[PATCH v2] proc: allow restricting /proc/pid/mem writes

[Matt Denton]

The SECCOMP_RET_USER_NOTIF sandbox is partially implemented but the
reason we needed it (glibc blocking signals during certain syscalls we
wanted to emulate) got reverted and we haven't had any important
issues with the SECCOMP_RET_TRAP sandbox since then. /proc/pid/mem was
always restricted on ChromeOS so the plan was to use
process_vm_readv() and process_vm_writev() in the unsandboxed broker
process. We knew about the pid race of course, but this would be far
from the only place that Chrome would be potentially vulnerable to the
race so it didn't seem any worse.

We did need to use process_vm_writev() for some syscalls, like
emulating stat() required us to write into the supervised process.

On Tue, Mar 5, 2024 at 3:03 AM Christian Brauner <brauner at kernel.org> wrote:
>
> On Tue, Mar 05, 2024 at 10:58:31AM +0100, Christian Brauner wrote:
> > On Tue, Mar 05, 2024 at 01:41:29AM -0800, Kees Cook 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()?
> >
> > Because it's inherently racy as I've explained in an earlier mail in
> > this thread. Opening /proc/<pid>/mem we can guard via:
> >
> > // Assume we hold @pidfd for supervised process
> >
> > int fd_mem = open("/proc/$pid/mem", O_RDWR);:
> >
> > if (pidfd_send_signal(pidfd, 0, ...) == 0)
> >         write(fd_mem, ...);
> >
> > But we can't exactly do:
> >
> > process_vm_writev(pid, WRITE_TO_MEMORY, ...);
> > if (pidfd_send_signal(pidfd, 0, ...) == 0)
> >         write(fd_mem, ...);
> >
> > That's always racy. The process might have been reaped before we even
> > call pidfd_send_signal() and we're writing to some random process
> > memory.
> >
> > If we wanted to support this we'd need to implement a proposal I had a
> > while ago:
> >
> > #define PROCESS_VM_RW_PIDFD (1 << 0)
> >
> > process_vm_readv(pidfd,  ..., PROCESS_VM_RW_PIDFD);
> > process_vm_writev(pidfd, ..., PROCESS_VM_RW_PIDFD);
> >
> > which is similar to what we did for waitid(pidfd, P_PIDFD, ...)
> >
> > That would make it possible to use a pidfd instead of a pid in the two
> > system calls. Then we can get rid of the raciness and actually use those
> > system calls. As they are now, we can't.
>
> What btw, is the Linux sandbox on Chromium doing? Did they finally move
> away from SECCOMP_RET_TRAP to SECCOMP_RET_USER_NOTIF? I see:
>
> https://issues.chromium.org/issues/40145101
>
> What ever became of this?