On Tue, Aug 11, 2020 at 09:56:50AM -0400, Mimi Zohar wrote: > On Tue, 2020-08-11 at 10:48 +0200, Mickaël Salaün wrote: > > On 11/08/2020 01:03, Jann Horn wrote: > > > On Tue, Aug 11, 2020 at 12:43 AM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > > > On 10/08/2020 22:21, Al Viro wrote: > > > > > On Mon, Aug 10, 2020 at 10:11:53PM +0200, Mickaël Salaün wrote: > > > > > > It seems that there is no more complains nor questions. Do you want me > > > > > > to send another series to fix the order of the S-o-b in patch 7? > > > > > > > > > > There is a major question regarding the API design and the choice of > > > > > hooking that stuff on open(). And I have not heard anything resembling > > > > > a coherent answer. > > > > > > > > Hooking on open is a simple design that enables processes to check files > > > > they intend to open, before they open them. From an API point of view, > > > > this series extends openat2(2) with one simple flag: O_MAYEXEC. The > > > > enforcement is then subject to the system policy (e.g. mount points, > > > > file access rights, IMA, etc.). > > > > > > > > Checking on open enables to not open a file if it does not meet some > > > > requirements, the same way as if the path doesn't exist or (for whatever > > > > reasons, including execution permission) if access is denied. > > > > > > You can do exactly the same thing if you do the check in a separate > > > syscall though. > > > > > > And it provides a greater degree of flexibility; for example, you can > > > use it in combination with fopen() without having to modify the > > > internals of fopen() or having to use fdopen(). > > > > > > > It is a > > > > good practice to check as soon as possible such properties, and it may > > > > enables to avoid (user space) time-of-check to time-of-use (TOCTOU) > > > > attacks (i.e. misuse of already open resources). > > > > > > The assumption that security checks should happen as early as possible > > > can actually cause security problems. For example, because seccomp was > > > designed to do its checks as early as possible, including before > > > ptrace, we had an issue for a long time where the ptrace API could be > > > abused to bypass seccomp filters. > > > > > > Please don't decide that a check must be ordered first _just_ because > > > it is a security check. While that can be good for limiting attack > > > surface, it can also create issues when the idea is applied too > > > broadly. > > > > I'd be interested with such security issue examples. > > > > I hope that delaying checks will not be an issue for mechanisms such as > > IMA or IPE: > > https://lore.kernel.org/lkml/1544699060.6703.11.camel@xxxxxxxxxxxxx/ > > > > Any though Mimi, Deven, Chrome OS folks? > > One of the major gaps, defining a system wide policy requiring all code > being executed to be signed, is interpreters. The kernel has no > context for the interpreter's opening the file. From an IMA > perspective, this information needs to be conveyed to the kernel prior > to ima_file_check(), which would allow IMA policy rules to be defined > in terms of O_MAYEXEC. This is kind of evading the point -- Mickaël is proposing a new flag to open() to tell IMA to measure the file being opened before the fd is returned to userspace, and Al is suggesting a new syscall to allow a previously-obtained fd to be measured. I think what you're saying is that you don't see any reason to prefer one over the other.
