From: Roberto Sassu <roberto.sassu@xxxxxxxxxx> LSMs should follow the conventions stated in include/linux/lsm_hooks.h for return values, as these conventions are followed by callers of the LSM infrastructure for error handling. The ability of an LSM to return arbitrary values could cause big troubles. For LSMs aiming at being upstreamed, this event is unlikely, as each LSM is carefully reviewed and it won't be accepted if it does not meet the return value conventions. However, the recent introduction of BPF LSM allows security modules (not part of the kernel) to inject arbitrary values, without BPF LSM verifying them. The initial idea was to fix BPF LSM itself. However, due to technical difficulties to determine the precise interval of return values from a static code analysis of eBPF programs, the new approach was to put the fix in the LSM infrastructure, so that all LSMs can benefit from this work as well. The biggest problem of allowing arbitrary return values is when an LSM returns a positive value, instead of a negative value, as it could be converted to a pointer. Since such pointer escapes the IS_ERR() check, its use later in the code can cause unpredictable consequences (e.g. invalid memory access). Another problem is returning zero when an LSM is supposed to have done some operations. For example, the inode_init_security hook expects that their implementations return zero only if they set the fields of the new xattr to be added to the new inode. Otherwise, other kernel subsystems might encounter unexpected conditions leading to a crash (e.g. evm_protected_xattr_common() getting NULL as argument). This problem is addressed separately in another patch set. Finally, there are LSM hooks which are supposed to return just 1 as positive value, or non-negative values. Also in these cases, although it seems less critical, it is safer to return to callers of the LSM infrastructure more precisely what they expect. Patches 1 and 2 ensure that the documentation of LSM return values is complete and accurate. Then, patch 3 introduces four flags (LSM_RET_NEG, LSM_RET_ZERO, LSM_RET_ONE, LSM_RET_GT_ONE), one for each interval of interest (< 0, = 0, = 1, > 1), and sets the correct flags for each LSM hook. Finally, patch 4 verifies for each return value from LSMs that it is an expected one. Roberto Sassu (4): lsm: Clarify documentation of vm_enough_memory hook lsm: Add missing return values doc in lsm_hooks.h and fix formatting lsm: Redefine LSM_HOOK() macro to add return value flags as argument security: Enforce limitations on return values from LSMs include/linux/bpf_lsm.h | 2 +- include/linux/lsm_hook_defs.h | 779 ++++++++++++++++++++-------------- include/linux/lsm_hooks.h | 136 ++++-- kernel/bpf/bpf_lsm.c | 5 +- security/bpf/hooks.c | 2 +- security/security.c | 38 +- 6 files changed, 589 insertions(+), 373 deletions(-) -- 2.25.1
