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glibc/sysdeps/x86/dl-cet.c
H.J. Lu 541641a3de x86/cet: Enable shadow stack during startup 
Previously, CET was enabled by kernel before passing control to user
space and the startup code must disable CET if applications or shared
libraries aren't CET enabled.  Since the current kernel only supports
shadow stack and won't enable shadow stack before passing control to
user space, we need to enable shadow stack during startup if the
application and all shared library are shadow stack enabled.  There
is no need to disable shadow stack at startup.  Shadow stack can only
be enabled in a function which will never return.  Otherwise, shadow
stack will underflow at the function return.

1. GL(dl_x86_feature_1) is set to the CET features which are supported
by the processor and are not disabled by the tunable.  Only non-zero
features in GL(dl_x86_feature_1) should be enabled.  After enabling
shadow stack with ARCH_SHSTK_ENABLE, ARCH_SHSTK_STATUS is used to check
if shadow stack is really enabled.
2. Use ARCH_SHSTK_ENABLE in RTLD_START in dynamic executable.  It is
safe since RTLD_START never returns.
3. Call arch_prctl (ARCH_SHSTK_ENABLE) from ARCH_SETUP_TLS in static
executable.  Since the start function using ARCH_SETUP_TLS never returns,
it is safe to enable shadow stack in ARCH_SETUP_TLS.
2024-01-01 05:22:48 -08:00

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/* x86 CET initializers function.
Copyright (C) 2018-2023 Free Software Foundation, Inc.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include <unistd.h>
#include <errno.h>
#include <libintl.h>
#include <ldsodefs.h>
#include <dl-cet.h>
#include <sys/single_threaded.h>
/* GNU_PROPERTY_X86_FEATURE_1_IBT and GNU_PROPERTY_X86_FEATURE_1_SHSTK
are defined in <elf.h>, which are only available for C sources.
X86_FEATURE_1_IBT and X86_FEATURE_1_SHSTK are defined in <sysdep.h>
which are available for both C and asm sources. They must match. */
#if GNU_PROPERTY_X86_FEATURE_1_IBT != X86_FEATURE_1_IBT
# error GNU_PROPERTY_X86_FEATURE_1_IBT != X86_FEATURE_1_IBT
#endif
#if GNU_PROPERTY_X86_FEATURE_1_SHSTK != X86_FEATURE_1_SHSTK
# error GNU_PROPERTY_X86_FEATURE_1_SHSTK != X86_FEATURE_1_SHSTK
#endif
struct dl_cet_info
{
const char *program;
/* Check how IBT and SHSTK should be enabled. */
enum dl_x86_cet_control enable_ibt_type;
enum dl_x86_cet_control enable_shstk_type;
/* If IBT and SHSTK were previously enabled. */
unsigned int feature_1_enabled;
/* If IBT and SHSTK should be enabled. */
unsigned int enable_feature_1;
/* If there are any legacy shared object. */
unsigned int feature_1_legacy;
/* Which shared object is the first legacy shared object. */
unsigned int feature_1_legacy_ibt;
unsigned int feature_1_legacy_shstk;
};
/* Check if the object M and its dependencies are legacy object. */
static void
dl_check_legacy_object (struct link_map *m,
struct dl_cet_info *info)
{
unsigned int i;
struct link_map *l = NULL;
i = m->l_searchlist.r_nlist;
while (i-- > 0)
{
/* Check each shared object to see if IBT and SHSTK are enabled. */
l = m->l_initfini[i];
if (l->l_init_called)
continue;
#ifdef SHARED
/* Skip check for ld.so since it has the features enabled. The
features will be disabled later if they are not enabled in
executable. */
if (l == &GL(dl_rtld_map)
|| l->l_real == &GL(dl_rtld_map)
|| (info->program != NULL && l == m))
continue;
#endif
/* IBT and SHSTK set only if enabled in executable and all DSOs.
NB: cet_always_on is handled outside of the loop. */
info->enable_feature_1 &= ((l->l_x86_feature_1_and
& (GNU_PROPERTY_X86_FEATURE_1_IBT
| GNU_PROPERTY_X86_FEATURE_1_SHSTK))
| ~(GNU_PROPERTY_X86_FEATURE_1_IBT
| GNU_PROPERTY_X86_FEATURE_1_SHSTK));
if ((info->feature_1_legacy
& GNU_PROPERTY_X86_FEATURE_1_IBT) == 0
&& ((info->enable_feature_1
& GNU_PROPERTY_X86_FEATURE_1_IBT)
!= (info->feature_1_enabled
& GNU_PROPERTY_X86_FEATURE_1_IBT)))
{
info->feature_1_legacy_ibt = i;
info->feature_1_legacy |= GNU_PROPERTY_X86_FEATURE_1_IBT;
}
if ((info->feature_1_legacy
& GNU_PROPERTY_X86_FEATURE_1_SHSTK) == 0
&& ((info->enable_feature_1
& GNU_PROPERTY_X86_FEATURE_1_SHSTK)
!= (info->feature_1_enabled
& GNU_PROPERTY_X86_FEATURE_1_SHSTK)))
{
info->feature_1_legacy_shstk = i;
info->feature_1_legacy |= GNU_PROPERTY_X86_FEATURE_1_SHSTK;
}
}
/* Handle cet_always_on. */
if ((info->feature_1_enabled
& GNU_PROPERTY_X86_FEATURE_1_IBT) != 0
&& info->enable_ibt_type == cet_always_on)
{
info->feature_1_legacy &= ~GNU_PROPERTY_X86_FEATURE_1_IBT;
info->enable_feature_1 |= GNU_PROPERTY_X86_FEATURE_1_IBT;
}
if ((info->feature_1_enabled
& GNU_PROPERTY_X86_FEATURE_1_SHSTK) != 0
&& info->enable_shstk_type == cet_always_on)
{
info->feature_1_legacy &= ~GNU_PROPERTY_X86_FEATURE_1_SHSTK;
info->enable_feature_1 |= GNU_PROPERTY_X86_FEATURE_1_SHSTK;
}
}
#ifdef SHARED
/* Enable IBT and SHSTK only if they are enabled in executable. Set
feature bits properly at the start of the program. */
static void
dl_cet_check_startup (struct link_map *m, struct dl_cet_info *info)
{
/* NB: IBT and SHSTK may be disabled by environment variable:
GLIBC_TUNABLES=glibc.cpu.hwcaps=-IBT,-SHSTK.
*/
if (CPU_FEATURE_USABLE (IBT))
{
if (info->enable_ibt_type == cet_always_on)
info->enable_feature_1 |= GNU_PROPERTY_X86_FEATURE_1_IBT;
else
info->enable_feature_1 &= ((m->l_x86_feature_1_and
& GNU_PROPERTY_X86_FEATURE_1_IBT)
| ~GNU_PROPERTY_X86_FEATURE_1_IBT);
}
else
info->enable_feature_1 &= ~GNU_PROPERTY_X86_FEATURE_1_IBT;
if (CPU_FEATURE_USABLE (SHSTK))
{
if (info->enable_shstk_type == cet_always_on)
info->enable_feature_1 |= GNU_PROPERTY_X86_FEATURE_1_SHSTK;
else
info->enable_feature_1 &= ((m->l_x86_feature_1_and
& GNU_PROPERTY_X86_FEATURE_1_SHSTK)
| ~GNU_PROPERTY_X86_FEATURE_1_SHSTK);
}
else
info->enable_feature_1 &= ~GNU_PROPERTY_X86_FEATURE_1_SHSTK;
if (info->enable_feature_1 != 0)
dl_check_legacy_object (m, info);
unsigned int disable_feature_1
= info->enable_feature_1 ^ info->feature_1_enabled;
if (disable_feature_1 != 0)
{
/* Clear the disabled bits. Sync dl_x86_feature_1 and
info->feature_1_enabled with info->enable_feature_1. */
info->feature_1_enabled = info->enable_feature_1;
GL(dl_x86_feature_1) = info->enable_feature_1;
}
}
#endif
/* Check feature bits when dlopening the shared object M. */
static void
dl_cet_check_dlopen (struct link_map *m, struct dl_cet_info *info)
{
/* Check if there are any legacy objects loaded. */
if (info->enable_feature_1 != 0)
{
dl_check_legacy_object (m, info);
/* Skip if there are no legacy shared objects loaded. */
if (info->feature_1_legacy == 0)
return;
}
unsigned int disable_feature_1 = 0;
unsigned int legacy_obj = 0;
const char *msg = NULL;
if ((info->feature_1_enabled
& GNU_PROPERTY_X86_FEATURE_1_IBT) != 0
&& (info->feature_1_legacy
& GNU_PROPERTY_X86_FEATURE_1_IBT) != 0)
{
/* Don't disable IBT if not single threaded since IBT may be still
enabled in other threads. */
if (info->enable_ibt_type != cet_permissive
|| !SINGLE_THREAD_P)
{
legacy_obj = info->feature_1_legacy_ibt;
msg = N_("rebuild shared object with IBT support enabled");
}
else
disable_feature_1 |= GNU_PROPERTY_X86_FEATURE_1_IBT;
}
/* Check the next feature only if there is no error. */
if (msg == NULL
&& (info->feature_1_enabled
& GNU_PROPERTY_X86_FEATURE_1_SHSTK) != 0
&& (info->feature_1_legacy
& GNU_PROPERTY_X86_FEATURE_1_SHSTK) != 0)
{
/* Don't disable SHSTK if not single threaded since SHSTK may be
still enabled in other threads. */
if (info->enable_shstk_type != cet_permissive
|| !SINGLE_THREAD_P)
{
legacy_obj = info->feature_1_legacy_shstk;
msg = N_("rebuild shared object with SHSTK support enabled");
}
else
disable_feature_1 |= GNU_PROPERTY_X86_FEATURE_1_SHSTK;
}
/* If there is an error, long jump back to the caller. */
if (msg != NULL)
_dl_signal_error (0, m->l_initfini[legacy_obj]->l_name, "dlopen",
msg);
if (disable_feature_1 != 0)
{
int res = dl_cet_disable_cet (disable_feature_1);
if (res)
{
if ((disable_feature_1
& GNU_PROPERTY_X86_FEATURE_1_IBT) != 0)
msg = N_("can't disable IBT");
else
msg = N_("can't disable SHSTK");
/* Long jump back to the caller on error. */
_dl_signal_error (-res, m->l_initfini[legacy_obj]->l_name,
"dlopen", msg);
}
/* Clear the disabled bits in dl_x86_feature_1. */
GL(dl_x86_feature_1) &= ~disable_feature_1;
THREAD_SETMEM (THREAD_SELF, header.feature_1,
GL(dl_x86_feature_1));
}
}
static void
dl_cet_check (struct link_map *m, const char *program)
{
struct dl_cet_info info;
/* CET is enabled only if RTLD_START_ENABLE_X86_FEATURES is defined. */
#if defined SHARED && defined RTLD_START_ENABLE_X86_FEATURES
/* Set dl_x86_feature_1 to features enabled in the executable. */
if (program != NULL)
GL(dl_x86_feature_1) = (m->l_x86_feature_1_and
& (X86_FEATURE_1_IBT
| X86_FEATURE_1_SHSTK));
#endif
/* Check how IBT and SHSTK should be enabled. */
info.enable_ibt_type = GL(dl_x86_feature_control).ibt;
info.enable_shstk_type = GL(dl_x86_feature_control).shstk;
info.feature_1_enabled = GL(dl_x86_feature_1);
/* No legacy object check if IBT and SHSTK are always on. */
if (info.enable_ibt_type == cet_always_on
&& info.enable_shstk_type == cet_always_on)
return;
/* Check if IBT and SHSTK were enabled. */
if (info.feature_1_enabled == 0)
return;
info.program = program;
/* Check which features should be enabled. */
info.enable_feature_1 = 0;
if (info.enable_ibt_type != cet_always_off)
info.enable_feature_1 |= (info.feature_1_enabled
& GNU_PROPERTY_X86_FEATURE_1_IBT);
if (info.enable_shstk_type != cet_always_off)
info.enable_feature_1 |= (info.feature_1_enabled
& GNU_PROPERTY_X86_FEATURE_1_SHSTK);
/* Start with no legacy objects. */
info.feature_1_legacy = 0;
info.feature_1_legacy_ibt = 0;
info.feature_1_legacy_shstk = 0;
#ifdef SHARED
if (program)
dl_cet_check_startup (m, &info);
else
#endif
dl_cet_check_dlopen (m, &info);
}
void
_dl_cet_open_check (struct link_map *l)
{
dl_cet_check (l, NULL);
}
/* Set GL(dl_x86_feature_1) to the enabled features and clear the
active bits of the disabled features. */
attribute_hidden void
_dl_cet_setup_features (unsigned int cet_feature)
{
/* NB: cet_feature == GL(dl_x86_feature_1) which is set to features
enabled from executable, not necessarily supported by kernel. */
if (cet_feature != 0)
{
cet_feature = dl_cet_get_cet_status ();
if (cet_feature != 0)
{
THREAD_SETMEM (THREAD_SELF, header.feature_1, cet_feature);
/* Lock CET if IBT or SHSTK is enabled in executable. Don't
lock CET if IBT or SHSTK is enabled permissively. */
if (GL(dl_x86_feature_control).ibt != cet_permissive
&& (GL(dl_x86_feature_control).shstk != cet_permissive))
dl_cet_lock_cet (cet_feature);
}
/* Sync GL(dl_x86_feature_1) with kernel. */
GL(dl_x86_feature_1) = cet_feature;
}
}
#ifdef SHARED
# ifndef LINKAGE
# define LINKAGE
# endif
LINKAGE
void
_dl_cet_check (struct link_map *main_map, const char *program)
{
dl_cet_check (main_map, program);
}
#endif /* SHARED */
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