[RFC][PATCH 0/4] security: Ensure LSMs return expected values

[Roberto Sassu]

From: Roberto Sassu <roberto.sassu at huawei.com>

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(-)

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2.25.1