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

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