Re: [RFC PATCH 00/11] Adding FreeBSD's Capsicum security framework (part 1)

From: Paolo Bonzini
Date: Thu Jul 03 2014 - 05:12:52 EST

Il 30/06/2014 12:28, David Drysdale ha scritto:
Hi all,

The last couple of versions of FreeBSD (9.x/10.x) have included the
Capsicum security framework [1], which allows security-aware
applications to sandbox themselves in a very fine-grained way. For
example, OpenSSH now (>= 6.5) uses Capsicum in its FreeBSD version to
restrict sshd's credentials checking process, to reduce the chances of
credential leakage.

Hi David,

we've had similar goals in QEMU. QEMU can be used as a virtual machine monitor from the command line, but it also has an API that lets a management tool drive QEMU via AF_UNIX sockets. Long term, we would like to have a restricted mode for QEMU where all file descriptors are obtained via SCM_RIGHTS or /dev/fd, and syscalls can be locked down.

Currently we do use seccomp v2 BPF filters, but unfortunately this didn't help very much. QEMU supports hotplugging hence the filter must whitelist anything that _might_ be used in the future, which is generally... too much.

Something like Capsicum would be really nice because it attaches capabilities to file descriptors. However, I wonder however how extensible Capsicum could be, and I am worried about the proliferation of capabilities that its design naturally leads to.

Given Linux's previous experience with BPF filters, what do you think about attaching specific BPF programs to file descriptors? Then whenever a syscall is run that affects a file descriptor, the BPF program for the file descriptor (attached to a struct file* as in Capsicum) would run in addition to the process-wide filter.

An equivalent of PR_SET_NO_NEW_PRIVS can also be added to file descriptors, so that a program that doesn't lock down syscalls can still lock down the operations (including fcntls and ioctls) on specific file descriptors.

Converting FreeBSD capabilities to BPF programs can be easily implemented in userspace.

[Capsicum also includes 'capability mode', which locks down the
available syscalls so the rights restrictions can't just be bypassed
by opening new file descriptors; I'll describe that separately later.]

This can also be implemented in userspace via seccomp and PR_SET_NO_NEW_PRIVS.

[Policing the rights checks anywhere else, for example at the system
call boundary, isn't a good idea because it opens up the possibility
of time-of-check/time-of-use (TOCTOU) attacks [2] where FDs are
changed (as openat/close/dup2 are allowed in capability mode) between
the 'check' at syscall entry and the 'use' at fget() invocation.]

In the case of BPF filters, I wonder if you could stash the BPF "environment" somewhere and then use it at fget() invocation. Alternatively, it can be reconstructed at fget() time, similar to your introduction of fgetr().


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