On Fri, Jul 05, 2024 at 07:54:16PM +0200, Mickaël Salaün wrote: > On Thu, Jul 04, 2024 at 05:18:04PM -0700, Kees Cook wrote: > > On Thu, Jul 04, 2024 at 09:01:34PM +0200, Mickaël Salaün wrote: > > > 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. > > > > If the threat model includes an attacker sitting at a shell prompt, we > > need to be very careful about how process perform enforcement. E.g. even > > on a locked down system, if an attacker has access to LD_PRELOAD or a > > LD_PRELOAD should be OK once ld.so will be patched to check the > libraries. We can still imagine a debug library used to bypass security > checks, but in this case the issue would be that this library is > executable in the first place. Ah yes, that's fair: the shell would discover the malicious library while using AT_CHECK during resolution of the LD_PRELOAD. > > seccomp wrapper (which you both mention here), it would be possible to > > run commands where the resulting process is tricked into thinking it > > doesn't have the bits set. > > As explained in the UAPI comments, all parent processes need to be > trusted. This meeans that their code is trusted, their seccomp filters > are trusted, and that they are patched, if needed, to check file > executability. But we have launchers that apply arbitrary seccomp policy, e.g. minijail on Chrome OS, or even systemd on regular distros. In theory, this should be handled via other ACLs. > > But this would be exactly true for calling execveat(): LD_PRELOAD or > > seccomp policy could have it just return 0. > > If an attacker is allowed/able to load an arbitrary seccomp filter on a > process, we cannot trust this process. > > > > > While I like AT_CHECK, I do wonder if it's better to do the checks via > > open(), as was originally designed with O_MAYEXEC. Because then > > enforcement is gated by the kernel -- the process does not get a file > > descriptor _at all_, no matter what LD_PRELOAD or seccomp tricks it into > > doing. > > Being able to check a path name or a file descriptor (with the same > syscall) is more flexible and cover more use cases. If flexibility costs us reliability, I think that flexibility is not a benefit. > The execveat(2) > interface, including current and future flags, is dedicated to file > execution. I then think that using execveat(2) for this kind of check > makes more sense, and will easily evolve with this syscall. Yeah, I do recognize that is feels much more natural, but I remain unhappy about how difficult it will become to audit a system for safety when the check is strictly per-process opt-in, and not enforced by the kernel for a given process tree. But, I think this may have always been a fiction in my mind. :) > > And this thinking also applies to faccessat() too: if a process can be > > tricked into thinking the access check passed, it'll happily interpret > > whatever. :( But not being able to open the fd _at all_ when O_MAYEXEC > > is being checked seems substantially safer to me... > > If attackers can filter execveat(2), they can also filter open(2) and > any other syscalls. In all cases, that would mean an issue in the > security policy. Hm, as in, make a separate call to open(2) without O_MAYEXEC, and pass that fd back to the filtered open(2) that did have O_MAYEXEC. Yes, true. I guess it does become morally equivalent. Okay. Well, let me ask about usability. Right now, a process will need to do: - should I use AT_CHECK? (check secbit) - if yes: perform execveat(AT_CHECK) Why not leave the secbit test up to the kernel, and then the program can just unconditionally call execveat(AT_CHECK)? Though perhaps the issue here is that an execveat() EINVAL doesn't tell the program if AT_CHECK is unimplemented or if something else went wrong, and the secbit prctl() will give the correct signal about AT_CHECK availability? -- Kees Cook
