[PATCH v26 24/30] x86/cet/shstk: Introduce shadow stack token setup/verify routines

[Yu-cheng Yu]

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, since those pointers point to executable code area.

The restore token can be used as an extra protection for signal handling.
To deliver a signal, create a shadow stack restore token and put the token
and the signal restorer address on the shadow stack.  In sigreturn, verify
the token and restore from it the shadow stack pointer.

Introduce token setup and verify routines.  Also introduce WRUSS, which is
a kernel-mode instruction but writes directly to user shadow stack.  It is
used to construct user signal stack as described above.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
---
v25:
- Update inline assembly syntax, use %[].
- Change token address from (unsigned long) to (u64/u32 __user *).
- Change -EPERM to -EFAULT.

 arch/x86/include/asm/cet.h           |   9 ++
 arch/x86/include/asm/special_insns.h |  32 +++++++
 arch/x86/kernel/shstk.c              | 126 +++++++++++++++++++++++++++
 3 files changed, 167 insertions(+)

diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
index 8b83ded577cc..ef6155213b7e 100644
--- a/arch/x86/include/asm/cet.h
+++ b/arch/x86/include/asm/cet.h
@@ -20,6 +20,10 @@ int shstk_setup_thread(struct task_struct *p, unsigned long clone_flags,
 		       unsigned long stack_size);
 void shstk_free(struct task_struct *p);
 void shstk_disable(void);
+int shstk_setup_rstor_token(bool ia32, unsigned long rstor,
+			    unsigned long *token_addr, unsigned long *new_ssp);
+int shstk_check_rstor_token(bool ia32, unsigned long token_addr,
+			    unsigned long *new_ssp);
 #else
 static inline int shstk_setup(void) { return 0; }
 static inline int shstk_setup_thread(struct task_struct *p,
@@ -27,6 +31,11 @@ static inline int shstk_setup_thread(struct task_struct *p,
 				     unsigned long stack_size) { return 0; }
 static inline void shstk_free(struct task_struct *p) {}
 static inline void shstk_disable(void) {}
+static inline int shstk_setup_rstor_token(bool ia32, unsigned long rstor,
+					  unsigned long *token_addr,
+					  unsigned long *new_ssp) { return 0; }
+static inline int shstk_check_rstor_token(bool ia32, unsigned long token_addr,
+					  unsigned long *new_ssp) { return 0; }
 #endif
 
 #endif /* __ASSEMBLY__ */
diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index 1d3cbaef4bb7..5a0488923cae 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_SHADOW_STACK
+#if defined(CONFIG_IA32_EMULATION) || defined(CONFIG_X86_X32)
+static inline int write_user_shstk_32(u32 __user *addr, u32 val)
+{
+	asm_volatile_goto("1: wrussd %[val], (%[addr])\n"
+			  _ASM_EXTABLE(1b, %l[fail])
+			  :: [addr] "r" (addr), [val] "r" (val)
+			  :: fail);
+	return 0;
+fail:
+	return -EFAULT;
+}
+#else
+static inline int write_user_shstk_32(u32 __user *addr, u32 val)
+{
+	WARN_ONCE(1, "%s used but not supported.\n", __func__);
+	return -EFAULT;
+}
+#endif
+
+static inline int write_user_shstk_64(u64 __user *addr, u64 val)
+{
+	asm_volatile_goto("1: wrussq %[val], (%[addr])\n"
+			  _ASM_EXTABLE(1b, %l[fail])
+			  :: [addr] "r" (addr), [val] "r" (val)
+			  :: fail);
+	return 0;
+fail:
+	return -EFAULT;
+}
+#endif /* CONFIG_X86_SHADOW_STACK */
+
 #define nop() asm volatile ("nop")
 
 static inline void serialize(void)
diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index d387df84b7f1..48a0c87414ef 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -20,6 +20,7 @@
 #include <asm/fpu/xstate.h>
 #include <asm/fpu/types.h>
 #include <asm/cet.h>
+#include <asm/special_insns.h>
 
 static void start_update_msrs(void)
 {
@@ -176,3 +177,128 @@ void shstk_disable(void)
 
 	shstk_free(current);
 }
+
+static unsigned long _get_user_shstk_addr(void)
+{
+	struct fpu *fpu = &current->thread.fpu;
+	unsigned long ssp = 0;
+
+	fpregs_lock();
+
+	if (fpregs_state_valid(fpu, smp_processor_id())) {
+		rdmsrl(MSR_IA32_PL3_SSP, ssp);
+	} else {
+		struct cet_user_state *p;
+
+		p = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
+		if (p)
+			ssp = p->user_ssp;
+	}
+
+	fpregs_unlock();
+	return ssp;
+}
+
+#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)
+
+/*
+ * 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 *token_addr)
+{
+	unsigned long addr;
+
+	*token_addr = 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((u64 __user *)addr, (u64)ssp))
+		return -EFAULT;
+
+	*token_addr = addr;
+	return 0;
+}
+
+/*
+ * Create a restore token on shadow stack, and then push the user-mode
+ * function return address.
+ */
+int shstk_setup_rstor_token(bool ia32, unsigned long ret_addr,
+			    unsigned long *token_addr, unsigned long *new_ssp)
+{
+	struct cet_status *cet = &current->thread.cet;
+	unsigned long ssp = 0;
+	int err = 0;
+
+	if (cet->shstk_size) {
+		if (!ret_addr)
+			return -EINVAL;
+
+		ssp = _get_user_shstk_addr();
+		err = _create_rstor_token(ia32, ssp, token_addr);
+		if (err)
+			return err;
+
+		if (ia32) {
+			*new_ssp = *token_addr - sizeof(u32);
+			err = write_user_shstk_32((u32 __user *)*new_ssp, (u32)ret_addr);
+		} else {
+			*new_ssp = *token_addr - sizeof(u64);
+			err = write_user_shstk_64((u64 __user *)*new_ssp, (u64)ret_addr);
+		}
+	}
+
+	return err;
+}
+
+/*
+ * Verify token_addr point to a valid token, and then set *new_ssp
+ * according to the token.
+ */
+int shstk_check_rstor_token(bool ia32, unsigned long token_addr, unsigned long *new_ssp)
+{
+	unsigned long token;
+
+	*new_ssp = 0;
+
+	if (!IS_ALIGNED(token_addr, 8))
+		return -EINVAL;
+
+	if (get_user(token, (unsigned long __user *)token_addr))
+		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) != token_addr) || token >= TASK_SIZE_MAX)
+		return -EINVAL;
+
+	*new_ssp = token;
+	return 0;
+}
-- 
2.21.0