[RFC PATCH v9 19/27] x86/cet/shstk: Handle signals for Shadow Stack

Yu-cheng Yu
2020-02-05T18:21:24Z

To deliver a signal, create a Shadow Stack (SHSTK) restore token and put
the token and the signal restorer address on the SHSTK. For sigreturn,
verify the token and restore the SHSTK pointer.

Introduce a signal context extension struct 'sc_ext', which is used to save
SHSTK 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.

v9:
- Update CET MSR access according to XSAVES supervisor state changes.
- Add 'wait_endbr' to struct 'sc_ext'.
- Update and simplify signal frame allocation, setup, and restoration.
- Update commit log text.

v2:
- Move CET status from sigcontext to a separate struct sc_ext, which is
located above the fpstate on the signal frame.
- Add a restore token for sigreturn address.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
arch/x86/ia32/ia32_signal.c | 17 +++
arch/x86/include/asm/cet.h | 7 ++
arch/x86/include/asm/fpu/internal.h | 2 +
arch/x86/include/uapi/asm/sigcontext.h | 9 ++
arch/x86/kernel/cet.c | 153 +++++++++++++++++++++++++
arch/x86/kernel/fpu/signal.c | 89 ++++++++++++++
arch/x86/kernel/signal.c | 10 ++
7 files changed, 287 insertions(+)

diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
index 30416d7f19d4..c0bb350a3d2d 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>

/*
* Do a signal return; undo the signal stack.
@@ -223,6 +224,7 @@ 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;
@@ -236,8 +238,23 @@ 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(*fpstate, (unsigned long)restorer, 1))
+ 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 c44c991ca91f..409d4f91a0dc 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
*/
@@ -18,8 +20,13 @@ struct cet_status {
#ifdef CONFIG_X86_INTEL_CET
int cet_setup_shstk(void);
void cet_disable_free_shstk(struct task_struct *p);
+int cet_restore_signal(bool ia32, struct sc_ext *sc);
+int cet_setup_signal(bool ia32, unsigned long rstor, struct sc_ext *sc);
#else
static inline void cet_disable_free_shstk(struct task_struct *p) {}
+static inline int cet_restore_signal(bool ia32, struct sc_ext *sc) { return -EINVAL; }
+static inline int cet_setup_signal(bool ia32, unsigned long rstor,
+ struct sc_ext *sc) { return -EINVAL; }
#endif

#define cpu_x86_cet_enabled() \
diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 42159f45bf9c..241521c0ed02 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -476,6 +476,8 @@ static inline void copy_kernel_to_fpregs(union fpregs_state *fpstate)
__copy_kernel_to_fpregs(fpstate, -1);
}

+extern int save_cet_to_sigframe(void __user *fp, unsigned long restorer,
+ int is_ia32);
extern int copy_fpstate_to_sigframe(void __user *buf, void __user *fp, int size);

/*
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 b4c7d88e9a8f..cba5c7656aab 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)
{
@@ -69,6 +71,80 @@ static unsigned long alloc_shstk(unsigned long size)
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.
+ */
+static int 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)
+ 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;
@@ -119,3 +195,80 @@ void cet_disable_free_shstk(struct task_struct *tsk)
cet->shstk_size = 0;
cet->shstk_enabled = 0;
}
+
+/*
+ * Called from __fpu__restore_sig() and XSAVES buffer is protected by
+ * set_thread_flag(TIF_NEED_FPU_LOAD).
+ */
+int cet_restore_signal(bool ia32, struct sc_ext *sc_ext)
+{
+ struct cet_user_state *cet_user_state;
+ struct cet_status *cet = &current->thread.cet;
+ unsigned long new_ssp = 0;
+ u64 msr_val = 0;
+ int err;
+
+ if (!cet->shstk_enabled)
+ return 0;
+
+ cet_user_state = get_xsave_addr(&current->thread.fpu.state.xsave,
+ XFEATURE_CET_USER);
+ if (!cet_user_state)
+ return -1;
+
+ if (cet->shstk_enabled) {
+ err = verify_rstor_token(ia32, sc_ext->ssp, &new_ssp);
+ if (err)
+ return err;
+
+ cet_user_state->user_ssp = new_ssp;
+ msr_val |= MSR_IA32_CET_SHSTK_EN;
+ }
+
+ cet_user_state->user_cet = msr_val;
+ return 0;
+}
+
+/*
+ * 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_enabled)
+ return 0;
+
+ if (cet->shstk_enabled) {
+ 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;
+ }
+
+ start_update_msrs();
+ if (cet->shstk_enabled)
+ 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 0d3e06a772b0..875cc0fadce3 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -52,6 +52,69 @@ static inline int check_for_xstate(struct fxregs_state __user *buf,
return 0;
}

+int save_cet_to_sigframe(void __user *fp, unsigned long restorer, int is_ia32)
+{
+ int err = 0;
+
+#ifdef CONFIG_X86_INTEL_CET
+ if (!current->thread.cet.shstk_enabled)
+ return 0;
+
+ if (fp) {
+ struct sc_ext ext = {0, 0, 0};
+
+ err = cet_setup_signal(is_ia32, restorer, &ext);
+ if (!err) {
+ void __user *p = fp;
+
+ ext.total_size = sizeof(ext);
+
+ if (is_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;
+ }
+ }
+#endif
+
+ return err;
+}
+
+static int restore_cet_from_sigframe(int is_ia32, void __user *fp)
+{
+ int err = 0;
+
+#ifdef CONFIG_X86_INTEL_CET
+ if (!current->thread.cet.shstk_enabled)
+ return 0;
+
+ if (fp) {
+ struct sc_ext ext = {0, 0, 0};
+ void __user *p = fp;
+
+ if (is_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;
+
+ err = cet_restore_signal(is_ia32, &ext);
+ }
+#endif
+
+ return err;
+}
+
/*
* Signal frame handlers.
*/
@@ -367,6 +430,10 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
pagefault_disable();
ret = copy_user_to_fpregs_zeroing(buf_fx, xfeatures_user, fx_only);
pagefault_enable();
+
+ if (!ret)
+ ret = restore_cet_from_sigframe(0, buf);
+
if (!ret) {
if (xfeatures_mask_supervisor())
copy_kernel_to_xregs(&fpu->state.xsave,
@@ -397,6 +464,10 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
sanitize_restored_user_xstate(&fpu->state, envp, xfeatures_user,
fx_only);

+ ret = restore_cet_from_sigframe((int)ia32_fxstate, buf);
+ if (ret)
+ goto err_out;
+
fpregs_lock();
if (unlikely(init_bv))
copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
@@ -468,12 +539,30 @@ int fpu__restore_sig(void __user *buf, int ia32_frame)
return __fpu__restore_sig(buf, buf_fx, size);
}

+static unsigned long fpu__alloc_sigcontext_ext(unsigned long sp)
+{
+ /*
+ * sigcontext_ext is at: fpu + fpu_user_xstate_size +
+ * FP_XSTATE_MAGIC2_SIZE, then aligned to 8.
+ */
+ if (cpu_x86_cet_enabled()) {
+ struct cet_status *cet = &current->thread.cet;
+
+ if (cet->shstk_enabled)
+ sp -= (sizeof(struct sc_ext) + 8);
+ }
+
+ return sp;
+}
+
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 ce9421ec285f..b26f5084a8a1 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -46,6 +46,7 @@

#include <asm/sigframe.h>
#include <asm/signal.h>
+#include <asm/cet.h>

#define COPY(x) do { \
get_user_ex(regs->x, &sc->x); \
@@ -246,6 +247,9 @@ 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))
@@ -277,6 +281,12 @@ 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(*fpstate, (unsigned long)restorer, 0);
+#endif
+
/* save i387 and extended state */
ret = copy_fpstate_to_sigframe(*fpstate, (void __user *)buf_fx, math_size);
if (ret < 0)
--
2.21.0