On Mon, Jul 8, 2024 at 11:48 AM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > On Mon, Jul 08, 2024 at 10:53:11AM -0700, Jeff Xu wrote: > > On Mon, Jul 8, 2024 at 9:17 AM Jeff Xu <jeffxu@xxxxxxxxxx> wrote: > > > > > > Hi > > > > > > On Thu, Jul 4, 2024 at 12:02 PM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > > > > > > > These new SECBIT_SHOULD_EXEC_CHECK, SECBIT_SHOULD_EXEC_RESTRICT, and > > > > their *_LOCKED counterparts are designed to be set by processes setting > > > > up an execution environment, such as a user session, a container, or a > > > > security sandbox. Like seccomp filters or Landlock domains, the > > > > securebits are inherited across proceses. > > > > > > > > When SECBIT_SHOULD_EXEC_CHECK is set, programs interpreting code should > > > > check executable resources with execveat(2) + AT_CHECK (see previous > > > > patch). > > > > > > > > When SECBIT_SHOULD_EXEC_RESTRICT is set, a process should only allow > > > > execution of approved resources, if any (see SECBIT_SHOULD_EXEC_CHECK). > > > > > > > Do we need both bits ? > > > When CHECK is set and RESTRICT is not, the "check fail" executable > > > will still get executed, so CHECK is for logging ? > > > Does RESTRICT imply CHECK is set, e.g. What if CHECK=0 and RESTRICT = 1 ? > > > > > The intention might be "permissive mode"? if so, consider reuse > > existing selinux's concept, and still with 2 bits: > > SECBIT_SHOULD_EXEC_RESTRICT > > SECBIT_SHOULD_EXEC_RESTRICT_PERMISSIVE > > SECBIT_SHOULD_EXEC_CHECK is for user space to check with execveat+AT_CHECK. > > SECBIT_SHOULD_EXEC_RESTRICT is for user space to restrict execution by > default, and potentially allow some exceptions from the list of > checked-and-allowed files, if SECBIT_SHOULD_EXEC_CHECK is set. > > Without SECBIT_SHOULD_EXEC_CHECK, SECBIT_SHOULD_EXEC_RESTRICT is to deny > any kind of execution/interpretation. > Do you mean "deny any kinds of executable/interpretation" or just those that failed with "AT_CHECK" ( I assume this)? > With only SECBIT_SHOULD_EXEC_CHECK, user space should just check and log > any denied access, but ignore them. So yes, it is similar to the > SELinux's permissive mode. > IIUC: CHECK=0, RESTRICT=0: do nothing, current behavior CHECK=1, RESTRICT=0: permissive mode - ignore AT_CHECK results. CHECK=0, RESTRICT=1: call AT_CHECK, deny if AT_CHECK failed, no exception. CHECK=1, RESTRICT=1: call AT_CHECK, deny if AT_CHECK failed, except those in the "checked-and-allowed" list. So CHECK is basically trying to form a allowlist? If there is a need for a allowlist, that is the task of "interruptor or dynamic linker" to maintain this list, and the list is known in advance, i.e. not something from execveat(AT_CHECK), and kernel shouldn't have the knowledge of this allowlist. Secondly, the concept of allow-list seems to be an attack factor for me, I would rather it be fully enforced, or permissive mode. And Check=1 and RESTRICT=1 is less secure than CHECK=0, RESTRICT=1, this might also be not obvious to dev. Unless I understood the CHECK wrong. > This is explained in the next patch as comments. > The next patch is a selftest patch, it is better to define them in the current commit and in the securebits.h. > The *_LOCKED variants are useful and part of the securebits concept. > The locked state is easy to understand. Thanks Best regards -Jeff > > > > > > -Jeff > > > > > > > > > > > > For a secure environment, we might also want > > > > SECBIT_SHOULD_EXEC_CHECK_LOCKED and SECBIT_SHOULD_EXEC_RESTRICT_LOCKED > > > > to be set. For a test environment (e.g. testing on a fleet to identify > > > > potential issues), only the SECBIT_SHOULD_EXEC_CHECK* bits can be set to > > > > still be able to identify potential issues (e.g. with interpreters logs > > > > or LSMs audit entries). > > > > > > > > It should be noted that unlike other security bits, the > > > > SECBIT_SHOULD_EXEC_CHECK and SECBIT_SHOULD_EXEC_RESTRICT bits are > > > > dedicated to user space willing to restrict itself. Because of that, > > > > they only make sense in the context of a trusted environment (e.g. > > > > sandbox, container, user session, full system) where the process > > > > changing its behavior (according to these bits) and all its parent > > > > processes are trusted. Otherwise, any parent process could just execute > > > > its own malicious code (interpreting a script or not), or even enforce a > > > > seccomp filter to mask these bits. > > > > > > > > Such a secure environment can be achieved with an appropriate access > > > > control policy (e.g. mount's noexec option, file access rights, LSM > > > > configuration) and an enlighten ld.so checking that libraries are > > > > allowed for execution e.g., to protect against illegitimate use of > > > > LD_PRELOAD. > > > > > > > > Scripts may need some changes to deal with untrusted data (e.g. stdin, > > > > environment variables), but that is outside the scope of the kernel. > > > > > > > > The only restriction enforced by the kernel is the right to ptrace > > > > another process. Processes are denied to ptrace less restricted ones, > > > > unless the tracer has CAP_SYS_PTRACE. This is mainly a safeguard to > > > > avoid trivial privilege escalations e.g., by a debugging process being > > > > abused with a confused deputy attack. > > > > > > > > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> > > > > Cc: Christian Brauner <brauner@xxxxxxxxxx> > > > > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > > > > Cc: Paul Moore <paul@xxxxxxxxxxxxxx> > > > > Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx> > > > > Link: https://lore.kernel.org/r/20240704190137.696169-3-mic@xxxxxxxxxxx > > > > ---
