[PATCH v8 21/27] x86/cet/shstk: Handle signals for shadow stack

From: Yu-cheng Yu
Date: Tue Aug 13 2019 - 17:03:42 EST

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When setting up a signal, the kernel creates a shadow stack restore
token at the current SHSTK address and then stores the token's
address in the signal frame, right after the FPU state. Before
restoring a signal, the kernel verifies and then uses the restore
token to set the SHSTK pointer.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
---
arch/x86/ia32/ia32_signal.c | 8 ++
arch/x86/include/asm/cet.h | 7 ++
arch/x86/include/asm/fpu/internal.h | 2 +
arch/x86/include/asm/fpu/signal.h | 2 +
arch/x86/include/uapi/asm/sigcontext.h | 15 +++
arch/x86/kernel/cet.c | 141 +++++++++++++++++++++++++
arch/x86/kernel/fpu/signal.c | 67 ++++++++++++
arch/x86/kernel/signal.c | 8 ++
8 files changed, 250 insertions(+)

diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
index 1cee10091b9f..73a0c1dc3123 100644
--- a/arch/x86/ia32/ia32_signal.c
+++ b/arch/x86/ia32/ia32_signal.c
@@ -34,6 +34,7 @@
#include <asm/sigframe.h>
#include <asm/sighandling.h>
#include <asm/smap.h>
+#include <asm/cet.h>

/*
* Do a signal return; undo the signal stack.
@@ -235,6 +236,7 @@ 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;

+ sp = fpu__alloc_sigcontext_ext(sp);
sp = fpu__alloc_mathframe(sp, 1, &fx_aligned, &math_size);
*fpstate = (struct _fpstate_32 __user *) sp;
if (copy_fpstate_to_sigframe(*fpstate, (void __user *)fx_aligned,
@@ -295,6 +297,9 @@ int ia32_setup_frame(int sig, struct ksignal *ksig,
restorer = &frame->retcode;
}

+ if (setup_fpu_system_states(1, (unsigned long)restorer, fpstate))
+ return -EFAULT;
+
put_user_try {
put_user_ex(ptr_to_compat(restorer), &frame->pretcode);

@@ -384,6 +389,9 @@ int ia32_setup_rt_frame(int sig, struct ksignal *ksig,
regs, set->sig[0]);
err |= __copy_to_user(&frame->uc.uc_sigmask, set, sizeof(*set));

+ if (!err)
+ err = setup_fpu_system_states(1, (unsigned long)restorer, fpstate);
+
if (err)
return -EFAULT;

diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
index c952a2ec65fe..422ccb8adbb7 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
*/
@@ -19,10 +21,15 @@ struct cet_status {
int cet_setup_shstk(void);
void cet_disable_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 int cet_setup_shstk(void) { return -EINVAL; }
static inline void cet_disable_shstk(void) {}
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 2ca5c36a77d5..8a40d676c448 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -472,6 +472,8 @@ static inline void copy_kernel_to_fpregs(union fpregs_state *fpstate)
__copy_kernel_to_fpregs(fpstate, -1);
}

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

/*
diff --git a/arch/x86/include/asm/fpu/signal.h b/arch/x86/include/asm/fpu/signal.h
index 7fb516b6893a..630a658aeea3 100644
--- a/arch/x86/include/asm/fpu/signal.h
+++ b/arch/x86/include/asm/fpu/signal.h
@@ -25,6 +25,8 @@ extern void convert_from_fxsr(struct user_i387_ia32_struct *env,
extern void convert_to_fxsr(struct fxregs_state *fxsave,
const struct user_i387_ia32_struct *env);

+unsigned long fpu__alloc_sigcontext_ext(unsigned long sp);
+
unsigned long
fpu__alloc_mathframe(unsigned long sp, int ia32_frame,
unsigned long *buf_fx, unsigned long *size);
diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
index 844d60eb1882..e3b08d1c0d3b 100644
--- a/arch/x86/include/uapi/asm/sigcontext.h
+++ b/arch/x86/include/uapi/asm/sigcontext.h
@@ -196,6 +196,21 @@ struct _xstate {
/* New processor state extensions go here: */
};

+/*
+ * Sigcontext extension (struct sc_ext) is located after
+ * sigcontext->fpstate. Because currently only the shadow
+ * stack pointer is saved there and the shadow stack depends
+ * on XSAVES, we can find sc_ext from sigcontext->fpstate.
+ *
+ * The 64-bit fpstate has a size of fpu_user_xstate_size, plus
+ * FP_XSTATE_MAGIC2_SIZE when XSAVE* is used. The struct sc_ext
+ * is located at the end of sigcontext->fpstate, aligned to 8.
+ */
+struct sc_ext {
+ unsigned long total_size;
+ unsigned long ssp;
+};
+
/*
* The 32-bit signal frame:
*/
diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
index 5e7af0cc75f9..f1cc8f4c57b8 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 int set_shstk_ptr(unsigned long addr)
{
@@ -51,6 +53,80 @@ static unsigned long get_shstk_addr(void)
return ptr;
}

+#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;
@@ -114,3 +190,68 @@ void cet_disable_free_shstk(struct task_struct *tsk)

tsk->thread.cet.shstk_enabled = 0;
}
+
+/*
+ * Called from __fpu__restore_sig() under the protection
+ * of fpregs_lock().
+ */
+int cet_restore_signal(bool ia32, struct sc_ext *sc_ext)
+{
+ unsigned long new_ssp = 0;
+ u64 msr_ia32_u_cet = 0;
+ int err;
+
+ if (current->thread.cet.shstk_enabled) {
+ err = verify_rstor_token(ia32, sc_ext->ssp, &new_ssp);
+ if (err)
+ return err;
+
+ msr_ia32_u_cet |= MSR_IA32_CET_SHSTK_EN;
+ }
+
+ wrmsrl(MSR_IA32_PL3_SSP, new_ssp);
+ wrmsrl(MSR_IA32_U_CET, msr_ia32_u_cet);
+ 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)
+{
+ unsigned long ssp = 0, new_ssp = 0;
+ u64 msr_ia32_u_cet = 0;
+ int err;
+
+ msr_ia32_u_cet = 0;
+ ssp = 0;
+
+ if (current->thread.cet.shstk_enabled) {
+ ssp = 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;
+
+ msr_ia32_u_cet |= MSR_IA32_CET_SHSTK_EN;
+ sc_ext->ssp = new_ssp;
+ }
+
+ modify_fpu_regs_begin();
+ wrmsrl(MSR_IA32_PL3_SSP, ssp);
+ wrmsrl(MSR_IA32_U_CET, msr_ia32_u_cet);
+ modify_fpu_regs_end();
+ return 0;
+}
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 4ecf1764a971..2673a2567174 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -52,6 +52,58 @@ static inline int check_for_xstate(struct fxregs_state __user *buf,
return 0;
}

+int setup_fpu_system_states(int is_ia32, unsigned long restorer,
+ void __user *fp)
+{
+ int err = 0;
+
+#ifdef CONFIG_X86_64
+ if (cpu_x86_cet_enabled() && fp) {
+ struct sc_ext ext = {0, 0};
+
+ err = cet_setup_signal(is_ia32, restorer, &ext);
+ if (!err) {
+ void __user *p;
+
+ ext.total_size = sizeof(ext);
+
+ p = fp + 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_fpu_system_states(int is_ia32, void __user *fp)
+{
+ int err = 0;
+
+#ifdef CONFIG_X86_64
+ if (cpu_x86_cet_enabled() && fp) {
+ struct sc_ext ext = {0, 0};
+ void __user *p;
+
+ p = fp + 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.
*/
@@ -347,6 +399,10 @@ 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, fx_only);
pagefault_enable();
+
+ if (!ret)
+ ret = restore_fpu_system_states(0, buf);
+
if (!ret) {
fpregs_mark_activate();
fpregs_unlock();
@@ -433,6 +489,17 @@ int fpu__restore_sig(void __user *buf, int ia32_frame)
return __fpu__restore_sig(buf, buf_fx, size);
}

+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())
+ 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)
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index ce9421ec285f..92a2c2b2dcd0 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); \
@@ -264,6 +265,7 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
sp = (unsigned long) ka->sa.sa_restorer;
}

+ sp = fpu__alloc_sigcontext_ext(sp);
sp = fpu__alloc_mathframe(sp, IS_ENABLED(CONFIG_X86_32),
&buf_fx, &math_size);
*fpstate = (void __user *)sp;
@@ -493,6 +495,9 @@ static int __setup_rt_frame(int sig, struct ksignal *ksig,
err |= setup_sigcontext(&frame->uc.uc_mcontext, fp, regs, set->sig[0]);
err |= __copy_to_user(&frame->uc.uc_sigmask, set, sizeof(*set));

+ if (!err)
+ err = setup_fpu_system_states(0, (unsigned long)ksig->ka.sa.sa_restorer, fp);
+
if (err)
return -EFAULT;

@@ -579,6 +584,9 @@ static int x32_setup_rt_frame(struct ksignal *ksig,
regs, set->sig[0]);
err |= __copy_to_user(&frame->uc.uc_sigmask, set, sizeof(*set));

+ if (!err)
+ err = setup_fpu_system_states(0, (unsigned long)ksig->ka.sa.sa_restorer, fpstate);
+
if (err)
return -EFAULT;

--
2.17.1

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