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stack_protector: Add tunable to bypass random cookies

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This is a stopgap measure to unbreak installer/VM/embedded boot issues
introduced (or at least exposed by) in r346250.

Add the new tunable, "security.stack_protect.permit_nonrandom_cookies," in
order to continue boot with insecure non-random stack cookies if the random
device is unavailable.

For now, enable it by default.  This is NOT safe.  It will be disabled by
default in a future revision.

There is follow-on work planned to use fast random sources (e.g., RDRAND on
x86 and DARN on Power) to seed when the early entropy file cannot be
provided, for whatever reason.  Please see D19928.

Some better hacks may be used to make the non-random __stack_chk_guard
slightly less predictable (from delphij@ and mjg@); those suggestions are
left for a future revision.  I think it may also be plausible to move stack
guard initialization far later in the boot process; potentially it could be
moved all the way to just before userspace is started.

Reported by:	many
Reviewed by:	delphij, emaste, imp (all w/ caveat: this is a stopgap fix)
Security:	yes
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D19927
This commit is contained in:
Conrad Meyer 2019-04-16 18:47:20 +00:00
parent f3d2512db6
commit ba57dad4b0
2 changed files with 55 additions and 3 deletions
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7
UPDATING
View File

@ -31,6 +31,13 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 13.x IS SLOW:
disable the most expensive debugging functionality run
"ln -s 'abort:false,junk:false' /etc/malloc.conf".)
20190416:
The tunable "security.stack_protect.permit_nonrandom_cookies" may be
set to a non-zero value to boot systems that do not provide early
entropy. Otherwise, such systems may see the panic message:
"cannot initialize stack cookies because random device is not yet
seeded."
20190416:
The loadable random module KPI has changed; the random_infra_init()
routine now requires a 3rd function pointer for a bool (*)(void)

51
sys/kern/stack_protector.c
View File

@ -4,12 +4,28 @@ __FBSDID("$FreeBSD$");
#include <sys/types.h>
#include <sys/param.h>
#include <sys/kernel.h>
#include <sys/random.h>
#include <sys/sysctl.h>
#include <sys/systm.h>
#include <sys/libkern.h>
long __stack_chk_guard[8] = {};
void __stack_chk_fail(void);
/*
* XXX This default is unsafe!!! We intend to change it after resolving issues
* with early entropy in the installer; some kinds of systems that do not use
* loader(8), such as riscv, aarch64, and power; and perhaps others that I am
* forgetting off the top of my head.
*/
static bool permit_nonrandom_cookies = true;
SYSCTL_NODE(_security, OID_AUTO, stack_protect, CTLFLAG_RW, 0,
"-fstack-protect support");
SYSCTL_BOOL(_security_stack_protect, OID_AUTO, permit_nonrandom_cookies,
CTLFLAG_RDTUN, &permit_nonrandom_cookies, 0,
"Allow stack guard to be used without real random cookies");
void
__stack_chk_fail(void)
{
@ -23,8 +39,37 @@ __stack_chk_init(void *dummy __unused)
size_t i;
long guard[nitems(__stack_chk_guard)];
arc4rand(guard, sizeof(guard), 0);
for (i = 0; i < nitems(guard); i++)
__stack_chk_guard[i] = guard[i];
if (is_random_seeded()) {
arc4rand(guard, sizeof(guard), 0);
for (i = 0; i < nitems(guard); i++)
__stack_chk_guard[i] = guard[i];
return;
}
if (permit_nonrandom_cookies) {
printf("%s: WARNING: Initializing stack protection with "
"non-random cookies!\n", __func__);
printf("%s: WARNING: This severely limits the benefit of "
"-fstack-protector!\n", __func__);
/*
* The emperor is naked, but I rolled some dice and at least
* these values aren't zero.
*/
__stack_chk_guard[0] = (long)0xe7318d5959af899full;
__stack_chk_guard[1] = (long)0x35a9481c089348bfull;
__stack_chk_guard[2] = (long)0xde657fdc04117255ull;
__stack_chk_guard[3] = (long)0x0dd44c61c22e4a6bull;
__stack_chk_guard[4] = (long)0x0a5869a354edb0a5ull;
__stack_chk_guard[5] = (long)0x05cebfed255b5232ull;
__stack_chk_guard[6] = (long)0x270ffac137c4c72full;
__stack_chk_guard[7] = (long)0xd8141a789bad478dull;
_Static_assert(nitems(__stack_chk_guard) == 8,
"__stack_chk_guard doesn't have 8 items");
return;
}
panic("%s: cannot initialize stack cookies because random device is "
"not yet seeded", __func__);
}
SYSINIT(stack_chk, SI_SUB_RANDOM, SI_ORDER_ANY, __stack_chk_init, NULL);