> On May 31, 2020, at 11:57 AM, Andy Lutomirski <l...@kernel.org> wrote: > > Using SECCOMP_RET_USER_NOTIF is likely to be considerably more > expensive than my scheme. On a non-PTI system, my approach will add a > few tens of ns to each syscall. On a PTI system, it will be worse. > But using any kind of notifier for all syscalls will cause a context > switch to a different user program for each syscall, and that will be > much slower.
There’s also no way (at least to my understanding) to modify register state from SECCOMP_RET_USER_NOTIF, which is how the existing -staging SIGSYS handler works: <https://github.com/wine-staging/wine-staging/blob/master/patches/ntdll-Syscall_Emulation/0001-ntdll-Support-x86_64-syscall-emulation.patch#L62> > I think that the implementation may well want to live in seccomp, but > doing this as a seccomp filter isn't quite right. It's not a security > thing -- it's an emulation thing. Seccomp is all about making > inescapable sandboxes, but that's not what you're doing at all, and > the fact that seccomp filters are preserved across execve() sounds > like it'll be annoying for you. Definitely. Regardless of what approach is taken, we don’t want it to persist across execve. > What if there was a special filter type that ran a BPF program on each > syscall, and the program was allowed to access user memory to make its > decisions, e.g. to look at some list of memory addresses. But this > would explicitly *not* be a security feature -- execve() would remove > the filter, and the filter's outcome would be one of redirecting > execution or allowing the syscall. If the "allow" outcome occurs, > then regular seccomp filters run. Obviously the exact semantics here > would need some care. Although if that’s running a BPF filter on every syscall, wouldn’t it also incur the ~10% overhead that Paul and Gabriel have seen with existing seccomp? Brendan Shanks CodeWeavers
