Re: [PATCH] mm: optionally disable brk()

[Topi Miettinen]

On 5.10.2020 15.18, David Hildenbrand wrote:
> On 05.10.20 13:21, David Laight wrote:
> > From: David Hildenbrand
> > > Sent: 05 October 2020 10:55
> > ...
> > > > If hardening and compatibility are seen as tradeoffs, perhaps there
> > > > could be a top level config choice (CONFIG_HARDENING_TRADEOFF) for this.
> > > > It would have options
> > > > - "compatibility" (default) to gear questions for maximum compatibility,
> > > > deselecting any hardening options which reduce compatibility
> > > > - "hardening" to gear questions for maximum hardening, deselecting any
> > > > compatibility options which reduce hardening
> > > > - "none/manual": ask all questions like before
> > >
> > > I think the general direction is to avoid an exploding set of config
> > > options. So if there isn't a *real* demand, I guess gluing this to a
> > > single option ("CONFIG_SECURITY_HARDENING") might be good enough.
> >
> > Wouldn't that be better achieved by run-time clobbering
> > of the syscall vectors?
>
> You mean via something like a boot parameter? Possibly yes.

This may be obvious, but a global seccomp filter which doesn't affect 
NNP can be installed in initrd with a simple program with no changes to 
kernel:

#include <errno.h>
#include <seccomp.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char **argv) {
        if (argc < 3) {
                fprintf(stderr, "Usage: %s syscall [syscall]... 
program\n", argv[0]);
                return EXIT_FAILURE;
        }

        scmp_filter_ctx ctx = seccomp_init(SCMP_ACT_ALLOW);

        if (ctx == NULL) {
                fprintf(stderr, "failed to init filter\n");
                return EXIT_FAILURE;
        }

        int r;
        r = seccomp_attr_set(ctx, SCMP_FLTATR_CTL_NNP, 0);
        if (r != 0) {
                fprintf(stderr, "failed to disable NNP\n");
                return EXIT_FAILURE;
        }

        fprintf(stderr, "filtering");
        for (int i = 1; i < argc - 1; i++) {
                const char *syscall = argv[i];

                int syscall_nr = seccomp_syscall_resolve_name(syscall);

                if (syscall_nr == __NR_SCMP_ERROR) {
                        //fprintf(stderr, "unknown syscall %s, 
ignoring\n", syscall);
                        continue;
                }
                r = seccomp_rule_add_exact(ctx, SCMP_ACT_ERRNO(ENOSYS), 
syscall_nr, 0);
                if (r != 0) {
                        //fprintf(stderr, "failed to filter syscall %s, 
ignoring\n", syscall);
                        continue;
                }
                fprintf(stderr, " %s", syscall);
        }
        fprintf(stderr, "\n");
        r = seccomp_load(ctx);
        if (r != 0) {
                fprintf(stderr, "failed to apply filter\n");
                return EXIT_FAILURE;
        }

        seccomp_release(ctx);

        char *program = argv[argc - 1];
        char *new_argv[] = { program, NULL };

        execv(program, new_argv);

        fprintf(stderr, "failed to exec %s\n", program);
        return EXIT_FAILURE;
}

This can be inserted in initrd to disable some obsolete and old system 
calls like this:
#!/bin/sh

exec /usr/local/sbin/seccomp-exec _sysctl afs_syscall bdflush break 
create_module ftime get_kernel_syms getpmsg gtty idle lock mpx prof 
profil putpmsg query_module security sgetmask ssetmask stty sysfs 
tuxcall ulimit uselib ustat vserver epoll_ctl_old epoll_wait_old 
old_adjtimex old_getpagesize oldfstat oldlstat oldolduname oldstat 
oldumount olduname osf_old_creat osf_old_fstat osf_old_getpgrp 
osf_old_killpg osf_old_lstat osf_old_open osf_old_sigaction 
osf_old_sigblock osf_old_sigreturn osf_old_sigsetmask osf_old_sigvec 
osf_old_stat osf_old_vadvise osf_old_vtrace osf_old_wait osf_oldquota 
vm86old brk /init

-Topi