[PATCH v23 23/28] x86/cet/shstk: Handle signals for shadow stack

Yu-cheng Yu
2021-03-16T15:12:31Z

To deliver a signal, create a shadow stack restore token and put the token
and the signal restorer address on the shadow stack. For sigreturn, verify
the token and restore from it the shadow stack pointer.

A shadow stack restore token marks a restore point of the shadow stack, and
the address in a token must point directly above the token, which is within
the same shadow stack. This is distinctively different from other pointers
on the shadow stack; those pointers point to executable code area.

In sigreturn, restoring from a token ensures the target address is the
location pointed by the token.

Introduce WRUSS, which is a kernel-mode instruction but writes directly to
user shadow stack. It is used to construct the user signal stack as
described above.

Currently there is no systematic facility for extending a signal context.
Introduce a signal context extension 'struct sc_ext', which is used to save
shadow stack restore token address and WAIT_ENDBR status. WAIT_ENDBR will
be introduced later in the Indirect Branch Tracking (IBT) series, but add
that into sc_ext now to keep the struct stable in case the IBT series is
applied later.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
arch/x86/ia32/ia32_signal.c | 17 +++
arch/x86/include/asm/cet.h | 8 ++
arch/x86/include/asm/fpu/internal.h | 10 ++
arch/x86/include/asm/special_insns.h | 32 ++++++
arch/x86/include/uapi/asm/sigcontext.h | 9 ++
arch/x86/kernel/cet.c | 152 +++++++++++++++++++++++++
arch/x86/kernel/fpu/signal.c | 100 ++++++++++++++++
arch/x86/kernel/signal.c | 10 ++
8 files changed, 338 insertions(+)

diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
index 5e3d9b7fd5fb..aee3e367e184 100644
--- a/arch/x86/ia32/ia32_signal.c
+++ b/arch/x86/ia32/ia32_signal.c
@@ -35,6 +35,7 @@
#include <asm/sigframe.h>
#include <asm/sighandling.h>
#include <asm/smap.h>
+#include <asm/cet.h>

static inline void reload_segments(struct sigcontext_32 *sc)
{
@@ -205,6 +206,7 @@ static void __user *get_sigframe(struct ksignal *ksig, struct pt_regs *regs,
void __user **fpstate)
{
unsigned long sp, fx_aligned, math_size;
+ void __user *restorer = NULL;

/* Default to using normal stack */
sp = regs->sp;
@@ -218,8 +220,23 @@ static void __user *get_sigframe(struct ksignal *ksig, struct pt_regs *regs,
ksig->ka.sa.sa_restorer)
sp = (unsigned long) ksig->ka.sa.sa_restorer;

+ if (ksig->ka.sa.sa_flags & SA_RESTORER) {
+ restorer = ksig->ka.sa.sa_restorer;
+ } else if (current->mm->context.vdso) {
+ if (ksig->ka.sa.sa_flags & SA_SIGINFO)
+ restorer = current->mm->context.vdso +
+ vdso_image_32.sym___kernel_rt_sigreturn;
+ else
+ restorer = current->mm->context.vdso +
+ vdso_image_32.sym___kernel_sigreturn;
+ }
+
sp = fpu__alloc_mathframe(sp, 1, &fx_aligned, &math_size);
*fpstate = (struct _fpstate_32 __user *) sp;
+
+ if (save_cet_to_sigframe(1, *fpstate, (unsigned long)restorer))
+ return (void __user *)-1L;
+
if (copy_fpstate_to_sigframe(*fpstate, (void __user *)fx_aligned,
math_size) < 0)
return (void __user *) -1L;
diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
index 5750fbcbb952..73435856ce54 100644
--- a/arch/x86/include/asm/cet.h
+++ b/arch/x86/include/asm/cet.h
@@ -6,6 +6,8 @@
#include <linux/types.h>

struct task_struct;
+struct sc_ext;
+
/*
* Per-thread CET status
*/
@@ -18,9 +20,15 @@ struct cet_status {
int cet_setup_shstk(void);
void cet_disable_shstk(void);
void cet_free_shstk(struct task_struct *p);
+int cet_verify_rstor_token(bool ia32, unsigned long ssp, unsigned long *new_ssp);
+void cet_restore_signal(struct sc_ext *sc);
+int cet_setup_signal(bool ia32, unsigned long rstor, struct sc_ext *sc);
#else
static inline void cet_disable_shstk(void) {}
static inline void cet_free_shstk(struct task_struct *p) {}
+static inline void cet_restore_signal(struct sc_ext *sc) { return; }
+static inline int cet_setup_signal(bool ia32, unsigned long rstor,
+ struct sc_ext *sc) { return -EINVAL; }
#endif

#endif /* __ASSEMBLY__ */
diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 8d33ad80704f..2c1f59ebe9d8 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -443,6 +443,16 @@ static inline void copy_kernel_to_fpregs(union fpregs_state *fpstate)
__copy_kernel_to_fpregs(fpstate, -1);
}

+#ifdef CONFIG_X86_CET
+extern int save_cet_to_sigframe(int ia32, void __user *fp,
+ unsigned long restorer);
+#else
+static inline int save_cet_to_sigframe(int ia32, void __user *fp,
+ unsigned long restorer)
+{
+ return 0;
+}
+#endif
extern int copy_fpstate_to_sigframe(void __user *buf, void __user *fp, int size);

/*
diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index 1d3cbaef4bb7..cc7b3c999366 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -234,6 +234,38 @@ static inline void clwb(volatile void *__p)
: [pax] "a" (p));
}

+#ifdef CONFIG_X86_CET
+#if defined(CONFIG_IA32_EMULATION) || defined(CONFIG_X86_X32)
+static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
+{
+ asm_volatile_goto("1: wrussd %1, (%0)\n"
+ _ASM_EXTABLE(1b, %l[fail])
+ :: "r" (addr), "r" (val)
+ :: fail);
+ return 0;
+fail:
+ return -EPERM;
+}
+#else
+static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
+{
+ WARN_ONCE(1, "%s used but not supported.\n", __func__);
+ return -EFAULT;
+}
+#endif
+
+static inline int write_user_shstk_64(unsigned long addr, unsigned long val)
+{
+ asm_volatile_goto("1: wrussq %1, (%0)\n"
+ _ASM_EXTABLE(1b, %l[fail])
+ :: "r" (addr), "r" (val)
+ :: fail);
+ return 0;
+fail:
+ return -EPERM;
+}
+#endif /* CONFIG_X86_CET */
+
#define nop() asm volatile ("nop")

static inline void serialize(void)
diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
index 844d60eb1882..cf2d55db3be4 100644
--- a/arch/x86/include/uapi/asm/sigcontext.h
+++ b/arch/x86/include/uapi/asm/sigcontext.h
@@ -196,6 +196,15 @@ struct _xstate {
/* New processor state extensions go here: */
};

+/*
+ * Located at the end of sigcontext->fpstate, aligned to 8.
+ */
+struct sc_ext {
+ unsigned long total_size;
+ unsigned long ssp;
+ unsigned long wait_endbr;
+};
+
/*
* The 32-bit signal frame:
*/
diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
index d25a03215984..08e43d9b5176 100644
--- a/arch/x86/kernel/cet.c
+++ b/arch/x86/kernel/cet.c
@@ -19,6 +19,8 @@
#include <asm/fpu/xstate.h>
#include <asm/fpu/types.h>
#include <asm/cet.h>
+#include <asm/special_insns.h>
+#include <uapi/asm/sigcontext.h>

static void start_update_msrs(void)
{
@@ -72,6 +74,80 @@ static unsigned long alloc_shstk(unsigned long size, int flags)
return addr;
}

+#define TOKEN_MODE_MASK 3UL
+#define TOKEN_MODE_64 1UL
+#define IS_TOKEN_64(token) (((token) & TOKEN_MODE_MASK) == TOKEN_MODE_64)
+#define IS_TOKEN_32(token) (((token) & TOKEN_MODE_MASK) == 0)
+
+/*
+ * Verify the restore token at the address of 'ssp' is
+ * valid and then set shadow stack pointer according to the
+ * token.
+ */
+int cet_verify_rstor_token(bool ia32, unsigned long ssp,
+ unsigned long *new_ssp)
+{
+ unsigned long token;
+
+ *new_ssp = 0;
+
+ if (!IS_ALIGNED(ssp, 8))
+ return -EINVAL;
+
+ if (get_user(token, (unsigned long __user *)ssp))
+ return -EFAULT;
+
+ /* Is 64-bit mode flag correct? */
+ if (!ia32 && !IS_TOKEN_64(token))
+ return -EINVAL;
+ else if (ia32 && !IS_TOKEN_32(token))
+ return -EINVAL;
+
+ token &= ~TOKEN_MODE_MASK;
+
+ /*
+ * Restore address properly aligned?
+ */
+ if ((!ia32 && !IS_ALIGNED(token, 8)) || !IS_ALIGNED(token, 4))
+ return -EINVAL;
+
+ /*
+ * Token was placed properly?
+ */
+ if (((ALIGN_DOWN(token, 8) - 8) != ssp) || token >= TASK_SIZE_MAX)
+ return -EINVAL;
+
+ *new_ssp = token;
+ return 0;
+}
+
+/*
+ * Create a restore token on the shadow stack.
+ * A token is always 8-byte and aligned to 8.
+ */
+static int create_rstor_token(bool ia32, unsigned long ssp,
+ unsigned long *new_ssp)
+{
+ unsigned long addr;
+
+ *new_ssp = 0;
+
+ if ((!ia32 && !IS_ALIGNED(ssp, 8)) || !IS_ALIGNED(ssp, 4))
+ return -EINVAL;
+
+ addr = ALIGN_DOWN(ssp, 8) - 8;
+
+ /* Is the token for 64-bit? */
+ if (!ia32)
+ ssp |= TOKEN_MODE_64;
+
+ if (write_user_shstk_64(addr, ssp))
+ return -EFAULT;
+
+ *new_ssp = addr;
+ return 0;
+}
+
int cet_setup_shstk(void)
{
unsigned long addr, size;
@@ -145,3 +221,79 @@ void cet_free_shstk(struct task_struct *tsk)
cet->shstk_base = 0;
cet->shstk_size = 0;
}
+
+/*
+ * Called from __fpu__restore_sig() and XSAVES buffer is protected by
+ * set_thread_flag(TIF_NEED_FPU_LOAD) in the slow path.
+ */
+void cet_restore_signal(struct sc_ext *sc_ext)
+{
+ struct cet_user_state *cet_user_state;
+ struct cet_status *cet = &current->thread.cet;
+ u64 msr_val = 0;
+
+ if (!static_cpu_has(X86_FEATURE_SHSTK))
+ return;
+
+ cet_user_state = get_xsave_addr(&current->thread.fpu.state.xsave,
+ XFEATURE_CET_USER);
+ if (!cet_user_state)
+ return;
+
+ if (cet->shstk_size) {
+ if (test_thread_flag(TIF_NEED_FPU_LOAD))
+ cet_user_state->user_ssp = sc_ext->ssp;
+ else
+ wrmsrl(MSR_IA32_PL3_SSP, sc_ext->ssp);
+
+ msr_val |= CET_SHSTK_EN;
+ }
+
+ if (test_thread_flag(TIF_NEED_FPU_LOAD))
+ cet_user_state->user_cet = msr_val;
+ else
+ wrmsrl(MSR_IA32_U_CET, msr_val);
+}
+
+/*
+ * Setup the shadow stack for the signal handler: first,
+ * create a restore token to keep track of the current ssp,
+ * and then the return address of the signal handler.
+ */
+int cet_setup_signal(bool ia32, unsigned long rstor_addr, struct sc_ext *sc_ext)
+{
+ struct cet_status *cet = &current->thread.cet;
+ unsigned long ssp = 0, new_ssp = 0;
+ int err;
+
+ if (cet->shstk_size) {
+ if (!rstor_addr)
+ return -EINVAL;
+
+ ssp = cet_get_shstk_addr();
+ err = create_rstor_token(ia32, ssp, &new_ssp);
+ if (err)
+ return err;
+
+ if (ia32) {
+ ssp = new_ssp - sizeof(u32);
+ err = write_user_shstk_32(ssp, (unsigned int)rstor_addr);
+ } else {
+ ssp = new_ssp - sizeof(u64);
+ err = write_user_shstk_64(ssp, rstor_addr);
+ }
+
+ if (err)
+ return err;
+
+ sc_ext->ssp = new_ssp;
+ }
+
+ if (ssp) {
+ start_update_msrs();
+ wrmsrl(MSR_IA32_PL3_SSP, ssp);
+ end_update_msrs();
+ }
+
+ return 0;
+}
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index a4ec65317a7f..270e4649f435 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -52,6 +52,74 @@ static inline int check_for_xstate(struct fxregs_state __user *buf,
return 0;
}

+#ifdef CONFIG_X86_CET
+int save_cet_to_sigframe(int ia32, void __user *fp, unsigned long restorer)
+{
+ int err = 0;
+
+ if (!current->thread.cet.shstk_size)
+ return 0;
+
+ if (fp) {
+ struct sc_ext ext = {};
+
+ err = cet_setup_signal(ia32, restorer, &ext);
+ if (!err) {
+ void __user *p = fp;
+
+ ext.total_size = sizeof(ext);
+
+ if (ia32)
+ p += sizeof(struct fregs_state);
+
+ p += fpu_user_xstate_size + FP_XSTATE_MAGIC2_SIZE;
+ p = (void __user *)ALIGN((unsigned long)p, 8);
+
+ if (copy_to_user(p, &ext, sizeof(ext)))
+ return -EFAULT;
+ }
+ }
+
+ return err;
+}
+
+static int get_cet_from_sigframe(int ia32, void __user *fp, struct sc_ext *ext)
+{
+ int err = 0;
+
+ memset(ext, 0, sizeof(*ext));
+
+ if (!current->thread.cet.shstk_size)
+ return 0;
+
+ if (fp) {
+ void __user *p = fp;
+
+ if (ia32)
+ p += sizeof(struct fregs_state);
+
+ p += fpu_user_xstate_size + FP_XSTATE_MAGIC2_SIZE;
+ p = (void __user *)ALIGN((unsigned long)p, 8);
+
+ if (copy_from_user(ext, p, sizeof(*ext)))
+ return -EFAULT;
+
+ if (ext->total_size != sizeof(*ext))
+ return -EFAULT;
+
+ if (current->thread.cet.shstk_size)
+ err = cet_verify_rstor_token(ia32, ext->ssp, &ext->ssp);
+ }
+
+ return err;
+}
+#else
+static int get_cet_from_sigframe(int ia32, void __user *fp, struct sc_ext *ext)
+{
+ return 0;
+}
+#endif
+
/*
* Signal frame handlers.
*/
@@ -295,6 +363,7 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
struct task_struct *tsk = current;
struct fpu *fpu = &tsk->thread.fpu;
struct user_i387_ia32_struct env;
+ struct sc_ext sc_ext;
u64 user_xfeatures = 0;
int fx_only = 0;
int ret = 0;
@@ -335,6 +404,10 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
if ((unsigned long)buf_fx % 64)
fx_only = 1;

+ ret = get_cet_from_sigframe(ia32_fxstate, buf, &sc_ext);
+ if (ret)
+ return ret;
+
if (!ia32_fxstate) {
/*
* Attempt to restore the FPU registers directly from user
@@ -349,6 +422,8 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
pagefault_enable();
if (!ret) {

+ cet_restore_signal(&sc_ext);
+
/*
* Restore supervisor states: previous context switch
* etc has done XSAVES and saved the supervisor states
@@ -423,6 +498,8 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
if (unlikely(init_bv))
copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);

+ cet_restore_signal(&sc_ext);
+
/*
* Restore previously saved supervisor xstates along with
* copied-in user xstates.
@@ -491,12 +568,35 @@ int fpu__restore_sig(void __user *buf, int ia32_frame)
return __fpu__restore_sig(buf, buf_fx, size);
}

+#ifdef CONFIG_X86_CET
+static unsigned long fpu__alloc_sigcontext_ext(unsigned long sp)
+{
+ struct cet_status *cet = &current->thread.cet;
+
+ /*
+ * sigcontext_ext is at: fpu + fpu_user_xstate_size +
+ * FP_XSTATE_MAGIC2_SIZE, then aligned to 8.
+ */
+ if (cet->shstk_size)
+ sp -= (sizeof(struct sc_ext) + 8);
+
+ return sp;
+}
+#else
+static unsigned long fpu__alloc_sigcontext_ext(unsigned long sp)
+{
+ return sp;
+}
+#endif
+
unsigned long
fpu__alloc_mathframe(unsigned long sp, int ia32_frame,
unsigned long *buf_fx, unsigned long *size)
{
unsigned long frame_size = xstate_sigframe_size();

+ sp = fpu__alloc_sigcontext_ext(sp);
+
*buf_fx = sp = round_down(sp - frame_size, 64);
if (ia32_frame && use_fxsr()) {
frame_size += sizeof(struct fregs_state);
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index ea794a083c44..1807379f1d86 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -46,6 +46,7 @@
#include <asm/syscall.h>
#include <asm/sigframe.h>
#include <asm/signal.h>
+#include <asm/cet.h>

#ifdef CONFIG_X86_64
/*
@@ -239,6 +240,9 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
unsigned long buf_fx = 0;
int onsigstack = on_sig_stack(sp);
int ret;
+#ifdef CONFIG_X86_64
+ void __user *restorer = NULL;
+#endif

/* redzone */
if (IS_ENABLED(CONFIG_X86_64))
@@ -270,6 +274,12 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
if (onsigstack && !likely(on_sig_stack(sp)))
return (void __user *)-1L;

+#ifdef CONFIG_X86_64
+ if (ka->sa.sa_flags & SA_RESTORER)
+ restorer = ka->sa.sa_restorer;
+ ret = save_cet_to_sigframe(0, *fpstate, (unsigned long)restorer);
+#endif
+
/* save i387 and extended state */
ret = copy_fpstate_to_sigframe(*fpstate, (void __user *)buf_fx, math_size);
if (ret < 0)
--
2.21.0