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Subject: Re: [PATCH] mm: optionally disable brk()
To:     David Hildenbrand <david@redhat.com>,
        David Laight <David.Laight@ACULAB.COM>,
        Michal Hocko <mhocko@suse.com>,
        Kees Cook <keescook@chromium.org>
Cc:     "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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From:   Topi Miettinen <toiwoton@gmail.com>
Message-ID: <ced9ebac-7222-1cd5-e1e4-d05b2f175984@gmail.com>
Date:   Sun, 1 Nov 2020 13:41:30 +0200
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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