On Tuesday, March 05, 2024 11:41 EET, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > On Tue, Mar 05, 2024 at 09:59:47AM +0100, Christian Brauner wrote: > > > > Uhm, this will break the seccomp notifier, no? So you can't turn on > > > > SECURITY_PROC_MEM_RESTRICT_WRITE when you want to use the seccomp > > > > notifier to do system call interception and rewrite memory locations of > > > > the calling task, no? Which is very much relied upon in various > > > > container managers and possibly other security tools. > > > > > > > > Which means that you can't turn this on in any of the regular distros. > > > > > > FWIW, it's a run-time toggle, but yes, let's make sure this works > > > correctly. > > > > > > > So you need to either account for the calling task being a seccomp > > > > supervisor for the task whose memory it is trying to access or you need > > > > to provide a migration path by adding an api that let's caller's perform > > > > these writes through the seccomp notifier. > > > > > > How do seccomp supervisors that use USER_NOTIF do those kinds of > > > memory writes currently? I thought they were actually using ptrace? > > > Everything I'm familiar with is just using SECCOMP_IOCTL_NOTIF_ADDFD, > > > and not doing fancy memory pokes. > > > > For example, incus has a seccomp supervisor such that each container > > gets it's own goroutine that is responsible for handling system call > > interception. > > > > If a container is started the container runtime connects to an AF_UNIX > > socket to register with the seccomp supervisor. It stays connected until > > it stops. Everytime a system call is performed that is registered in the > > seccomp notifier filter the container runtime will send a AF_UNIX > > message to the seccomp supervisor. This will include the following fds: > > > > - the pidfd of the task that performed the system call (we should > > actually replace this with SO_PEERPIDFD now that we have that) > > - the fd of the task's memory to /proc/<pid>/mem > > > > The seccomp supervisor will then perform the system call interception > > including the required memory reads and writes. > > Okay, so the patch would very much break that. Some questions, though: > - why not use process_vm_writev()? > - does the supervisor depend on FOLL_FORCE? > > Perhaps is is sufficient to block the use of FOLL_FORCE? > > I took a look at the Chrome OS exploit, and I _think_ it is depending > on the FOLL_FORCE behavior (it searches for a symbol to overwrite that > if I'm following correctly is in a read-only region), but some of the > binaries don't include source code, so I couldn't easily see what was > being injected. Mike or Adrian can you confirm this? I can't speak for what is acceptable for ChromeOS security because I'm not part of that project, so I'll let Mike answer whether blocking writes is mandatory for them or blocking FOLL_FORCE is enough. >From a design perspective, the question is whether to 1. block writes and allow known good exceptions or 2. allow writes and block known bad/exploitable exceptions. I am looking into reproducing and adding an exception for the container syscall intercept use-case raised by Christian, because I think it's easier to justify allowing known good exceptions from a security perspective. Otherwise I'm fine with both approaches. @Mike WDYT ?
