr, it has been dropped and the remaining > users, ocfs2 and reiserfs, switch to security_inode_init_security(). > However, due to the complexity of the changes required to fully exploit the > ability of security_inode_init_security() to set multiple xattrs, those > users can still set only one xattr (the first set in the xattr array) where > previously they called security_old_inode_init_security(). > > Furthermore, while EVM is invoked unlike before, its xattr will not be set > as it would not be the first set in the xattr array, or if it is the first, > there would not be protected xattrs to calculate the HMAC on. > > The second problem this patch set addresses is the limitation of the > call_int_hook() of stopping the loop when the return value from a hook > implementation is not zero. Unfortunately, for the inode_init_security hook > it is a legitimate case to return -EOPNOTSUPP, but this would not > necessarily mean that there is an error to report to the LSM infrastructure > but just that an LSM does not will to set an xattr. Other LSMs should be > still consulted as well. > > The solution for this specific case is to replace the call_int_hook() with > the loop itself, so that -EOPNOTSUPP can be ignored. > > Next, this patch set removes the limitation of creating only two xattrs, > one by an active LSM and another by EVM. This patch set extends the > reservation mechanism of the LSM infrastructure, to allow each LSM to > request one or multiple xattrs. While this could potentially lead to > reaching the filesystem limits of number/size of the xattrs, it seems not > an issue that need to be solved by the LSM infrastructure but by the > filesystems themselves. Currently, if the limit is reached, the only > workaround would be to use fewer LSMs. > > The reservation mechanism concept makes it very easy for LSMs to position > themselves correctly in the xattr array, as the LSM infrastructure at > initialization time changes the number of xattrs requested by each LSM with > an offset. LSMs can just take that offset as the starting index in the > xattr array and fill the next slots depending on how many xattrs they > requested. > > However, while this concept is intuitive, it needs extra care. While for > security blobs (the main reason of the reservation mechanism) it is not > relevant for an LSM if other LSMs filled their portion, it matters for > xattrs, as both EVM and initxattrs() callbacks scan the entire array until > a terminator (xattr with NULL name). If an LSM did not provide an xattr, > which could happen if it is loaded but not initialized, consumers of the > xattr array would stop prematurely. > > This patch set avoids this problem by compacting the xattr array each time > after an LSM executed its implementation of the inode_init_security hook. > It needs to be done after each LSM, and not after all, since there might be > LSMs scanning that xattr array too. Compacting the array after all LSMs > would be too late. > > Finally, this patch set modifies the evm_inode_init_security() definition > to be compatible with the inode_init_security hook definition and adds > support for scanning the whole xattr array and for calculating the HMAC > on all xattrs provided by LSMs. > > 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, one providing multiple xattrs > and another providing an xattr but in a disabled state). The patch is not > included in this set but it is available here: > > https://github.com/robertosassu/linux/commit/311b83a98757915cc3ccb363f545578e7c38df54 > > The test, added to ima-evm-utils, is available here: > > https://github.com/robertosassu/ima-evm-utils/blob/evm-multiple-lsms-v8-devel-v2/tests/evm_multiple_lsms.test > > The test takes a UML kernel built by Github Actions and launches it several > times, each time with a different combination of LSMs and filesystems (ext4, > reiserfs, ocfs2). After boot, it first checks that there is an xattr for each > LSM providing it (for reiserfs and ocfs2 just the first LSM), and then (for > ext4) 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://github.com/robertosassu/ima-evm-utils/actions/runs/4403223954/jobs/7722101662 > > The patch set has been tested with both the SElinux and Smack test suites. > Below, there is the summary of the test results: > > SELinux Test Suite result (without patches): > All tests successful. > Files=76, Tests=1343, 255 wallclock secs ( 0.51 usr 0.19 sys + 6.30 cusr 58.15 csys = 65.15 CPU) > Result: PASS > > SELinux Test Suite result (with patches): > All tests successful. > Files=76, Tests=1343, 258 wallclock secs ( 0.49 usr 0.21 sys + 6.46 cusr 59.27 csys = 66.43 CPU) > Result: PASS Side note: https://github.com/SELinuxProject/selinux-testsuite/blob/master/tests/mmap/test#L51 This test fails if CONFIG_LSM_MMAP_MIN_ADDR=0 Roberto > Smack Test Suite result (without patches): > 95 Passed, 0 Failed, 100% Success rate > > Smack Test Suite result (with patches): > 95 Passed, 0 Failed, 100% Success rate > > Changelog > > v7: > - Add a patch dependency comment in patch 1 (suggested by Mimi) > - Restore check of -EOPNOTSUPP status in ocfs2_mknod() and ocfs2_symlink() > (reported by Mimi) > - Add explanation in evm_inode_init_security() why walking through the > xattrs array is safe (suggested by Mimi) > - Document the lbs_xattr field of struct lsm_blob_sizes (suggested by > Casey) > - Move documentation changes of the inode_init_security hook to security.c, > after LSM documentation reorganization by Paul > - Use attributes in plural form in the description of the xattrs parameter > of smack_inode_init_security() > - Check xattr name instead of xattr value in evm_inode_init_security(), > for consistency with evm_init_hmac(); equivalent, since > security_check_compact_filled_xattrs() rejects xattrs with xattr name > NULL and value not NULL, and viceversa > > v6: > - Add a comment in Smack to introduce its xattrs (suggested by Casey) > - Document the overloaded meaning of -EOPNOTSUPP in > security_inode_init_security() (suggested by Mimi) > > v5: > - Modify the cover letter to explain that the goal of this patch set is > supporting multiple per LSM xattrs in EVM, and not moving IMA and EVM to > the LSM infrastructure (suggested by Mimi) > - Remove references in the patches description about moving IMA and EVM > to the LSM infrastructure (suggested by Mimi) > - Explain that the additional EVM invocation due to the switch to > security_inode_init_security() will not cause the EVM xattr to be added > (suggested by Mimi) > > v4: > - Remove patch to call reiserfs_security_free(), already queued > - Switch ocfs2 and reiserfs to security_inode_init_security() (suggested by > Mimi) > - Remove security_old_inode_init_security() (suggested by Paul) > - Rename security_check_compact_xattrs() to > security_check_compact_filled_xattrs() and add function description > (suggested by Mimi) > - Rename checked_xattrs parameter of security_check_compact_filled_xattrs() > to num_filled_xattrs (suggested by Mimi) > - Rename cur_xattrs variable in security_inode_init_security() to > num_filled_xattrs (suggested by Mimi) > > v3: > - Don't free the xattr name in reiserfs_security_free() > - Don't include fs_data parameter in inode_init_security hook > - Don't change evm_inode_init_security(), as it will be removed if EVM is > stacked > - Fix inode_init_security hook documentation > - Drop lsm_find_xattr_slot(), use simple xattr reservation mechanism and > introduce security_check_compact_xattrs() to compact the xattr array > - Don't allocate xattr array if LSMs didn't reserve any xattr > - Return zero if initxattrs() is not provided to > security_inode_init_security(), -EOPNOTSUPP if value is not provided to > security_old_inode_init_security() > - Request LSMs to fill xattrs if only value (not the triple) is provided to > security_old_inode_init_security(), to avoid unnecessary memory > allocation > > v2: > - rewrite selinux_old_inode_init_security() to use > security_inode_init_security() > - add lbs_xattr field to lsm_blob_sizes structure, to give the ability to > LSMs to reserve slots in the xattr array (suggested by Casey) > - add new parameter base_slot to inode_init_security hook definition > > v1: > - add calls to reiserfs_security_free() and initialize sec->value to NULL > (suggested by Tetsuo and Mimi) > - change definition of inode_init_security hook, replace the name, value > and len triple with the xattr array (suggested by Casey) > - introduce lsm_find_xattr_slot() helper for LSMs to find an unused slot in > the passed xattr array > > Roberto Sassu (6): > reiserfs: Switch to security_inode_init_security() > ocfs2: Switch to security_inode_init_security() > security: Remove security_old_inode_init_security() > security: Allow all LSMs to provide xattrs for inode_init_security > hook > evm: Align evm_inode_init_security() definition with LSM > infrastructure > evm: Support multiple LSMs providing an xattr > > fs/ocfs2/namei.c | 2 + > fs/ocfs2/xattr.c | 30 ++++++- > fs/reiserfs/xattr_security.c | 23 +++-- > include/linux/evm.h | 12 +-- > include/linux/lsm_hook_defs.h | 3 +- > include/linux/lsm_hooks.h | 1 + > include/linux/security.h | 12 --- > security/integrity/evm/evm.h | 2 + > security/integrity/evm/evm_crypto.c | 9 +- > security/integrity/evm/evm_main.c | 33 +++++-- > security/security.c | 131 +++++++++++++++++++++------- > security/selinux/hooks.c | 19 ++-- > security/smack/smack_lsm.c | 33 ++++--- > 13 files changed, 224 insertions(+), 86 deletions(-) >
From: Roberto Sassu <roberto.sassu@huawei.com> One of the major goals of LSM stacking is to run multiple LSMs side by side without interfering with each other. The ultimate decision will depend on individual LSM decision. Several changes need to be made to the LSM infrastructure to be able to support that. This patch set tackles one of them: gives to each LSM the ability to specify one or multiple xattrs to be set at inode creation time and, at the same time, gives to EVM the ability to access all those xattrs and calculate the HMAC on them. The first problem that this patch set addresses is to make the inode_init_security hook definition suitable to use with EVM which, unlike other LSMs, needs to have visibility of all xattrs and not only the one that the LSM infrastructure passes to the LSM to be set. The solution is to replace in the inode_init_security definition the name/value/len parameters with the beginning of the array containing all xattrs set by LSMs. Due to security_old_inode_init_security() API limitation of setting only one xattr, it has been dropped and the remaining users, ocfs2 and reiserfs, switch to security_inode_init_security(). However, due to the complexity of the changes required to fully exploit the ability of security_inode_init_security() to set multiple xattrs, those users can still set only one xattr (the first set in the xattr array) where previously they called security_old_inode_init_security(). Furthermore, while EVM is invoked unlike before, its xattr will not be set as it would not be the first set in the xattr array, or if it is the first, there would not be protected xattrs to calculate the HMAC on. The second problem this patch set addresses is the limitation of the call_int_hook() of stopping the loop when the return value from a hook implementation is not zero. Unfortunately, for the inode_init_security hook it is a legitimate case to return -EOPNOTSUPP, but this would not necessarily mean that there is an error to report to the LSM infrastructure but just that an LSM does not will to set an xattr. Other LSMs should be still consulted as well. The solution for this specific case is to replace the call_int_hook() with the loop itself, so that -EOPNOTSUPP can be ignored. Next, this patch set removes the limitation of creating only two xattrs, one by an active LSM and another by EVM. This patch set extends the reservation mechanism of the LSM infrastructure, to allow each LSM to request one or multiple xattrs. While this could potentially lead to reaching the filesystem limits of number/size of the xattrs, it seems not an issue that need to be solved by the LSM infrastructure but by the filesystems themselves. Currently, if the limit is reached, the only workaround would be to use fewer LSMs. The reservation mechanism concept makes it very easy for LSMs to position themselves correctly in the xattr array, as the LSM infrastructure at initialization time changes the number of xattrs requested by each LSM with an offset. LSMs can just take that offset as the starting index in the xattr array and fill the next slots depending on how many xattrs they requested. However, while this concept is intuitive, it needs extra care. While for security blobs (the main reason of the reservation mechanism) it is not relevant for an LSM if other LSMs filled their portion, it matters for xattrs, as both EVM and initxattrs() callbacks scan the entire array until a terminator (xattr with NULL name). If an LSM did not provide an xattr, which could happen if it is loaded but not initialized, consumers of the xattr array would stop prematurely. This patch set avoids this problem by compacting the xattr array each time after an LSM executed its implementation of the inode_init_security hook. It needs to be done after each LSM, and not after all, since there might be LSMs scanning that xattr array too. Compacting the array after all LSMs would be too late. Finally, this patch set modifies the evm_inode_init_security() definition to be compatible with the inode_init_security hook definition and adds support for scanning the whole xattr array and for calculating the HMAC on all xattrs provided by LSMs. 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, one providing multiple xattrs and another providing an xattr but in a disabled state). The patch is not included in this set but it is available here: https://github.com/robertosassu/linux/commit/311b83a98757915cc3ccb363f545578e7c38df54 The test, added to ima-evm-utils, is available here: https://github.com/robertosassu/ima-evm-utils/blob/evm-multiple-lsms-v8-devel-v2/tests/evm_multiple_lsms.test The test takes a UML kernel built by Github Actions and launches it several times, each time with a different combination of LSMs and filesystems (ext4, reiserfs, ocfs2). After boot, it first checks that there is an xattr for each LSM providing it (for reiserfs and ocfs2 just the first LSM), and then (for ext4) 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://github.com/robertosassu/ima-evm-utils/actions/runs/4403223954/jobs/7722101662 The patch set has been tested with both the SElinux and Smack test suites. Below, there is the summary of the test results: SELinux Test Suite result (without patches): All tests successful. Files=76, Tests=1343, 255 wallclock secs ( 0.51 usr 0.19 sys + 6.30 cusr 58.15 csys = 65.15 CPU) Result: PASS SELinux Test Suite result (with patches): All tests successful. Files=76, Tests=1343, 258 wallclock secs ( 0.49 usr 0.21 sys + 6.46 cusr 59.27 csys = 66.43 CPU) Result: PASS Smack Test Suite result (without patches): 95 Passed, 0 Failed, 100% Success rate Smack Test Suite result (with patches): 95 Passed, 0 Failed, 100% Success rate Changelog v7: - Add a patch dependency comment in patch 1 (suggested by Mimi) - Restore check of -EOPNOTSUPP status in ocfs2_mknod() and ocfs2_symlink() (reported by Mimi) - Add explanation in evm_inode_init_security() why walking through the xattrs array is safe (suggested by Mimi) - Document the lbs_xattr field of struct lsm_blob_sizes (suggested by Casey) - Move documentation changes of the inode_init_security hook to security.c, after LSM documentation reorganization by Paul - Use attributes in plural form in the description of the xattrs parameter of smack_inode_init_security() - Check xattr name instead of xattr value in evm_inode_init_security(), for consistency with evm_init_hmac(); equivalent, since security_check_compact_filled_xattrs() rejects xattrs with xattr name NULL and value not NULL, and viceversa v6: - Add a comment in Smack to introduce its xattrs (suggested by Casey) - Document the overloaded meaning of -EOPNOTSUPP in security_inode_init_security() (suggested by Mimi) v5: - Modify the cover letter to explain that the goal of this patch set is supporting multiple per LSM xattrs in EVM, and not moving IMA and EVM to the LSM infrastructure (suggested by Mimi) - Remove references in the patches description about moving IMA and EVM to the LSM infrastructure (suggested by Mimi) - Explain that the additional EVM invocation due to the switch to security_inode_init_security() will not cause the EVM xattr to be added (suggested by Mimi) v4: - Remove patch to call reiserfs_security_free(), already queued - Switch ocfs2 and reiserfs to security_inode_init_security() (suggested by Mimi) - Remove security_old_inode_init_security() (suggested by Paul) - Rename security_check_compact_xattrs() to security_check_compact_filled_xattrs() and add function description (suggested by Mimi) - Rename checked_xattrs parameter of security_check_compact_filled_xattrs() to num_filled_xattrs (suggested by Mimi) - Rename cur_xattrs variable in security_inode_init_security() to num_filled_xattrs (suggested by Mimi) v3: - Don't free the xattr name in reiserfs_security_free() - Don't include fs_data parameter in inode_init_security hook - Don't change evm_inode_init_security(), as it will be removed if EVM is stacked - Fix inode_init_security hook documentation - Drop lsm_find_xattr_slot(), use simple xattr reservation mechanism and introduce security_check_compact_xattrs() to compact the xattr array - Don't allocate xattr array if LSMs didn't reserve any xattr - Return zero if initxattrs() is not provided to security_inode_init_security(), -EOPNOTSUPP if value is not provided to security_old_inode_init_security() - Request LSMs to fill xattrs if only value (not the triple) is provided to security_old_inode_init_security(), to avoid unnecessary memory allocation v2: - rewrite selinux_old_inode_init_security() to use security_inode_init_security() - add lbs_xattr field to lsm_blob_sizes structure, to give the ability to LSMs to reserve slots in the xattr array (suggested by Casey) - add new parameter base_slot to inode_init_security hook definition v1: - add calls to reiserfs_security_free() and initialize sec->value to NULL (suggested by Tetsuo and Mimi) - change definition of inode_init_security hook, replace the name, value and len triple with the xattr array (suggested by Casey) - introduce lsm_find_xattr_slot() helper for LSMs to find an unused slot in the passed xattr array Roberto Sassu (6): reiserfs: Switch to security_inode_init_security() ocfs2: Switch to security_inode_init_security() security: Remove security_old_inode_init_security() security: Allow all LSMs to provide xattrs for inode_init_security hook evm: Align evm_inode_init_security() definition with LSM infrastructure evm: Support multiple LSMs providing an xattr fs/ocfs2/namei.c | 2 + fs/ocfs2/xattr.c | 30 ++++++- fs/reiserfs/xattr_security.c | 23 +++-- include/linux/evm.h | 12 +-- include/linux/lsm_hook_defs.h | 3 +- include/linux/lsm_hooks.h | 1 + include/linux/security.h | 12 --- security/integrity/evm/evm.h | 2 + security/integrity/evm/evm_crypto.c | 9 +- security/integrity/evm/evm_main.c | 33 +++++-- security/security.c | 131 +++++++++++++++++++++------- security/selinux/hooks.c | 19 ++-- security/smack/smack_lsm.c | 33 ++++--- 13 files changed, 224 insertions(+), 86 deletions(-)