Date: Tue, 03 Sep 2019 14:07:50 -0000 From: Conrad Meyer <cem@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r346292 - in head: . sys/kern Message-ID: <201904161847.x3GIlKir077013@repo.freebsd.org>
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Author: cem Date: Tue Apr 16 18:47:20 2019 New Revision: 346292 URL: https://svnweb.freebsd.org/changeset/base/346292 Log: stack_protector: Add tunable to bypass random cookies 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 Modified: head/UPDATING head/sys/kern/stack_protector.c Modified: head/UPDATING ============================================================================== --- head/UPDATING Tue Apr 16 18:32:07 2019 (r346291) +++ head/UPDATING Tue Apr 16 18:47:20 2019 (r346292) @@ -32,6 +32,13 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 13.x IS SLOW: "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) method that returns true if the random device is seeded (and Modified: head/sys/kern/stack_protector.c ============================================================================== --- head/sys/kern/stack_protector.c Tue Apr 16 18:32:07 2019 (r346291) +++ head/sys/kern/stack_protector.c Tue Apr 16 18:47:20 2019 (r346292) @@ -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);
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